# Solid Shed build,vapor barrier question



## yuzi87 (9 Jul 2020)

Hi Guys,

i just finished a 2 year project building a 2 story wrap round extension and complete renovation of existing house and now some of the rooms are full of tools which means i need a shed!

As i love building things and wood work i am building my own and i just have a question regarding the vapor barrier.

The shed is 4.5m x 2.2m the floor and roof will be made from 5x2" 400 centres frame sitting on 6 concrete block piers approx 15cm from ground.

I am thinking for the floor i will have 100mm celotex within the floor frame sealed off by expanding foam and aluminium tape flush to bottom, leaving air gap then roofing breathable membrane(upside down) then 18mm osb

The walls starting inside will be 4x2" 400 centre framework, 18mm osb, 100mm insulation, 18mm osb, breathable membrane, battons, feather edge.

My question is do i need vapor barrier on the internal stud side behind the osb? or the floor?

The shed will have an electric distribution board in and thats the only possible heat source.

Many thanks, 
some pics attached progress so far


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## MikeG. (10 Jul 2020)

Blocks with expanding foam, on the ground, and then upside- down felt. Why not do the job properly? Air gaps in floors refer to the space below the bottom of the joists....and you should have a 150 space there. I can't begin to think why you are planning to do what you have in mind.

OSB is vapour impermeable (to all practical intents). It therefore should not be used on the outside of insulation. Used on the inside it forms an effective vapour barrier in the correct place in the wall, and you don't need a secondary layer. 18mm is unnecessarily thick for walls. 10mm is more than adequate.

I suggest you have a read of the two threads linked to in my signature, and you'll maybe have a bit of a re-think before you go any further.


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## yuzi87 (10 Jul 2020)

Im not putting blocks on the floor........ im putting blocks of 100mm celotex flush within the floor frame which is raised around 150mm from ground....

Then i will seal all gaps between celotex and frame with expandable foam and aluminium tape, the roofing felt repels water while still breathable thus it needs to be reversed when put on the floor under the osb. 
10mm may be enough but this is a solid shed build like i said and double layer 18mm osb will certainly achieve that


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## MikeG. (10 Jul 2020)

Apologies, I misread the floor stuff about Celotex....but I note your edit. I ask again, how are you going to keep vermin out from the underside of the shed?

Your choice, but you shouldn't have a vapour barrier on the outside of the insulation, which is what you are proposing with the unnecessary OSB on the outside of the studwork. You risk interstitial condensation, mould, and eventual rotting. Leave it out and put a breather membrane there in its place, and of course, batten out before applying your external wall covering. I just saved you hundreds of pounds.

The felt under the floor achieves nothing (positive) at all. Also, it isn't breathable. The reason it has been superseded as a roofing material is that it didn't allow the passage of vapour out of loft spaces, and without adequate ventilation rooves actually rotted as a result. That's why their replacement, roofing membranes, are known as "breather membrane". Felt where you are proposing it in the floor would be another example of a vapour barrier on the wrong side of the insulation.

Please read the threads I have linked to. You could save yourself some unnecessary problems.


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## yuzi87 (10 Jul 2020)

I've watched many shed build videos now and many off them have framed wall, osb on outside then covered in membrane followed by battons and cladding, the inside then has insulation and more ply/osb or plasterboard. 

Mine would be the same as this so im not sure why it would not work?

Also i will skip the membrane on the floor as foil backed celotex sealed with metal tape acts as a vapor barrier in itself. Rodents hopefully wont be a problem i'll set traps if they are. Or perhaps i could put a wire mesh on underside of frame

I have already bought sheets of 18mm osb so i wont be saving anything, i've looked at your detailed drawings they are very informative, however the shed is not as solid as i require thus the 2x walls of 18mm osb


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## MikeG. (10 Jul 2020)

You crack on. Youtube obviously trumps architectural training and 20 years experience, and 200- odd workshop builds around the world. I'm never really sure why people come looking for advice and are then impervious to the answers they get.


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## yuzi87 (10 Jul 2020)

Hi Mike i'm certainly not discounting your valuble advice, but the internet is a good tool and there are many experienced people as well as yourself and so i am just taking all the info in and questioning everything.

Like these videos here, one who seems very professional - https://youtu.be/UWQqGBsqfI4
https://youtu.be/LP67MAoihZk


Is that a bad shed build because thats pretty much what i was planning apart from i would have battons behind the cladding, but this build has ply both sides.


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## MikeG. (10 Jul 2020)

Yep, it's wrong. As I say, you carry on and do it how you were going to do it anyway. What you aren't understanding is that almost no properly trained people bother working down at the level of domestic outbuildings, so I am the dissenting voice amongst a bunch of builders and DIYers who don't know any better. And there is no comeback, because Youtube hasn't been around long enough for these buildings to start failing yet. I am happy to help people build decent workshops FOC, but I don't have a Youtube channel so clearly can't have a clue what I am talking about.


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## yuzi87 (10 Jul 2020)

what about this guy? He seems to have a good company from it? https://youtu.be/UWQqGBsqfI4

I just want some more science behind it, so i can understand better, like there must be a way that you can achieve what i am after surely, can i not add those periscope vents in your drawings? Based on my idea what will happen in regards to vapor etc so we can work a way around it


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## MikeG. (10 Jul 2020)

Good luck with your build.

And will you stop editing your posts after they've been responded to. You've changed the video in your previous post, and that is potentially misleading for anyone following along. That's really pretty poor behaviour.


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## yuzi87 (10 Jul 2020)

I was already editing the post to add another video as well as the original while you replied. 

So you have no response or answers to my last post? There is a plethora of shed build videos and many vary, you cant just say "they are all wrong im right listen to me" i would just appreciate some science or facts behind why it wont work and how vapor/condensation would react to that build so i can understand better and find a work around because like everything there is always a way.


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## spb (10 Jul 2020)

Condensation happens when warm, humid air meets a cold surface. To prevent it, you need the vapour barrier, which will keep the humid air on one side of it, to be at the same temperature as that air. Hence, the OSB goes on the inside of the insulation, because the outside of the insulation is colder. If you have two impermeable layers then there's no way for the air in between them to follow seasonal changes in humidity, and you'll still get condensation on the inside of the outer one. This is why double glazing panels are completely sealed and filled with inert gas, because if they weren't then they'd get condensation between the layers whenever it got cold out.

