Garden shed - ventilation for foil based cold roof

[BerryBarn]

Hi all,

New member here. I have recently erected a pent Power Shed, which I want to use a little workshop and house my little off grid power solution. I have a fair few questions about insulation and temp/damp/mould control, but I am most immediately concerned about the following.

The shed has a pent roof, about 2m wide, and the deck is essentially tongue in groove cladding nailed to a frame to make roof panels. The framing timbers are 40mm high, so extending from the warm side of the deck down into the shed 40mm, with lengths running across the inside width of the shed spaced about 60cm apart, which I will refer to as "rafters". Having read a fair bit of material online, I have decided to insulate the roof by simply stapling foil to the underside of the roof rafters. I would then use foil tape to seal the foil insulation to the PIR board fitted to the wall panels. This will create 40mm void in the roof, segmented by the rafters at roughly 60cm intervals. I appreciate 50mm is supposed to be the minumum air gap for a cold roof, but... I don't have that now, and creating it would reduce the internal head height too much.

So, I am hoping that by introducing as much airflow as possible in each void would be enough to prevent moisture build up whilst still providing a benefit from having the insutlion. My idea is to fit trickle vents at either end of the void, so the tall end of the shed, and the shorter end. I could go on and put holes in the timbers, aka "rafters" but I woul prefer no weaken the roof stucture at all, as I only have 40mm to play with. Will 40cm trickle vents on either end allow for enough airflow to take away moisture build up before it causes any damage?

The other idea I have is to paint the warms side of the deck and framing timbers with anti mould paint, to further discourage a possible mould infestation. Does this sound a) useful, b) harmful, or c) not going to do anything other than waste time and money?

Thanks!

 

[Fitzroy]

Hi all,

New member here. I have recently erected a pent Power Shed, which I want to use a little workshop and house my little off grid power solution. I have a fair few questions about insulation and temp/damp/mould control, but I am most immediately concerned about the following.

The shed has a pent roof, about 2m wide, and the deck is essentially tongue in groove cladding nailed to a frame to make roof panels. The framing timbers are 40mm high, so extending from the warm side of the deck down into the shed 40mm, with lengths running across the inside width of the shed spaced about 60cm apart, which I will refer to as "rafters". Having read a fair bit of material online, I have decided to insulate the roof by simply stapling foil to the underside of the roof rafters. I would then use foil tape to seal the foil insulation to the PIR board fitted to the wall panels. This will create 40mm void in the roof, segmented by the rafters at roughly 60cm intervals. I appreciate 50mm is supposed to be the minumum air gap for a cold roof, but... I don't have that now, and creating it would reduce the internal head height too much.

So, I am hoping that by introducing as much airflow as possible in each void would be enough to prevent moisture build up whilst still providing a benefit from having the insutlion. My idea is to fit trickle vents at either end of the void, so the tall end of the shed, and the shorter end. I could go on and put holes in the timbers, aka "rafters" but I woul prefer no weaken the roof stucture at all, as I only have 40mm to play with. Will 40cm trickle vents on either end allow for enough airflow to take away moisture build up before it causes any damage?

The other idea I have is to paint the warms side of the deck and framing timbers with anti mould paint, to further discourage a possible mould infestation. Does this sound a) useful, b) harmful, or c) not going to do anything other than waste time and money?

Thanks!

Video is great, so much easier to see what you are faced with. My opinion:
- Will adding multifoil insulation help? Yes.
- Will ventilating the cavity be better that not ventilating? Yes
- Mould paint? No point, if it gets that wet you have bigger issues.

Best thing you can do it manage the moisture in the building, ie don't let warm moist air build up inside. If you're working in there get a thermometer and keep it below15degC, open window/door if it's getting too warm or your doing heavy sawing/planing and sweating. When you finish in the building open a door and let the moist air out before shutting up for the day/night.

Fitz.

 

[BerryBarn]

Video is great, so much easier to see what you are faced with. My opinion:
- Will adding multifoil insulation help? Yes.
- Will ventilating the cavity be better that not ventilating? Yes
- Mould paint? No point, if it gets that wet you have bigger issues.

Best thing you can do it manage the moisture in the building, ie don't let warm moist air build up inside. If you're working in there get a thermometer and keep it below15degC, open window/door if it's getting too warm or your doing heavy sawing/planing and sweating. When you finish in the building open a door and let the moist air out before shutting up for the day/night.

Fitz.

Thanks Fitz. I'll go on and order the trickle vents.

 

[sams93]

I have a wooden shed like yours although it has a pitched roof rather than pent. Initially I was quite concerned about rust etc damaging both hand tools and machines. I live near the coast so the air is full of salty water which isn't great for corrosion and rust. I installed 2 'high airflow' (rather than high static pressure) 140mm 12v computer fans in louvre vents in the gable ends of my shed in a push-pull configuration. Supposedly they each move 110m3 per hour which in theory is plenty although I am sure they are much more affected by the wind etc being in the environment they are in compared to what they are designed for... They each draw 1.5w maximum so probably would be suitable for an off-grif power system.

 

[BerryBarn]

Thanks Sam93! I have considered fans. I guess my main concerns are that I would have to install a pair for each void (so that would be a fair few fans), and once I staple the insulation onto the rafters and fit the ceiling boards (3mm ply I am thinking) I would not have easy access to repair replace them. The holes/slots I would make in the fascias to allow air to flow through the vents would only be about 1cm high, so they wouldn't really help in providing access.

But perhaps I ought to give fans more thought though. It would be reassuring to know there was a guaranteed source of airflow.

 

[Sachakins]

Could you not board and fill void with expanding spray foam. Insulated and no void requiring air flow?

