# Garden building insulation advice



## phantombantam (27 Jul 2021)

Hi everyone,

im looking for some advice with insulating my ew garden building. It's single dense concrete block 8 m x 4m with an internal partition wall to separate the shed and garden room area. This leaves approx 6 x 4 m room to insulate.
Here is my basic plan. 80mm tf70 thermafloor on top of DPM on concrete slab floor then 22mm caberfloor tongue and groove floating floor. 50mm pir on walls with 50mm air gap and then 100mm pir between rafters leaving a 50mm gap to the roof boards.
Does this sound ok? just trying to plan in my mind before ordering materials.

slight problem with how to work around internal double block pillars x 2 which are one either side of french doors and two on the opposite long wall.

outside was treated with storm dry before winterand has since been rendered with parex which will also be treated with paraguard so it's watertight


----------



## phantombantam (27 Jul 2021)




----------



## Jameshow (27 Jul 2021)

How about celotex on wall, and foil on the pillars. 

Just a thought 

Cheers James


----------



## phantombantam (27 Jul 2021)

What kind of foil please? I'm just trying to avoid cold spots as much as possible but it will be more insulated being double block I'm assuming anyway


----------



## RobinBHM (27 Jul 2021)

I’m not sure about your cold roof detailing.

it looks to me as though you would struggle to get decent air flow down each joist gap.

Id be tempted to do a hybrid roof - push celetex tight to roof, leave say 5mm gap all round and then foam up with gun set low.

when set, cut foam off flush, foil tape all around joists so foil on boards is joined.

then use a vcl and fully foil taped around perimeter before boarding.

if you are plasterboarding then use foil backed too.


----------



## RobinBHM (27 Jul 2021)

phantombantam said:


> What kind of foil please? I'm just trying to avoid cold spots as much as possible but it will be more insulated being double block I'm assuming anyway


I would do 50mm on walls and 12mm or 20mm on pillars….if you need it at all. 

TBH the u value of a multi quilt is compromised without an air gap


----------



## Jameshow (27 Jul 2021)

Just a multifoil. You would need to prevent it being crushed for maximum insulation. 

Cheers James


----------



## phantombantam (27 Jul 2021)

The rafters are 6 inch deep. Will be drilling through for cross ventilation and venting the soffits on all 4 sides.


----------



## phantombantam (27 Jul 2021)

Originally I was going to fit the celotex just between the pillars and leave them exposed and painted with ply or similar between on top of the celotex, but if the benefit of insulating the pillars is minimal I won't bother


----------



## clogs (27 Jul 2021)

nice looking shed tho......


----------



## phantombantam (27 Jul 2021)

Another thing is like advice on from those that know is whether I need to go to expense of the thermafloor tf70? Does it have better compressive strength over standard pir board or is it just a sales tool?


----------



## phantombantam (28 Jul 2021)

The other thing I can't seem to find definite info on is which order to fit the insulation. Is it floor, ceiling then walls or another way?


----------



## MARK.B. (28 Jul 2021)

Good looking shed , Are those signs of damp/mold on the ply ceiling or just mucky spots ?. I don't know if there is a right or wrong order to do the insulation, but there are those on here that will guide you


----------



## RobinBHM (28 Jul 2021)

phantombantam said:


> Originally I was going to fit the celotex just between the pillars and leave them exposed and painted with ply or similar between on top of the celotex, but if the benefit of insulating the pillars is minimal I won't bother


are the walls cavity construction or a single skin?

it looks like it may be single with returns at the door openings -so no cavity acting as a thermal break.

I guess it wont be too bad given that lightweight blocks are quite insulating -I suppose they are standard 3.6N blocks like thermailites.

I would have a slight concern about damp transfer on a single skin in which case you might want to isolate with foil insulation -including the pillars to help prevent any damp or vapour issues. I know the outside is rendered but that isnt 100% guaranteed to not let moisture through.