As for why so many professionals build sheds that way, because it's slightly faster and easier, and by the time the problems become apparent they'll be long gone. Mike is telling you, based on formal training and years of experience, the way to build a shed that will last as long as you do. People on YouTube are showing you how to build a shed that will last long enough that the customer won't ask for their money back. Which one you want to follow is up to you.


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## yuzi87 (11 Jul 2020)

Thanks for your reply, so can't i just put some vents in the framework to help airflow and any condensation? Or put the vapour barrier on the outside of the outer osb?


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## MikeG. (11 Jul 2020)

There's good reason not to get involved with some people. :roll: 



yuzi87":2qs3q7n1 said:


> ........... can't i just......put the vapour barrier on the outside of the outer osb?





MikeG.":2qs3q7n1 said:


> .......you shouldn't have a vapour barrier outside of the insulation.....


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## yuzi87 (11 Jul 2020)

Im sorry but i like to understand things not just be told things, instead of explaining what would happen etc your just my way or the highway.


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## MikeG. (11 Jul 2020)

yuzi87":33tlr2ck said:


> Im sorry but i like to understand things not just be told things, instead of explaining what would happen etc your just my way or the highway.





MikeG.":33tlr2ck said:


> .........You risk interstitial condensation, mould, and eventual rotting.........





spb":33tlr2ck said:


> Condensation happens when warm, humid air meets a cold surface. To prevent it, you need the vapour barrier, which will keep the humid air on one side of it, to be at the same temperature as that air. Hence, the OSB goes on the inside of the insulation, because the outside of the insulation is colder. If you have two impermeable layers then there's no way for the air in between them to follow seasonal changes in humidity, and you'll still get condensation on the inside of the outer one......


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## yuzi87 (12 Jul 2020)

I know how consendation works but i wanted to know how it would react to my build idea and then i could come up with a work around.

Based on the fact this is literally just a shed that wont be heated and i will hardly be in it do i even need a vapour barrier? Would it not be better to just leave it out and let the air regulate itsself?


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## CHJ (12 Jul 2020)

yuzi87":3rxog6sl said:


> Im sorry but i like to understand things not just be told things, instead of explaining what would happen etc your just my way or the highway.



Then you need to learn how to ask politely for clarification of principle, not pose confrontational replies that purport to show that an ‘experts’ advice is wrong.

If after an ‘expert’ has explained the principals behind the reasoning for the advice and the science behind it, you can then go on to prove that the practice as applied in the UK environment is not optimum your research would add to the collective knowledge.



yuzi87":3rxog6sl said:


> I know how consendation works but i wanted to know how it would react to my build idea and then i could come up with a work around.



If you KNOW how it works surly you KNOW what the results will be.

You don't need a 'work around' for something if you don't insist on making the mistake in the first instance, you appear to be perpetuating a problem that only exists in your mind.


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## TheTiddles (12 Jul 2020)

Just a quick note/rant about “professionals”....

“Professionals” built my house... missing mortar, missing vents, missing shower tray supports, disconnected extraction hoses, missing screws on hinges, grass over manhole covers I really don’t have enough time to list everything, our neighbours porch fell off the house and would have killed someone if they were under it at the time, based on what I’ve seen of other new builds, we got lucky.

I recently saw a “professional” plumber bodge a neighbours new boiler, the condensation pipe is about 6’ of copper snaking down the front of their house to the drain including some level sections, that’ll work well in the winter...

I had some “professional” landscapers do some work outside, they tried to convince me that steps shouldn’t be level side to side for water runoff (after they failed to put them in level).

So, builders and other “professionals”... you have to repeatedly tell them how to do their own jobs as they don’t know, they have no qualifications and no professional standards and no limitations to them calling themselves these names and no penalties for failure. They are anything but “professional”.

I’ve been watching that guys videos too, I like how he constructs the foundations, that about where the admiration ends, last nights video had “bodge-job” written all over it.

Aidan


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## yuzi87 (12 Jul 2020)

You are completely correct, that is why i researched every single aspect of my house build project managed it and built most of it myself. 

Please don't take offence to my line of questioning, i dont make chit chat, i say please and thank you and get to the point, until i understand something completely i will continue to ask questions from every angle.

Im a security engineer and i want a double skinned wall for extra security, i can not believe that there is no way of achieving this.

I have yet to be told how a double skinned wall will react to condensation with/without vapor barrier and whether vents etc could bypass any potential problems


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## TheTiddles (12 Jul 2020)

You want brick then! If someone can smash through a single layer of wood, two layers won’t stop them

And your doors and windows are weaker still


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## MikeG. (12 Jul 2020)

yuzi87":1dtubp0l said:


> ...... this is literally just a shed that wont be heated.....



Why do you want insulation then?


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## yuzi87 (13 Jul 2020)

With a lot of expensive kit in the shed i dont want moisture getting in etc.

I am leaning towards just the skin inside but do i even need the vapor barrier if its not heated? If i had two skins your saying moisture wont be able to escape from in the frame, but wouldnt vents on outer skin behind membrane in each section of frame help airflow and dissipate moisture?


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## TheTiddles (13 Jul 2020)

I think you are getting a bit confused.

If you don’t want moisture getting in, you need to seal it shut with some desiccant inside and never open it again, seems a bit of a waste.
Or...
You need to desiccate the air perpetually with either heaters or dehumidifiers and then push that humid air or condensate out mechanically, this is called a tumblr dryer.

Alternatively, the usual method has been described, you are at liberty to try something else.

All sheds are heated, unless you put them in a cave.

Aidan


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## owen (14 Jul 2020)

I can't understand how you would get interstitial condensation inside a shed without there being a shower/ tumble drier etc in there to make the air humid in the first place. 
Aren't most timberframe homes built with osb on the inside and the outside? Are they all wrong too? Genuine question this by the way, I have no experience of timber frame homes having worked in old/listed buildings the last 15 years.


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## Inspector (14 Jul 2020)

Modern timber frames here (big timber mortice and tenoned) are often skinned with SIPs, (Structural Insulated Panels) That usually have OSB bonded to styrofoam or similar foams on inside and outside. The inside would usually get drywalled inside and house wrapped outside then strapped and sided. The styrofoam is not only insulation but the vapour barrier as well. SIPs can be used pretty much by themselves to build a house without the need of a timer frame. That's what I wanted but talked out by the builder in favour of double wall construction. Shouldn't have listened and found someone else. SIPs get used on timber frames as they can be had in panels up to 28' long x 8' high and up to 12" thick with all the doors and window openings pre-cut by large CNC routers at the factory. They span the distances of the timbers well and are fast to put up (1 to 3 days). They are much like the insulated metal and foam panels used to build refrigerated warehouses for the food industry but the OSB SIPs are better suited to make a house with. They are so tight that whole house ventilation is needed to keep the air fresh and clean. 