 

[BerryBarn]

I did consider putting 40mm PIR board in the void and sealing that with foam and tape, but from what I read and learned from YouTube, unless I created a true warm roof, then the next best thing was a cold roof. Otherwise, you are creating a hybrid roof, which is not advisable unless you have someone run the numbers for you, and I don't want to have to be paying for engineers if I can help it.

Happy to be told I am wrong though. Filling the void so no air gap would be far easier. What I would not want is to create a scenario whereby condesation can form under the deck, in any small airtight gaps I have created in the voids which touch the timbers, and there be no airflow to carry that condensation away. Plus, the timbers would be touching the deck forming a bridge to the outside. I would expect condensation will form on the timbers wherever an airtight gap existed, and with no airflow to carry the moisture aware problems might arise. So I figured a simple cold deck made of foil and some ventilation might be the way to go, albeit a more fiddly way to go what with having to install trickle vents.

As I say, I would be more than happy to be talked out of it...

 

[Fitzroy]

I did consider putting 40mm PIR board in the void and sealing that with foam and tape, but from what I read and learned from YouTube, unless I created a true warm roof, then the next best thing was a cold roof. Otherwise, you are creating a hybrid roof, which is not advisable unless you have someone run the numbers for you, and I don't want to have to be paying for engineers if I can help it.

Happy to be told I am wrong though. Filling the foid so no air gap would be far easier. What I would not want is for that to create a scenario whereby condesate can form on the deck under the felt, or any tiny airtight gaps I have created in the voids which touch the timbers, and there be no airflow to carry that condensation away. So I figured a simple cold deck made of foil and some ventilation might be the way to go, albeit a more fiddly way to go what with having to install trickle vents.

As I say, I would be more than happy to be talked out of it...

If you were building a house with a £50k roof structure and which you may want to sell in the future then the hybrid roof issue would be very important. eg surveyors don't like spray foam as they can't assess the timber structure, then the mortgage companies won't give mortgages and before you know it your house is unsalable unless you tear it all out. Total bonkers but it happens!

With your shed, what's the worst outcome, a roof replacement in 10 years if you get it wrong for a few hundred £. The timbers are all treated and the base is in constant contact with a damp floor. I expect no matter what you do the base of the shed will degrade long before the roof due to condensation. Shed roofs tend to fail due to leaks through the surface cover, keeping this in good condition is a bigger priority.

The reason I would be against PIR between the rafters is that is is difficult to make a good job of it and without a good job it is expensive and the benefit is considerably reduced. For a goodjob you really need to be using some thing like Gapotape so that the contact between PIR and rafter is good across the full 40mm depth, which adds more expense. I my build I went with rockwool slabs as they conform much better than rigid insulation, but at 40mm thick the insulation properties are not great.

If you could sprayfoam the cavities I think that would be a great solution, however I've no idea of how and the expense of doing it.

Fitz

 

[BerryBarn]

Hi Fitzroy, the base is sitting on raised heavy duty decking risers floating above a concrete base. I am not overly concerned about it getting damp from the ground.

Like you, I also can't imagine how I would effectively and assuredy completely fill a void with expanding foam, and it seems possibly quite expensive.

Why would it be hard to make a good job of PIR between the rafters? Cut them well, squeeze them in tight, fill any tiny gaps with foam and cover with decent foil tape. Gapotape if that's the best. Might be expensive by I am not exactly insulating a very big area. I think I would probaby only need about 20m of tape. And it is what I would have done, but I was afraid of the things I describe above. So I figured, create a ventilated void. Not what I wanted, but seemed "better practice".

I appreciate the cost of repairing a mould shed roof can't be that much. Perhaps that is the clincher then? Fill the void, go hybrid, and hope for the best. If it fails, it will be in many years to come and I can easily save up for it?

Thanks.

 

[BerryBarn]

Perhaps the better thing to do would be to take the felt off the roof, make a warm roof and cover with new felt... that feels excessive, but maybe not that much more work and a better solution?

 

[Jameshow]

How about 40 insulation pie or polyurethane and then hardboard plywood screwed up. That way you don't have any gaps. It looks better and it's not much more than the cost of tape.

Just a thought.

 

[BerryBarn]

Hi Jameshow, thanks for the reply. 40 inch PIR insulation and plywood board screwed up was my original idea. Although, I would have run foil tape along the underside of the rafters and the abutting edges of the PIR boards, to create a seal. But was concerned about creating a hybrid roof, as from what I have read online they should be avoided unless you have had an engineer do the calculations. But, as Fitzroy has suggested, perhaps a dose of perspective is needed? If I create a hybrid roof and if it goes wrong, it will take many years and ultimately not cost that much to deal with?

 

[sams93]

Out of interest, how big is the shed?

 

[BerryBarn]

8 foot by 14 foot

 

[BerryBarn]

Hi all,

So just before I move on with this, can I just get a confirmation that the general consensus would be to fill the void and not overly worry about potential hyrbrid roof problems? Thanks.

 

[Sachakins]

Yes BUT. Consider the temp diff between shed and outside, don't want condensation forming in the inside of the roof either! So plenty of air movement in the shed too.

 

[BerryBarn]

Ok, thanks.

 

[perrymac]

I have the very same shed, 2 years ago I filled all around the shed walls with 25mm pir board, haven't had a problem until I fitted mdf shelves 6 months ago , just noticed mould build up . Will clean and treat as required . Thanks for info on varnish etc

 

[HamsterJam]

I put foil backed PIR board directly on top of the roof boards and felted over it with felt shingles as recommended by the manufacturer (Tuin).
Time will tell whether this creates any issues but I have done the same previously without any problems.

 

[HappyHacker]

I built a log cabin with a pitched roof using T&G as the roof bed. I put insulation boards with a ply skin on one side on top of the T&G and then put Wickes felt shingles on top. It has worked ok for the last ten years despite the high winds we often get.