----------



## RobinBHM (28 Jul 2021)

phantombantam said:


> The rafters are 6 inch deep. Will be drilling through for cross ventilation and venting the soffits on all 4 sides.


if you are insulating in between the rafter, you still have a hybrid roof, Id be nervous about a cold roof construction in that scenario.

Will you be running cables in the ceiling and having lights? -if so you would have to be 100% sure you have no penetrations through the vcl and insulation if you are having a cold roof.

There seem to be noggins close to the wall, which would really impede ventilation of a cold roof.


lovely structure by the way -thatll be a great garden building.


----------



## phantombantam (28 Jul 2021)

They are single skin paint grade 7.3n blocks. The pillars are two blocks thick one either side of french doors at front and directly opposite wall. Was originally going to be painted but had to treat after it was up last year. The out side had two coats of storm dry masonry protection cream as it was like a sponge. That's explains the mould patches on the inside of the roof boards I had to wait 4 months to get the proper roof on and there was no window until early April. It's been seated with mould killer and wiped down and since then no more has appeared.
The render went on about 6 weeks ago and will be treated with Paraguard which is stain and water repellent.
Rafters will be cross drilled and all 4 soffits vented. The short ends are not blocked up fully to the roof boards so already get some air flow that way without the soffit vents fitted and the front and back blocks will be drilled at the tops to allow flow front to back of building.

all cabling etc will be surface mounted and run in conduit on the out side of the internal cladding to avoid having to come through the insulation. Cladding will be plywood on 2 walls and also reclaimed pallets slats on 2 walls


----------



## phantombantam (28 Jul 2021)

The plan was to insulate the wall all around between pillars with 50 mm PIR board which leaves a gap of 50 mm to the external wall apart from at the pillars where the block is doubled up. Still not sure which order to start in. Was looking at floor, ceiling then walls


----------



## phantombantam (28 Jul 2021)

RobinBHM said:


> are the walls cavity construction or a single skin?
> 
> it looks like it may be single with returns at the door openings -so no cavity acting as a thermal break.
> 
> ...





RobinBHM said:


> if you are insulating in between the rafter, you still have a hybrid roof, Id be nervous about a cold roof construction in that scenario.
> 
> Will you be running cables in the ceiling and having lights? -if so you would have to be 100% sure you have no penetrations through the vcl and insulation if you are having a cold roof.
> 
> ...



how will a hybrid roof avoid condensation though if it leaves no gap to the roof boarding and will be closer to the cold side of the roof. I've never done anything like this before and would like to get it right first time


----------



## RobinBHM (28 Jul 2021)

phantombantam said:


> how will a hybrid roof avoid condensation though if it leaves no gap to the roof boarding and will be closer to the cold side of the roof. I've never done anything like this before and would like to get it right first time



It is to do with interstitial condensation.

the important factor, that is true of both warm roofs and cold roof designs is that warm air laden with moisture must not be able to get from the warm side to the cold side.

And the solution to that is by having a vapour control layer on the warm side.

Ideally a warm roof will have insulation above the joists and a cold roof will have the insulation below the joists -those scenarios mean no thermal bridge and a nice flat area to lay the VCL

with insulation between the rafters its important to have a continuous vapour barrier -which can be achieved by foil taping across every joist connecting all the foil covered insulation sheets together and in your case, I would tape the to the insulation sheets on the walls to ensure no gaps where roof and walls meet. 

you need to be careful to avoid penetrating the VCL with electical cable holes, downlighters etc etc.


Insulation companies dont test hybrid roofs and architects wont specify them - they are a compromise and its important to take a belt and braces approach to the VCL


----------



## phantombantam (28 Jul 2021)

RobinBHM said:


> It is to do with interstitial condensation.
> 
> the important factor, that is true of both warm roofs and cold roof designs is that warm air laden with moisture must not be able to get from the warm side to the cold side.
> 
> ...


Thanks, yes I was always planning on using a plastic layer as well.

does the rest of my plan sound ok to you?


----------



## RobinBHM (28 Jul 2021)

phantombantam said:


> Thanks, yes I was always planning on using a plastic layer as well.
> 
> does the rest of my plan sound ok to you?