If you are talking about stick built (2x construction) houses that's a different beast.

Pete


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## Hornbeam (14 Jul 2020)

Understanding humidity, relative humidity and condensation risk factors is a science but there are some very simple approaches
In construction if you have a cavity filled with a loose fill insulation, it is virtually impossible to fully seal it so allow it to breathe out. If you do seal it fully you will have sealed in an amount of moisture at that temprature/humidity. On a colder day the air wont hold that much and you get condensation/ mould rot
With SIP panels that are usually PIR foam (dont touch styrofoam or polystyrene due to fire risk) the foam is closed cell and doesnt allow vapour movement so you can construct with a solid construction
With either construction you will be putting on an external cladding which again must allow air movement
If you want a further layer for security then you could build as
Internal OSB, Stud frame with quilt. breather membrane battens, profile steel sheet laid vertically with vented filler blocks top and bottom to allow air movement behind steel sheet and air to breathe out of the insulation cavity. OSB provides internal air tightness layer. Always place a separation layer such as dpc membrane between steel and timber due to tanning/chemical treatments of timber and corrosion risks


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## yuzi87 (15 Jul 2020)

Thanks Hornbeam for the great breakdown of information, so just to clarify, could i not build osb/dpc/frame+insulation/osb(with vents in each frame section)/breather membrane/battons/cladding?

Or if i did just do the one internal osb wall would i even need the dpc as its more of a storage shed than a regular workshop/heated room?

Thanks in advance


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## yuzi87 (15 Jul 2020)

Some progress pics


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## That would work (15 Jul 2020)

I like this idea. And glad that you finally got a measured and civil response!


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## yuzi87 (15 Jul 2020)

That would work":1e59sg85 said:


> I like this idea. And glad that you finally got a measured and civil response!



Which idea do you like mate?


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## That would work (15 Jul 2020)

The wire mesh as security


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## yuzi87 (15 Jul 2020)

That would work":3bc2zq7b said:


> The wire mesh as security



Oh I actually used it under the floor frame to stop any mice/rats getting in insulation lol


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## MikeG. (15 Jul 2020)

That would work":aw7sawou said:


> The wire mesh as security


I do that around the walls of bikes sheds for security. Stainless steel EML rather than galvanised. Pointless, of course, unless you also have a steel door and high security locking system.


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## That would work (15 Jul 2020)

I keep most of mine in a large understair cupboard :wink:


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## MikeG. (15 Jul 2020)

We've 7 bikes, I think. That'd be some pretty impressive stairs! :lol:


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## That would work (15 Jul 2020)

Front wheels out and hung up... 4 in there and two in the kitchen at night. It's quite an operation at times. Need just one more tho....


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## Hornbeam (15 Jul 2020)

owen said:


> I can't understand how you would get interstitial condensation inside a shed without there being a shower/ tumble drier etc in there to make the air humid in the first place.
> The issue is not about humidity it is about relative humidity
> All air contains water vapour. The amount of water vapour that can be held depends upon the temperature. The warmer the air the more water vapour it can hold
> So back to the shed on a moderately warm damp winters day the air will be fairly saturated with water vapour and this air will slowly permeate into the cavity. If we then have a cold clear night the air temperature and the temperature of the outer surface of the shed will drop.
> If this temperature drops to a point such that the air can no longer hold all the vapour that was originally in it and so the water condenses out. The temperature at which this occurs is known as the dew point


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## yuzi87 (15 Jul 2020)

MikeG.":3tk268bj said:


> That would work":3tk268bj said:
> 
> 
> > The wire mesh as security
> ...



I'm in security, so alarm, magnetic lock, cctv, beam sensors and smoke cannon will protect mine 8)


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## yuzi87 (16 Jul 2020)

Can somebody still answer my question whether air vents in the outer osb skin will help solve double skin problem?


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## Hornbeam (16 Jul 2020)

Some of your pictures have confused me slightly. The studwork has small blocks in to locate the PIR foam against but does this mean that there is a cavity in the construction between the PIR board and OSB
If there is no cavity then the holes wont provide any ventilation
If the cavity is absolutely full of PIR then in theory it doesnt require ventilation (as it is now acting like a SIP panel)
I too am struggling as to why you want an extra layer of OSB . You dont need it and unless your construction is absolutely spot on you will potentially get condensation. If you have already bought the OSB use it for internal shelves etc
What is the external cladding for your shed?


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## Hornbeam (16 Jul 2020)

MikeG.":3bpr44yj said:


> We've 7 bikes, I think. That'd be some pretty impressive stairs! :lol:


Only 7. My daughter has that many on her own


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## yuzi87 (16 Jul 2020)

Hornbeam":36eeevt2 said:


> Some of your pictures have confused me slightly. The studwork has small blocks in to locate the PIR foam against but does this mean that there is a cavity in the construction between the PIR board and OSB
> If there is no cavity then the holes wont provide any ventilation
> If the cavity is absolutely full of PIR then in theory it doesnt require ventilation (as it is now acting like a SIP panel)
> I too am struggling as to why you want an extra layer of OSB . You dont need it and unless your construction is absolutely spot on you will potentially get condensation. If you have already bought the OSB use it for internal shelves etc
> What is the external cladding for your shed?



You are confused because i laid the floor frame upright to install the mesh under the frame, only the floor and roof will be pir insulation the walls will be rockwall etc. 
Once i put noggins in i could put a vent in outer osb for each section to allow air flow?

I like the idea of double osb as it will be absolutley solid and i like to over engineer things.

If i dont though will i def need vapor barrier inside as its £65 for a roll :shock: 

External cladding is just feather edge boards


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## MikeG. (16 Jul 2020)

yuzi87":ukcist16 said:


> ...... this is literally just a shed that wont be heated.....





MikeG.":ukcist16 said:


> Why do you want insulation then?





yuzi87":ukcist16 said:


> ......... only the floor and roof will be pir insulation the walls will be rockwall etc.....



Why are you insulating an unheated shed?


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## yuzi87 (16 Jul 2020)

MikeG.":39i7x9ig said:


> yuzi87":39i7x9ig said:
> 
> 
> > ...... this is literally just a shed that wont be heated.....
> ...



