Solid Shed build,vapor barrier question

[Hornbeam]

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?

 

[Hornbeam]

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

Only 7. My daughter has that many on her own

 

[yuzi87]

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

 

[MikeG.]

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

Why do you want insulation then?

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

Why are you insulating an unheated shed?

 

[yuzi87]

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

Why do you want insulation then?

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

Why are you insulating an unheated shed?

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

 

[Hornbeam]

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

 

[Inspector]

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

 

[owen]

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

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.

 

[MikeG.]

.........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.

 

[MikeG.]

........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.

 

[CHJ]

Why are you insulating an unheated shed?

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

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)

 

[owen]

........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.

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:

 

[MikeG.]

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.

 

[MikeG.]

........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.

 

[yuzi87]

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?

 

[TheTiddles]

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

 

[MikeG.]

......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.

 

[RogerS]

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

Only 7. My daughter has that many on her own

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

 

[MikeG.]

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.

 

[yuzi87]

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:



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