I dont know if it makes any difference but Id be slightly tempted to do the wall insulation before the floor dpm and floor -only because if any moisture gets through the wall -it will run down to the slab and not the floor.

Am I right in saying you are having on walls: a 50mm wall cavity, then 50mm celetex then plywood?

I think on a solid external wall, it might be better to put 50mm insulation tight to the wall, with all gaps filled and the joints taped and if you want cover with VCL. Then fix 50 x 25mm battens flat and screw in place through the insulation -then fix plywood to the battens. 

If you google "internal insulation of solid walls" I think this is the recommended method. 

eg:
Wall insulation types: solid and cavity wall insulation


I would definitely insulate the pillars as they made of Hi 7s -they will be cold.

it might be a good idea to put a vertical dpc where the doors are -a length of 100mm dpc would so, folded so it creates a say 70mm x 30mm corner -tape in place at the doors, put insulation in place then clad with plywood -afterwards cut off any dpc poling out in front of the plywood. Why do that? -well if you get any driving rain that gets through between door and frame, it will hit the dpc and run down and cant make the ply damp.


----------



## phantombantam (28 Jul 2021)

The reason I was going to do the 50 mm cavity was that kingspans installation guide say minimum 50 mm air gap if solid wall is less than 200 mm thick. Mine is 100 mm plus the coat of render


----------



## phantombantam (28 Jul 2021)

The door and window was fully sealed and foamed in before the render was done. Its what and how to insulate at the pillars next to the door frame that I'm struggling with as the others I could just use the 50mm around but can't do that at the door


----------



## phantombantam (28 Jul 2021)

This although a cavity wall would suggest that the walls should be done first. And they show both methods for fitting with either battens on first the PIR and then PIR first to walls. Is there a definite way that gives least chance of interstitial condensation ? And if masonry is breathable like concrete block rendered is should any moisture that tracks through disperse more quickly than it can accumulate?


----------



## phantombantam (28 Jul 2021)

I feel like I'm overthinking it but don't want to have to revisit later as the insulation is going to set me back quite a bit


----------



## phantombantam (28 Jul 2021)

Was thinking if I can get some 25 mm PIR for the pillars I could fit that in without being too close to the edge of the door frame ?


----------



## phantombantam (29 Jul 2021)

If the VCL is so important why do all installation methods have you screwing through it to mount battens or your cladding??


----------



## Fitzroy (29 Jul 2021)

I feel your pain, I have a shed I built that I want to insulate, different construction to yours but I've been through much of the same thought processes. My main take-aways from all the thinking are:
- You need to design for insulation, retrofitting, or fitting to a building after the fact is going to be sub-optimal and lead to compromises.
- Most buildings designed for insulation incorporate 'belt and braces; ie stop the transmission of moist air and provide ventilation if the moisture air passes, so small penetrations (screws etc) should not be an issue.

On the flip side I've settled myself that
- It's a shed not my main residence, I'll do my best but if it breaks/goes mouldy it's a pain but it's not my families health at risk.
- It's a shed that will not be occupied 24/7 so it'll have many more hours in an ambient state, rather than a humid state, hopefully allowing it to dry out if I get it wrong.

Hopefully any compromises / miss judgements will be so minor that they will not create an issue.

My building is a framed wall construction rather than a solid walls. but a very similar roof. My plan is to:
- 100mm Insulate between the rafters (150mm), vent the 50mm space above them at the soffits and drill vent holes through the noggins.
- 100mm Insulate between the 100mm wall studs, no air gap, rear ventilation compromised by OSB on the exterior of the wall frames.
- Door and window returns will have no insulation around them, but they are wood not stone, so hopefully no condensation.
- Internal vapour barrier with taped joints.
- 12mm ply wood internal walls, screwed in place (through the vapor barrier, ho-hum)
- All electrics and lights surface mounted to avoid large penetrations.
- I am insulating in rockwool not PIR board, to provide more sound deadening.