Why not? I will have lots of expensive tools in there and i dont want them rusting up or getting condensation inside so i wanted to build a nice shed that can keep warm etc


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## Hornbeam (17 Jul 2020)

I think you are missing the point and seem to be disagree with the information people are giving you. I am confused because you didnt say the walls would be rockwool and yes I can tell that the image you showed was the floor.
Simple
OSB inner leaf
No air tightnes membrane required as the OSB provides that
Rockwook insulation
Breather membrane Make sure you get the right one
Cavity which must be vented top and bottom
External cladding

Additional external layer of OSB adds cost and introduces additional issues regarding vapour movement

Insulation for an unheated shed will not keep things warm. Insulation slows down heat transfer so your insulated unheated shed will be just as cold as an uninsulated unheated shed. Insulation will slow down the rate of temperature changes in the shed but you still run the risk of condensation on highly conductive materials like metal tools. You should consider building as airtight as possible and running a dehumidifier


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## Inspector (17 Jul 2020)

Up to the last post I was under the impression that there was no heat. In the opening post you wrote "The shed will have an electric distribution board in and thats the only possible heat source." I took that to mean any heat at all would come from the panel and not something wired to or plugged into it to heat the space. Since a panel shouldn't generate heat it follows that the shed is unheated.

Pete


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## owen (17 Jul 2020)

Hornbeam":18mq9slr said:


> owen":18mq9slr said:
> 
> 
> > I can't understand how you would get interstitial condensation inside a shed without there being a shower/ tumble drier etc in there to make the air humid in the first place.
> ...



Thanks for that, so when the sheds filled with damp air, because youve had the doors open while working in there, how does it get out once you shut the doors when the walls are lined with OSB, if osb is a vapour barrier? And how would the damp air get into the wall if the outside and inside are lined with OSB.


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## MikeG. (17 Jul 2020)

Hornbeam":1au08i5u said:


> .........Cavity which must be vented top and bottom.........



I'm content that cavities behind timber boarding need only be vented at the bottom. If it is rendered, or clad with a sheet material, then yes, it needs venting at the top too, but timber has enough air leakage, I think, to mean this isn't necessary. My empirical view only, without much evidence.


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## MikeG. (17 Jul 2020)

owen":yuch9g2c said:


> ........so when the sheds filled with damp air, because youve had the doors open while working in there, how does it get out once you shut the doors when the walls are lined with OSB, if osb is a vapour barrier?



If you've had the doors open you are likely to be reducing the amount of dampness in the shed rather than increasing it.



> And how would the damp air get into the wall if the outside and inside are lined with OSB.



There is air and thus moisture in the wall when you construct it. The OP will be sealing damp into the wall with this proposed build-up. And secondly, no vapour barrier is perfect. The rule of thumb is to have 5 times the vapour resistance on the inner face of the wall compared with the outer, so that any vapour that gets in has a route out without condensing.


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## CHJ (17 Jul 2020)

yuzi87":gqteefs6 said:


> MikeG.":gqteefs6 said:
> 
> 
> > Why are you insulating an unheated shed?
> ...



No amount of Insulation will prevent metal machinery and tools from eventually gathering a surface rust film in an unheated storage facility, in fact even heating it is no guaranty that atmospheric air moisture will not settle (condense out) on metal surfaces unless you keep it above atmospheric temp. at all times.

In a cold weather spell, eventually, regardless of the insulation any metal will act as a cold sink and drop in temperature, when the external atmospheric temperature rises with its increased humidity potential as soon as you let that warmer air into the building air moisture is going to settle out on the cold metal surfaces up until such time as the metal temperature rises to atmospheric air temp.

The only way to prevent this regardless of whether the building is insulated or open to the elements is to ensure that any metal surfaces that are not fully protected by a none oxidizing coating is to ensure that the metal itself is warmer than any atmospheric air that is likely to impinge on it.

Even exhaled breathing air or damp clothing is enough to cause condensation on tools and machinery if the metal it is colder than the surrounding air mass. Just popping into a shed for a few minutes on a wet day can introduce excessive condensate onto cold metal surfaces.

To this end many workshops and stores deliberately fit their machines and tool storage cabinets with low wattage heating running 24/7 to keep them 1-2 deg. above ambient air temp. 

Many UKW members resort to low wattage bulbs, vivarium heater mats, direct contact low voltage powered resistors etc. to achieve this so that they don't have to keep the whole workshop or store above ambient temperature.

(I was doing this with 7watt car sidelight bulb running off a transformer in the bottom of my tool cupboard in the early 1960's)


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## owen (17 Jul 2020)

MikeG.":2d7dmsd2 said:


> owen":2d7dmsd2 said:
> 
> 
> > ........so when the sheds filled with damp air, because youve had the doors open while working in there, how does it get out once you shut the doors when the walls are lined with OSB, if osb is a vapour barrier?
> ...



Thanks Mike that makes more sense, so if you did put OSB on the outside, as long as there was ventilation behind it, it would be ok wouldn't it? I'm trying to make sense of it in my head, and some of it makes sense some of it doesn't :shock: :lol:


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## MikeG. (17 Jul 2020)

If you put ventilation into insulation you may as well not have the insulation......(if that's what you meant by "ventilation behind it").

It is interesting how a seemingly simple subject is a lot more complex than people think.


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## MikeG. (17 Jul 2020)

CHJ":2jdit5pe said:


> ........The only way to prevent this regardless of whether the building is insulated or open to the elements is to ensure that any metal surfaces that are not fully protected by a none oxidizing coating is to ensure that the metal itself is warmer than any atmospheric air that is likely to impinge on it...........



Or keep the air moving. Ventilation solves a lot of these issues.


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## yuzi87 (17 Jul 2020)

MikeG.":2fohfehc said:


> If you put ventilation into insulation you may as well not have the insulation......(if that's what you meant by "ventilation behind it").
> 
> It is interesting how a seemingly simple subject is a lot more complex than people think.



What i dont understand is by your proposed build after the insulation there is a big air gap until the cladding.

If i but osb after insulation with a vent hole inside each frame section this would allow any trapped air to release and mix with the air space behind cladding just like your build?

I dont understand why it wouldnt work doing that?


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## TheTiddles (17 Jul 2020)

You’ve said several times that you’re an engineer, but seem to be struggling with very basic physics, just draw the thermal gradient through your proposed wall and you’ll see where and why condensation is likely and why.

Consider also that outdoors is rather well ventilated but on a clear summer’s morning there is often condensation over everything.

Aidan


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## MikeG. (17 Jul 2020)

TheTiddles":1dvw4npz said:


> ......Consider also that outdoors is rather well ventilated but on a clear summer’s morning there is often condensation over everything.......



On a clear *still* summers morning. You won't find dew if it has been windy. I'm afraid my physics breaks down at that point, so I can't give a good explanation, but this is why ventilation is only effective if it involves moving air, not just "fresh" air.


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## RogerS (17 Jul 2020)

Hornbeam":3h1x3oco said:


> MikeG.":3h1x3oco said:
> 
> 
> > We've 7 bikes, I think. That'd be some pretty impressive stairs! :lol:
> ...


Why the aggression ? 'Mine's bigger than yours' type of thing ?


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## MikeG. (17 Jul 2020)

I saw that as gentle teasing/ humour, Roger. Cyclists are supposed to aspire to N+1 bikes, where N is the number you have now, so we tend to take the mickey out of people who don't quite have enough. Harmless fun, I thought.


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## yuzi87 (17 Jul 2020)

TheTiddles":14oowncj said:


> You’ve said several times that you’re an engineer, but seem to be struggling with very basic physics



Sorry i didn't realise there is only one type of engineer who is master of all sciences and trades. :roll: 





Roof next


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## TheTiddles (17 Jul 2020)

Good maths? Strong understanding of physics? Able to apply those principles? Capable of reviewing arguments and making judgments based on them? If yes, you may be an engineer.


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## Max Power (18 Jul 2020)

MikeG.":1w3fmohx said:


> Hornbeam":1w3fmohx said:
> 
> 
> > .........Cavity which must be vented top and bottom.........
> ...


Would it be ok to vent at the top rather than the bottom, less chance of mice getting in


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## MikeG. (18 Jul 2020)

No, because any condensate or wind-driven water ingress needs a direct way out. It can't be allowed to pool. The gap should be sealed with insect mesh at the bottom to allow air in, water out, and to prevent insects and vermin from getting in.


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## Hornbeam (18 Jul 2020)

This is why on larger buildings we always vent top and bottom. Firstly it allows air in and out. Second the opening at the bottom allows any condensate that may form a route out, Third warm air rises so if there is condensate in teh cavity as it heats up in the sun and evaporated the warm air rises in the cavity and exits at teh top also having 2 openings helps to drive ventilation


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## Hornbeam (18 Jul 2020)

RogerS":exwoqvs9 said:


> Hornbeam":exwoqvs9 said:
> 
> 
> > MikeG.":exwoqvs9 said:
> ...


No Aggression I just have bikes everywhere


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## MikeG. (18 Jul 2020)

Hornbeam":2qbpv8t3 said:


> This is why on larger buildings we always vent top and bottom. Firstly it allows air in and out. Second the opening at the bottom allows any condensate that may form a route out, Third warm air rises so if there is condensate in teh cavity as it heats up in the sun and evaporated the warm air rises in the cavity and exits at teh top also having 2 openings helps to drive ventilation



Absolutely. The one exception I make to that is when the rainscreen is rough timber boarding on a small building, where there is enough air leakage through it to make up for the lack of venting at the top (in my non-proven view).


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## Hornbeam (18 Jul 2020)

MikeG.":3sy1snhe said:


> I saw that as gentle teasing/ humour, Roger. Cyclists are supposed to aspire to N+1 bikes, where N is the number you have now, so we tend to take the mickey out of people who don't quite have enough. Harmless fun, I thought.


That the way it was intended Mike. Funnily my daughter is Nicole and we often call her N
She has gone back to university today and taken a bike with her so we now have 1-N


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## yuzi87 (18 Jul 2020)

TheTiddles":1ptygfjb said:


> Good maths? Strong understanding of physics? Able to apply those principles? Capable of reviewing arguments and making judgments based on them? If yes, you may be an engineer.



Well i just designed and built a house minus brickwork with no previous experience, i run 3 companys and i have been building computers for 17 years so i guess i must be good at something. :lol: 

Having pieced together multiple comments on this thread i have come to the conclusion that double skin osb is certainly do-able based on the osb leaving a gap at top and bottom of each seperate frame section to allow air flow and warm air to escape and water pooling to escape.

I think i will probably just do one skin anyway due to the time involved and i think i have under ordered enough boards. #-o 

I appreciate everyones time and knowledge given on the matter and i will continue to update with pictures.

One final question i still am unsure of though is do i actually need inner vapor barrier as one suggestion was that the osb is good enough barrier, based on the fact that its just a storage shed really


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## MikeG. (18 Jul 2020)

Jesus H........ :roll:

Anyone following along in years to come........just get out of this thread and go and look at one where the advice given and taken is correct. Following along here you will end up with a cock-up.


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## Hornbeam (18 Jul 2020)

One final question i still am unsure of though is do i actually need inner vapor barrier as one suggestion was that the osb is good enough barrier, based on the fact that its just a storage shed really[/quote]

No


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## TheTiddles (18 Jul 2020)

yuzi87":3l99fe3s said:


> TheTiddles":3l99fe3s said:
> 
> 
> > Good maths? Strong understanding of physics? Able to apply those principles? Capable of reviewing arguments and making judgments based on them? If yes, you may be an engineer.
> ...



Clearly you don’t need advice on a damp membrane then, do you!


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## yuzi87 (18 Jul 2020)

TheTiddles":yyvu0jht said:


> yuzi87":yyvu0jht said:
> 
> 
> > TheTiddles":yyvu0jht said:
> ...



Are you just being obtuse for the hell of it?


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## yuzi87 (18 Jul 2020)

MikeG.":fl8xxe4s said:


> Jesus H........ :roll:
> 
> Anyone following along in years to come........just get out of this thread and go and look at one where the advice given and taken is correct. Following along here you will end up with a cock-up.



What cock up is that? Following the same design that you specify but adding some external boarding that still leaves a gap in each frame section top and bottom, so basically changing nothing but providing extra security and strength.

What is the downside to that idea? I would love to hear it. So many people on here with a stick up their buttocks, the refusal to discuss or solve problems, absolute belittling of anyone who wants to try something new, and sly remarks made when questions are asked.


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## yuzi87 (18 Jul 2020)

TheTiddles":2izz0hhp said:


> Clearly you don’t need advice on a damp membrane then, do you!