PS: Please do not digress from the OPs questions, if you have comment that solely pertains to my build feel free to 'throw rocks at me' on my build thread. I'm not saying what I am doing is right or wrong, just trying to provide support and where I got to with my compromises. 

Very messy build pic, but shows the construction.





Regards

Fitz.


----------



## phantombantam (29 Jul 2021)

What's your plans for the floor? I'm considering order of fitting with walls and can't make my mind up whether to do floor or walls first. Robin made a valid point about the DPM being inside so condensation will run behind if there is any and not down onto top of floor but I want/need my floor DPM to go against the wall and my existing damp proof course above the bottom course of bricks


----------



## Fitzroy (29 Jul 2021)

phantombantam said:


> What's your plans for the floor? I'm considering order of fitting with walls and can't make my mind up whether to do floor or walls first. Robin made a valid point about the DPM being inside so condensation will run behind if there is any and not down onto top of floor but I want/need my floor DPM to go against the wall and my existing damp proof course above the bottom course of bricks


I’ve probably cocked that up as well. I insulated the floor whilst building. And the final floor is laid. Take a look at my build thread for pictures.


----------



## phantombantam (29 Jul 2021)

Ah right I didn't see the link on your previous reply at first  very nice looking shed though


----------



## phantombantam (30 Jul 2021)

Is there any benefit in batenning out walls first so that the wall insulation cavity and roof cavity share the ventilation from the soffits I wonder? Then the warm part of the room is further away from the cold wall?


----------



## Fitzroy (30 Jul 2021)

phantombantam said:


> Is there any benefit in batenning out walls first so that the wall insulation cavity and roof cavity share the ventilation from the soffits I wonder? Then the warm part of the room is further away from the cold wall?



Building an insulated box inside a rain proof structure, with a well ventilated and insect proof cavity between the two is pretty much what you’re aiming for. It’s just that the air gap and battens steal space from the living area. You would want to ventilate the bottom of the wall also to prevent a stagnant area.

This is actually how Aberdeen houses are built (less the insulation). There is a granite shell, then an air gap and then a timber framed lathe and plaster secondary box inside. The gap behind the lathe is vented outside at the bottom through 6” holes in the bottom stones and connected to the vented roof space at the top. This is excellent for managing penetrating moisture from wind blown rain and allows internal moist air to be carried away. However without insulation it results in a cold house or a huge heating bill. The problems arise when debris bridges the gap and moisture wicks to the internal box, resulting in damp patches.

In our case the bridges we are concerned about are thermal bridges bringing internal cold spots where condensation can occur.

Fitz.


----------



## phantombantam (30 Jul 2021)

Yeah I can't ventilate the bottom unfortunately as the render is now on and don't want to disturb it. So maybe better to insulate direct against the wall and seal the roof void off then. My head is battered with this


----------



## phantombantam (30 Jul 2021)

Although I am pretty confident of the water tightness of the block and render. So not concerned of penetrating damp. If there's no more likelihood of interstitial condensation on the walls with it against the wall than spaced off then i would prefer to keep the internal space.


----------



## RobinBHM (30 Jul 2021)

phantombantam said:


> I feel like I'm overthinking it but don't want to have to revisit later as the insulation is going to set me back quite a bit


You are right to think about it, I’ve seen the damage interstitial condensation causes.

this is my concern as I’m not sure you will have enough air flow.

the best source of info is Steve roofer on youtube


this guy does a hybrid roof which is criticised by Steve roofer


----------



## phantombantam (30 Jul 2021)

Thanks, I have seen some of his videos. I'm more concerned about the walls to be honest Robin as i think if I have to I can insulate between and below the battens and feel confident of sealing off well and still retain enough ceiling height.
ive seen the one where he states that fitting cross battens before the roof boards would have allowed cross ventilation before fitting the roof but mines on already so I need to look at another method. I've seen one of his where he shows the cross drilling as a solution after the fact.

I really need to know about the walls and it seems like an air gap would probably make the likelihood of condensation worse or at least the consequences of it.