No i don't because im God and I know everything :roll: .....

What a ridiculous thing to say, i say im a security engineer yet you then think i should be a specialist in physics and moisture and try to belittle me like im an silly person which i then disprove and now you say i dont need help with the one thing i asked for help. 

Trolls exist in shed building forums as well as social media platforms it seems


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## Hornbeam (18 Jul 2020)

I have generally found that people on this forum offer clear constructive advice. I think this is true for virtually everything written so far on this thread. However when given advice rather than take it you seem to question the advice and ask why your method wont work. There are plenty of ways of skinning a cat but my general advice is stick to what has been proven to work well and be aware that not following standard approaches can lead to a lot of potential problems (look at how much problems there were with rot/mould timber framed housing when the detailing wasnt right


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## MikeG. (19 Jul 2020)

yuzi87":1490ta3f said:


> MikeG.":1490ta3f said:
> 
> 
> > Jesus H........ :roll:
> ...



I told you very early on that I am not discussing it with you because of your insistence that you knew best and that some guy on Youtube backed you up. Crack on, I said. There have since been four pages of people explaining to you how vapour movement works within a wall. You've ignored everything. Or you haven't understood anything. One or the other. Your completely muddled approach to insulation shows this beautifully. You got the "Jesus H" for taking the venting arrangements for a void and applying them to an element stuffed full of insulation, then trying to turn it into some sort of agreement with what we've been saying. It's bizarre behaviour, before we even start on the "stick up backside" insults. Nobody here is trolling. You have been offered pages of advice by people who know what they are talking about, and you have learnt absolutely nothing at all. Your loss, as this is just about the only place on the English-speaking part of the internet where you can get quality advice on the construction of small outbuildings.


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## yuzi87 (19 Jul 2020)

MikeG.":12na37t8 said:


> I told you very early on that I am not discussing it with you because of your insistence that you knew best and that some guy on Youtube backed you up. Crack on, I said. There have since been four pages of people explaining to you how vapour movement works within a wall. You've ignored everything. Or you haven't understood anything. One or the other. Your completely muddled approach to insulation shows this beautifully. You got the "Jesus H" for taking the venting arrangements for a void and applying them to an element stuffed full of insulation, then trying to turn it into some sort of agreement with what we've been saying. It's bizarre behaviour, before we even start on the "stick up backside" insults. Nobody here is trolling. You have been offered pages of advice by people who know what they are talking about, and you have learnt absolutely nothing at all. Your loss, as this is just about the only place on the English-speaking part of the internet where you can get quality advice on the construction of small outbuildings.




I have actually learnt a lot thanks, i just ask a lot of questions. The internet is full of companies and individuals building sheds and workshops and many of them are double skinned, now you guys on here say they are all wrong which is fine if thats the case but dont expect me to just take your word for it because my brain doesnt work that way and i also know you guys as much as the youtube guys, hence i ask a lot of questions and come up with a lot of ideas to come to a conclusion.

My conclusion is you have the ideal shed build all sussed out and a tried and tested method, this is how i will also build my shed minus the vapor barrier. 
However there are always ways to adapt things to suit your needs and based on the sciences that have been explained here i can not see why my idea with osb across the outer frame with gaps top and bottom of each frame section would be of major detriment to a shed build.

All the hundreds of builds on youtube im sure are still standing or the companies would not exist still due to so many negative reviews, yet my idea is an improvement on the negative aspect of their builds by allowing drainage, air flow and warm air to escape.

Its a work around on my proposed build that takes into account what ive been told on here and rather than discuss the idea or add to it its just "you obviously dont listen" "nobody listen to this guy" 

New inventions and technologys exist because people try new things and ask questions and think outside the box.

P.s i never once insisted i am right or said that a youtube guy backs me up


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## MikeG. (19 Jul 2020)

Keep on telling yourself that you are thinking outside the box. Keep on telling yourself that what you are proposing concords with what you've been told here. The reality is that you are making a fundamental mistake, and don't have enough knowledge or understanding to realise it.


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## yuzi87 (19 Jul 2020)

MikeG.":qpyv5ngd said:


> Keep on telling yourself that you are thinking outside the box. Keep on telling yourself that what you are proposing concords with what you've been told here. The reality is that you are making a fundamental mistake, and don't have enough knowledge or understanding to realise it.



Then tell me how what i am suggesting will majorly affect your standard shed build


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## Hornbeam (19 Jul 2020)

Believe me new developments on cladding systems do not happen overnight. From my work which covers most aspects of large industrial building cladding, we have proven calculated and tested standard details. New details would go through a formal process of concept design, calculation, accelerated testing and possibly long term exposure testing on a test rig.
For a simple garden shed stick to the tried and tested approach


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## yuzi87 (19 Jul 2020)

Hornbeam":1ca5c94u said:


> I have generally found that people on this forum offer clear constructive advice. I think this is true for virtually everything written so far on this thread. However when given advice rather than take it you seem to question the advice and ask why your method wont work. There are plenty of ways of skinning a cat but my general advice is stick to what has been proven to work well and be aware that not following standard approaches can lead to a lot of potential problems (look at how much problems there were with rot/mould timber framed housing when the detailing wasnt right



Thanks for your reply, i find you to be the most plesant of people on here, your posts are informative and you give reason without the need to belittle or insult. 
People ask questions thats how they learn and most people on here seem to get highly offended by it and throw their dummys out the pram "fine do it yourself then see if i care"

Personally my brain will not allow me to believe there is only one way to build a shed or even that that example can not be added to in anyway. So unless somebody can explain why what i have suggested above with my rudimentary drawing would not work i will continue to think it.


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## Ttrees (19 Jul 2020)

Good morning all
Instead of this bickering, wouldn't it be more constructive for someone to suggest a
decent youtube video series on the correct way to do it for the UK environment?

It probably would clear things up for a lot of folks.

Tom


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## MikeG. (19 Jul 2020)

yuzi87":1wauozt7 said:


> .......... So unless somebody can explain why what i have suggested above with my rudimentary drawing would not work i will continue to think it.



It won't work from a number of different aspects. Firstly, the outer cladding is a rainscreen only, and you can assume that wind-driven water will not only penetrate it, but will blow onto the inner face of the void. This has been proven numerous times with arthroscopic videos. So, you have to consider both sides of the void to be wet. Now, imagine your novel arrangement but with the cladding removed. What would happen if you turned a hose on it?