----------



## phantombantam (31 Jul 2021)

This was the make up before boarding.


----------



## jimmy_s (31 Jul 2021)

It would help if the cavity in the walls was getting some ventilation from the ventilated roof and ideally some vents at low level.

I'm going to try and explain how interstitial condensation occurs so hopefully its easier to figure out how best to avoid it. Interstitial condensation will depend a lot on how you are using the building and how much moisture is being generated. 

Basically you have a temperature gradient over the wall. The overall vapour pressure depends on the difference in temperatures over the construction and also the water vapour content of the air at each side. The pressures are always trying to equalise so you get a pressure gradient over the constructions. Similarly, you have a temperature difference over the walls and roof etc and this produces temp gradients. The lower the temperature the less moisture the air can hold and you get a corresponding dew point temp. Without going into what causes this basically the temp at each interface has a corresponding saturated vapour pressure - this is the maximum corresponding water vapour pressure that can be held without condensation occurring. So, without a vapour control layer you can end up with a vapour pressure at an interface that is higher than the saturation pressure and you will have condensation.

So, the things that you can do to help overcome issues:

Incorporate vapour control layer on warm side
Not insulate it to death 
Ventilate the room - with trickle vent etc - allows some outside air in so vapour pressures can even out (diffusion)
Not heat the it excessively all the time as this will encourage a higher vapour pressure to form inside
Dont fill it with loads of damp stuff - again raises vapour pressure

You ideally would want a ventilated cavity wall as the outer leaf works like a rain jacket to cast water away - anything that gets through will hopefully diffuse away by the ventilation. If it cant get away you might end up with problems as it can get driven in the other direction into the structure. As vapour transport can and does happen in both directions.

I hope this helps


Jimmy


----------



## phantombantam (31 Jul 2021)

jimmy_s said:


> It would help if the cavity in the walls was getting some ventilation from the ventilated roof and ideally some vents at low level.
> 
> I'm going to try and explain how interstitial condensation occurs so hopefully its easier to figure out how best to avoid it. Interstitial condensation will depend a lot on how you are using the building and how much moisture is being generated.
> 
> ...



I do have a double trickle vent above the french doors. My end window has two openers which can be locked on night vent. There is no way I can vent at lower level that I can think of . I'll be using PIR boards taped and foamed with vapour layer inside before cladding. Kingspan states a minimum 50 mm gap if wall thickness is less than 200 mm and mine is 100 mm block plus render coat.
if I was to try to vent a 50 mm cavity behind the insulation with the soffit venitilation I am going to install along with cross drilling the roof rafters will it be sufficient?

also you say to not over insulate. So plan was 50 mm PIR on walls with or without cavity depending on best advice from here.
100mm in between rafters all panels joined across with tape after being foamed in at edges and joins. Then VCL inside. Floor 75 mm on top of DPM.
does that sound like too much. It will not be occupied constantly or at night unless the kids have an odd sleepover in there very occasionally. Was planning on using a small oil filled radiator or solar to introduce a little heat in autumn/winter.

can you advise on that specifically please?

And thanks for the explanation too it does help me to understand it more clearly


----------



## jimmy_s (31 Jul 2021)

Personally I would try and get some form of cavity - could you not core cut a few vents in at low level?

Also its better not to put all the insulation in between the rafters general rule is half between and rest under as the rafter is a cold bridge but might be ok. 

I have to go out but if you want to PM me the build ups I can run the interstitial calcs for you as thankfully I dont have to do them manually anymore.

I'm an engineer not an architect so others here may be able to advise better on build ups but I'll try and give you a hand

Jimmy


----------



## phantombantam (31 Jul 2021)

Thank you. The reason for not venting low level is the building has just been parex rendered at a decent cost and I'd really not like to damage it. I need to see when I'm in from work as to whether I can put any insulation under the rafters as I'm not sure whether I will have the height once the floor is in. I also need to think about how I can get so holes through the blocks at the front and rear where it was blocked in up to the roof boards to allow the air to get through. And I can't use standard round 70 mm soffit vents as my soffits are not that wide.


----------



## phantombantam (31 Jul 2021)

I think I might be able to get away with 75 mm between rafters and 25 mm below rafters but will need to measure. Increase in cost is fairly small.
If I thought I could drill through the render and block successfully would several of these do?


----------



## jimmy_s (31 Jul 2021)

phantombantam said:


> I think I might be able to get away with 75 mm between rafters and 25 mm below rafters but will need to measure. Increase in cost is fairly small.
> If I thought I could drill through the render and block successfully would several of these do? View attachment 115018


cant see why not - looks like a round weep vent. Maybe get a 30mm core cutter to lessen the chance of damaging the render?


----------



## phantombantam (31 Jul 2021)

I'm not comfortable with drilling into the render barring my only other part which is my gutter down pipe which will only be small holes. Can you please let me know what you need for calcs and I'll send you a pm when I can get the relevant info. If the render becomes damaged it WILL end in divorce


----------



## phantombantam (31 Jul 2021)

I can however get away with 75 mm between rafters and 25 mm right across the underside of them


----------



## RobinBHM (31 Jul 2021)

phantombantam said:


> Thanks, I have seen some of his videos. I'm more concerned about the walls to be honest Robin as i think if I have to I can insulate between and below the battens and feel confident of sealing off well and still retain enough ceiling height.
> ive seen the one where he states that fitting cross battens before the roof boards would have allowed cross ventilation before fitting the roof but mines on already so I need to look at another method. I've seen one of his where he shows the cross drilling as a solution after the fact.
> 
> I really need to know about the walls and it seems like an air gap would probably make the likelihood of condensation worse or at least the consequences of it.



my apologies if I’ve created more confusion! - but it’s worth thinking it through, because getting it wrong can lead to long term issues.

I apologise if I made suggestions of a hybrid roof - I realise these can fail, but I’ve had experience of cold roofs fail.


----------



## RobinBHM (31 Jul 2021)

One thing I slightly wonder: if the wall insulation is fitted tight to the wall with no gap and runs down to the concrete slab….would any condensation actually matter - one face is masonry, one is foil.


----------



## RobinBHM (31 Jul 2021)

jimmy_s said:


> . As vapour transport can and does happen in both directions



In regards to garden buildings, Ive always struggled to work out what actually happens.

These buildings aren’t like houses, they are generally used occasionally, so my guess is there are days when the inside may be colder than the outside and vica versa.


----------



## Fitzroy (31 Jul 2021)

RobinBHM said:


> In regards to garden buildings, Ive always struggled to work out what actually happens.
> 
> These buildings aren’t like houses, they are generally used occasionally, so my guess is there are days when the inside may be colder than the outside and vica versa.


That was a can of thinking worms I was avoiding opening. It struck me a few months back that the building will seldom be is a state of equilibrium unless it’s heated permanently, so what actually happens is uncertain.


----------



## phantombantam (31 Jul 2021)

I need to order the gear prob Monday or Tuesday at the latest to get it for my window of holiday from work so I'm hoping I have a definite plan by then. It's definitely a minefield. Thanks for all of the info and points of view so far it's definitely helped me understand more about it.


----------



## RobinBHM (31 Jul 2021)

Fitzroy said:


> That was a can of thinking worms I was avoiding opening. It struck me a few months back that the building will seldom be is a state of equilibrium unless it’s heated permanently, so what actually happens is uncertain.


I built my own garden cabin and I couldn’t get my head around it…..so ignored it in the end.

I have to say though, even leaving it for a week or two unheated in the winter I’ve not seen even a hint of condensation on any metal or glass surface, ever. It’s timber frame, featheredge cladding with 100mm insulation in walls and 120mm in warm roof.