Secondly, you've been told pages back that putting a vapour barrier outside insulation provides a site for interstitial condensation, which will run down the inner face and pool at the bottom, eventually rotting your sole plate and the bottom ends of your studs, but long before that, providing an ideal breeding ground for moulds. You've also been told that the inside of a wall should provide at least 5 times the vapour resistance of the outside. Given that you are proposing putting the same material on the inside and the outside of the wall (18mm OSB), then logically you should have a maximum of one fifth of the amount of OSB on the outside as you do on the inside. In other words, instead of you proposed slots at the top and bottom, you need to remove 80% of the OSB from the outside. This of course leaves 80% of your insulation exposed to the wind driven water ingress that we talked about in the previous paragraph. As insulation performance drops to virtually nil when soaked, you will need to protect it by the installation of a breather membrane. Breather membranes only work if they have a void outside them, so you would need to counterbatten the OSB off the face of studs & membrane, before counterbattening your external cladding. Your walls just got 25mm thicker.

Slots at the top and bottom with insulation pushed up hard behind them will not be interlinked. In other words, they don't provide ventilation for the rest of the wall which is covered by OSB. Yes, they allow moisutre out at those locations, but they do nothing about the vapour that arrives at the back of the board a foot away.

Now, with 80 percent of your external OSB removed, can you tell us again what purpose it serves.

All this you have been told. It's all there in the previous pages, and in the threads you have been linked to. Please do not come back again with "this is what I am doing, now tell me why it's wrong" (in terms). Work with the principles you've been given repeatedly.


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## MikeG. (19 Jul 2020)

Ttrees":2yoy560u said:


> Good morning all
> Instead of this bickering, wouldn't it be more constructive for someone to suggest a
> decent youtube video series on the correct way to do it for the UK environment?
> 
> ...



There isn't one, as far as I know. You could look here and see one being built properly. Or here.


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## yuzi87 (19 Jul 2020)

MikeG.":3vla0556 said:


> yuzi87":3vla0556 said:
> 
> 
> > .......... So unless somebody can explain why what i have suggested above with my rudimentary drawing would not work i will continue to think it.
> ...



Thank you for the informative post, the 80% rule is interesting i will research that further, regarding your other points i would have put the breather membrane over the osb not under, does breather membrane also require clearance behind? I guess not due to it normally backing onto insulation


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## MikeG. (19 Jul 2020)

TRADA and the BRE produced some of the early research on this, but you might prefer to just take a look at this Architect's Journal article referring to the 5:1 minimum ratio. The Centre for Alternative Technology publish a series of useful and informative pamphlets, and it was through them that I first learnt of the 5:1 rule of thumb 30 odd years ago. They don't have much online, but there is a reference here. Scroll down and click on "What is Breathability". You'll find I'm not making this stuff up. 

-

If you put breather membrane on top of a vapour barrier such as OSB, it clearly no longer serves a "breather" function. It will do no more than keep the OSB dry from the outside.


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## TheTiddles (19 Jul 2020)

What a nice chap you are.


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## Hornbeam (19 Jul 2020)

Please do not confuse a vapour control layer and a breather membrane. They are 2 different things and go in different places
difference between breather membrane and vcl
Google it or check out the link below

https://blog.celotex.co.uk/technical/va ... l-celotex/


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## yuzi87 (19 Jul 2020)

Hornbeam":1l77jbj1 said:


> Please do not confuse a vapour control layer and a breather membrane. They are 2 different things and go in different places
> difference between breather membrane and vcl
> Google it or check out the link below
> 
> https://blog.celotex.co.uk/technical/va ... l-celotex/



I understand the differences i was just wondering about behind the membrane and whether that could be solid, but as Mike stated i guess it cant and it needs a void to breathe and lose fill insulation wouldnt really affect that.


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## yuzi87 (19 Jul 2020)

TheTiddles":7cg9i026 said:


> What a nice chap you are.



I am indeed a very nice chap and i don't appreciate sarcastic remarks thrown at me.


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## yuzi87 (19 Jul 2020)




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## MikeG. (19 Jul 2020)

There's no lintel over the door opening.


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## yuzi87 (19 Jul 2020)

MikeG.":1tpu1hdt said:


> There's no lintel over the door opening.



You mean reinforced double header etc above where the door will be? Yea i have not built that yet, last bit of framing to do


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## yuzi87 (19 Jul 2020)

i do notice there is still some wobble with the frame, will this be rectified once the inner osb is installed which will offer more stability and strength or will i require some diagonal braces of some sort?


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## yuzi87 (19 Jul 2020)

I guess your going to say that breather membrane is pointless due to it being not able to breath and butted up to osb and i should of raised osb roof?
If anything though it serves to stops water if somehow it gets through roof


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## MikeG. (19 Jul 2020)

yuzi87":14a4szdd said:


> MikeG.":14a4szdd said:
> 
> 
> > There's no lintel over the door opening.
> ...



No, not header. We're not American. When I said lintel, I meant lintel.


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## MikeG. (19 Jul 2020)

yuzi87":2wx3xeu6 said:


> I guess your going to say that breather membrane is pointless due to it being not able to breath and butted up to osb and i should of raised osb roof?
> If anything though it serves to stops water if somehow it gets through roof



A breather membrane under OSB!! Oh boy. This really is head-on-brick-wall territory.


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## Inspector (19 Jul 2020)

For your next build leave the top plates off until the walls are up and put them on overlapping the corners. A touch stronger that way.

Pete


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## Blackswanwood (19 Jul 2020)

yuzi87":393fy11u said:


> I guess your going to say that breather membrane is pointless due to it being not able to breath and butted up to osb and i should of raised osb roof?
> If anything though it serves to stops water if somehow it gets through roof



Okay but if water does come through the roof it’s just going to then pool on top of the breather membrane? And if vapour passes through the other way as the OSB is a vapour barrier surely that doesn’t sound too good ...moisture trapped and then mould?

Obviously it’s your shed and I maybe missing something but I don’t follow your logic.

On the racking question yes the OSB will make it much more rigid.

Good luck!


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## yuzi87 (20 Jul 2020)

Blackswanwood":2lrlavvv said:


> yuzi87":2lrlavvv said:
> 
> 
> > I guess your going to say that breather membrane is pointless due to it being not able to breath and butted up to osb and i should of raised osb roof?
> ...