----------



## phantombantam (31 Jul 2021)

Some more pics in case it helps anyone with any fresh ideas


----------



## johnny (1 Aug 2021)

wood has a natural low thermal conductivity and therefore would not be a source of 'cold bridging ' It is an excellent thermal insulator. You certainly do not need to insulate the timber rafters or put any insulation beneath the them. 
As others have said with such a variable environment a shed is difficult to insulate in order to avoid condensation issues successfully. You only have to witness the condensation problems most of us have in our houses throughout the Winter to see that..

You might consider an alternative approach. You could install an efficient humidity controlled extraction in the shed together with perhaps a small dehumidifier .


----------



## phantombantam (1 Aug 2021)

johnny said:


> wood has a natural low thermal conductivity and therefore would not be a source of 'cold bridging ' It is an excellent thermal insulator. You certainly do not need to insulate the timber rafters or put any insulation beneath the them.
> As others have said with such a variable environment a shed is difficult to insulate in order to avoid condensation issues successfully. You only have to witness the condensation problems most of us have in our houses throughout the Winter to see that..
> 
> You might consider an alternative approach. You could install an efficient humidity controlled extraction in the shed together with perhaps a small dehumidifier .


Unfortunately it's fully rendered now so without cutting a hole through the wall and render  extraction isn't really an option.


----------



## phantombantam (1 Aug 2021)

Can anyone shed (excuse the pun) any more light on my best course of action please? I will need to order the materials probably Tuesday latest to get it in time for my week off work as that's my only opportunity to get consecutive days working on it.


----------



## phantombantam (1 Aug 2021)

johnny said:


> wood has a natural low thermal conductivity and therefore would not be a source of 'cold bridging ' It is an excellent thermal insulator. You certainly do not need to insulate the timber rafters or put any insulation beneath the them.
> As others have said with such a variable environment a shed is difficult to insulate in order to avoid condensation issues successfully. You only have to witness the condensation problems most of us have in our houses throughout the Winter to see that..
> 
> You might consider an alternative approach. You could install an efficient humidity controlled extraction in the shed together with perhaps a small dehumidifier .



the difference being that it's not going to be occupied anywhere near as often as our house.
I want to minimise any issues but also need to balance that with cost and fitting in with what is already there as I don't want to damage render that was quite expensive.

it won't be occupied overnight except very occasionally if kids want to sleep out. So maybe could be used for 2-3 hours a day but not every day. And sometimes longer maybe during school hols weekends


----------



## Fitzroy (2 Aug 2021)

Phantom. It feels like you’ve reached the end of our advice/thinking. There is no perfect answer to your problem, so it’s decision time for you on the level of compromise. Ventilated but render holes. Hybrid roof and moisture barrier but no ventilation etc.

Personally I’d insulate the roof with a gap above and vent this gap as best as possible at the soffits. Insulate the walls with no cavity, then spend the effort on the vapour barrier. History will judge. 
fitz.


----------



## johnny (2 Aug 2021)

phantombantam said:


> the difference being that it's not going to be occupied anywhere near as often as our house.
> I want to minimise any issues but also need to balance that with cost and fitting in with what is already there as I don't want to damage render that was quite expensive.
> 
> it won't be occupied overnight except very occasionally if kids want to sleep out. So maybe could be used for 2-3 hours a day but not every day. And sometimes longer maybe during school hols weekends



the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .

Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .

You can either install an extractor in the roof of the shed ,(preferably at the highest point ,) or you can install a dehumidifier which will extract and store the moisture and will require emptying periodically.

These are simple,cheap and very effective options for your situation that will cause minimal installation disturbance .

The other thing you might consider is how hot the inside of the shed will become when the outside temperature rises and the sun is beating down on the timber flat roof. Some form of extraction system will be essential if you intend to work in the shed in comfort during Summer months . Opening the door and windows is unlikely to provide adequate through ventilation


----------



## phantombantam (2 Aug 2021)

johnny said:


> the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .
> 
> Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .
> 
> ...


Hi, no petulance here from me. I'm just trying to weigh up the different opinions and advice which seem to be complete opposites.
Hindsight is a marvellous thing and I obviously did not know what I have since been told about a number of things


----------



## phantombantam (2 Aug 2021)

johnny said:


> the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .
> 
> Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .
> 
> ...


I'm pretty confident of the water tightness of the wall and render so I wonder if this will be helpful or not. What do you think please?


----------



## phantombantam (2 Aug 2021)

johnny said:


> the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .
> 
> Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .
> 
> ...


Hi Johnny,

it is going to be used as a garden room as opposed to a workshop so will have furniture, TV etc in. I was under the impression rightly or wrongly that it would be cooler in summer and warmer in winter once insulated. Is that not likely to be true?


----------



## RobinBHM (2 Aug 2021)

phantombantam said:


> Hi, no petulance here from me. I'm just trying to weigh up the different opinions and advice which seem to be complete opposites.
> Hindsight is a marvellous thing and I obviously did not know what I have since been told about a number of things


TBH most builders don’t know, thousands of flat roofs are built every year…wrongly. In fact there are plenty of new build houses that have suffered such severe interstitial condensation, the walls have had to be gutted and redone.

My gut feeling is, if you can’t get much ventilation in the roof void, is to not have a void at all and use a high performance vapour control layer like resitrix 600


----------



## johnny (3 Aug 2021)

phantombantam said:


> Hi Johnny,
> 
> it is going to be used as a garden room as opposed to a workshop so will have furniture, TV etc in. I was under the impression rightly or wrongly that it would be cooler in summer and warmer in winter once insulated. Is that not likely to be true?


Insulation is fine for cold weather when you want to retain heat and minimise heating needs and costs however in the hot weather the reverse is true. Insulation is going to keep warm air in your shed so you will need some form of induced air flow to make it bearable. 
In a traditional stone cottage with 18" walls and a tiled roof the inside will stay cool all day long without any thermal insulation whatsoever during hot weather. The construction is also quite effective it retaining heat due to its mass. Your shed roof has no dense mass to absorb heat.

With a shed that has a timber flat roof you have no effective means of reflecting heat from the sun so it will rapidly heat up. Internal thermal insulation is not going to help too much . Ideally what you need is a through ventilated secondary layer fixed over your roof covering which will keep your roof from getting hot . Think Landrover where they have an additional layer over the roof and cab ,open at the edges to keep the interior cool in hot climates. Alternatively you are going to need forced ventilation and extraction in hot weather or you'll bake in there .


----------



## johnny (3 Aug 2021)

RobinBHM said:


> TBH most builders don’t know, thousands of flat roofs are built every year…wrongly. In fact there are plenty of new build houses that have suffered such severe interstitial condensation, the walls have had to be gutted and redone.
> 
> My gut feeling is, if you can’t get much ventilation in the roof void, is to not have a void at all and use a high performance vapour control layer like resitrix 600


I had to Survey several new houses on a housing estate in Yeovil not so long ago where occupants complained of running water in the lofts...it transpired the builder had not installed any ventilation at all and the insulation was pushed up tight to the eaves so all the condensation in the house got trapped in the lofts. It was like an Amazon rainforest in the roof .


----------



## phantombantam (3 Aug 2021)

johnny said:


> I had to Survey several new houses on a housing estate in Yeovil not so long ago where occupants complained of running water in the lofts...it transpired the builder had not installed any ventilation at all and the insulation was pushed up tight to the eaves so all the condensation in the house got trapped in the lofts. It was like an Amazon rainforest in the roof .


A friend of mine bought a relatively newly built house and I was helping him move some stuff into the loft. It was wet inside and we discovered his bathroom extractor fan was sucking straight into the loft space.


----------



## phantombantam (22 Aug 2021)

Is it recommended to seal the concrete floor before laying the dpm and insulation?


----------



## peterw3035 (22 Aug 2021)

It's not necessary, just make sure the DPM is continuous and tape any joints


----------



## phantombantam (22 Aug 2021)

peterw3035 said:


> It's not necessary, just make sure the DPM is continuous and tape any joints


Great thanks


----------