Its fine i knew the answer as soon as i finshed building it lol, ill just raise the osb with some battons i guess


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## yuzi87 (20 Jul 2020)

MikeG.":160pc45a said:


> yuzi87":160pc45a said:
> 
> 
> > MikeG.":160pc45a said:
> ...



:lol: :lol: :lol:


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## yuzi87 (20 Jul 2020)

Inspector":2ht9j7rh said:


> For your next build leave the top plates off until the walls are up and put them on overlapping the corners. A touch stronger that way.
> 
> Pete



Ah good point!


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## Inspector (20 Jul 2020)

Given that you want double skins of OSB I propose you build as the others suggest to a point. OSB, insulation, vapour barrier, strapping, but at this point instead of siding put your outer OSB on, strap again and then the siding. No need for any additional membranes. Caulk and paint the inside of the OSB for better moisture resistance. Use long screws when you put the OSB on the outside that go through the strapping and deep into the studs. The downside is because you already have the walls up the siding will overhang the slab more. Upside is getting the more secure wall. With the advent of cordless tools like reciprocating saws and chainsaws crooks won't be slowed for long if they get past the electronics, so I don't know whether the extra OSB is worth it. Up to you though.

Pete

PS. Don't know why you guys don't like bigger roof overhangs to keep the rain off the walls? Also great place to hang wind chimes from.


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## MikeG. (20 Jul 2020)

No, Pete, you can't put a vapour barrier on the outside of a wall, unless you live somewhere very hot and intend air conditioning the building.

What do you mean by "strapping"?


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## MikeG. (20 Jul 2020)

yuzi87":3384j4f7 said:


> MikeG.":3384j4f7 said:
> 
> 
> > No, not header. We're not American. When I said lintel, I meant lintel.
> ...



And it's "vapour", not "vapor". :wink:


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## Inspector (20 Jul 2020)

Look once more Mike. I did write vapour. :wink: 
Whoops. That wasn’t meant for me. Sorry. 

Okay incorrectly wrote vapour barrier instead of breathable membrane. Here I would call it house wrap. 

By strapping I mean narrow boards to create an airspace to vent and drain moisture and water between two surfaces. 

Pete


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## MikeG. (20 Jul 2020)

Inspector":3tf9x6mk said:


> Look once more Mike. I did write vapour. :wink:



You did, but our OP, who I quoted, didn't.



> Okay incorrectly wrote vapour barrier instead of breathable membrane. Here I would call it house wrap.



That's a really important mistake! Pretty much the subject of this entire thread.....glad you corrected yourself.



> By strapping I mean narrow boards to create an airspace to vent and drain moisture and water between two surfaces....



Thanks for that. They're counterbattens over here.


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## yuzi87 (20 Jul 2020)

Inspector":2m59tp73 said:


> Given that you want double skins of OSB I propose you build as the others suggest to a point. OSB, insulation, vapour barrier, strapping, but at this point instead of siding put your outer OSB on, strap again and then the siding. No need for any additional membranes. Caulk and paint the inside of the OSB for better moisture resistance. Use long screws when you put the OSB on the outside that go through the strapping and deep into the studs. The downside is because you already have the walls up the siding will overhang the slab more. Upside is getting the more secure wall. With the advent of cordless tools like reciprocating saws and chainsaws crooks won't be slowed for long if they get past the electronics, so I don't know whether the extra OSB is worth it. Up to you though.
> 
> Pete
> 
> PS. Don't know why you guys don't like bigger roof overhangs to keep the rain off the walls? Also great place to hang wind chimes from.



I am just doing the one osb skin inside now.
I didnt have a larger overhang as the wood was 2.4m so i just went with that to save time and money


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## yuzi87 (20 Jul 2020)

So should i raise this OSB roof up with some battons then?


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## MikeG. (20 Jul 2020)

Batons are what a conductor uses to conduct an orchestra. You mean battens.

I have no idea of your roof construction so I couldn't begin to comment on what you need to do to make it work. Is it a warm deck or a cold deck? What is your construction, inside to outside?


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## yuzi87 (20 Jul 2020)

MikeG.":p9nhk86c said:


> Batons are what a conductor uses to conduct an orchestra. You mean battens.
> 
> I have no idea of your roof construction so I couldn't begin to comment on what you need to do to make it work. Is it a warm deck or a cold deck? What is your construction, inside to outside?



It will be osb inside, pir insulation enough to leave 50mm void then breather and osb


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## MikeG. (20 Jul 2020)

So long as the 50mm void is open both ends (via insect mesh) then that's fine. Apart of course from the silly breather membrane location.

What is going on top to keep the water out?


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## yuzi87 (20 Jul 2020)

MikeG.":3ui99kv3 said:


> So long as the 50mm void is open both ends (via insect mesh) then that's fine. Apart of course from the silly breather membrane location.
> 
> What is going on top to keep the water out?



What is wrong with breather location? On a traditional roof breather goes on rafters then battens then tiles. Except in this case its osb instead of tiles?

The osb will be covered in felt which i will wrap around the fascia


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## MikeG. (20 Jul 2020)

yuzi87":1rfbkjdw said:


> MikeG.":1rfbkjdw said:
> 
> 
> > So long as the 50mm void is open both ends (via insect mesh) then that's fine. Apart of course from the silly breather membrane location.
> ...



You've already been told by Blackswanwood. Again, this is basic stuff. Tiles are vapour permeable (through the gaps), the void below them is ventilated, and they are spaced 25mm above the "breather" membrane. That is fundamentally different, indeed opposite, to what you have done with the OSB.


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## yuzi87 (20 Jul 2020)

MikeG.":p77hag4s said:


> yuzi87":p77hag4s said:
> 
> 
> > MikeG.":p77hag4s said:
> ...



There are a lot of messages on here saying a lot of things! It might be basic to you with your years of shed building but its new to me so stop being condesending. 

So should i raise the osb with 25mm battens or not? I notice there was no membrane on your shed diagram


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## MikeG. (20 Jul 2020)

Look, these things have been explained to you repeatedly in this thread. Telling you that is not being condescending, and is nothing to do with my years of experience. All the information you need is here in this thread, and in the threads I have linked you to. All you have to do is follow the advice you have been given. You are remarkably lucky that anyone is still helping you. You may want to note the comments made by our moderator early on in the thread, and note too the positivity of the other thread I linked you to, here. That's how 99% of threads go here, if people approach them with the right attitude.

-

No, don't raise the OSB. Cut the membrane away from underneath. It is doing more harm than good.


----------

