# Insulating a flat shed roof. (Cold roof design)



## irishthump (1 May 2021)

Quick question guys....

Recently completed the main structure of my new shed/workshop. I've now moved on to the insulation and have a question about the roof. 
It's basically a cold roof design, 6x2 joists with 18mm OSB deck and felt on top. I was planning to use 50mm Kingspan between the joists but I'm wondering is it possible to fix the insulation directly to the underside of the joists? I have enough headroom that I don't mind losing a couple of inches to the insulation.

Any opinions would be welcome....


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## Adam W. (1 May 2021)

Yes, but it will cause condensation to build up under the OSB, so you need ventilation in there.

It's easier to make a warm roof and put the insulation on top of the joists and screw the OSB through that to the joist with 100mm screws.


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## peterw3035 (1 May 2021)

A usual detail would be to fit between as well as a thinner layer underneath.


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## Fitzroy (1 May 2021)

As Peter alludes, yes you can affix directly to the joists, it’ll prevent them acting as ‘cold bridges’. However putting some insulation between and full sheets below the joists is the gold standard for a cold roof design. With lots of ventilation and insect screening for the void above the insulation.
Fitz.


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## irishthump (1 May 2021)

Adam W. said:


> Yes, but it will cause condensation to build up under the OSB, so you need ventilation in there.
> 
> It's easier to make a warm roof and put the insulation on top of the joists and screw the OSB through that to the joist with 100mm screws.



Unfortunately the roof is already built so I cant do that!
But I think I'm ok ventilation-wise, I have vents installed in the front and rerar soffits and the excternal cladding is also mounted to 2x1 battens leaving an air gap that also extends up to the roof.


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## Inspector (1 May 2021)

The best place would have been on top of the roof deck with the roofing on top. How much would it cost to have someone come in and spray foam under? 

Pete


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## irishthump (1 May 2021)

peterw3035 said:


> A usual detail would be to fit between as well as a thinner layer underneath.


OK I see. Is a vapour barrier also necessary or will the foil side of the Kingspan perform that function?
Obviously any exposed joints would need to be sealed with foil tape....


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## irishthump (1 May 2021)

Inspector said:


> The best place would have been on top of the roof deck with the roofing on top. How much would it cost to have someone come in and spray foam under?
> 
> Pete


I don't think that's an option!


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## peterw3035 (1 May 2021)

irishthump said:


> OK I see. Is a vapour barrier also necessary or will the foil side of the Kingspan perform that function?
> Obviously any exposed joints would need to be sealed with foil tape....


Under tacking with a single layer with taped joints creates the continuous vapour barrier. Just make sure you leave a ventilated air gap between insulation & decking, minimum 50mm.


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## Inspector (1 May 2021)

irishthump said:


> I don't think that's an option!



It is often done here but not on your rolled felt roofs. It is used under membrane roofs, both commercial and residential. You have unbroken insulation and no need to cover the underside of the framing unless you want too, it would then need air circulation back into the room below. One of the woodturners is having his 58 year old tar and gravel roof redone that way. It never had insulation before so he will be warm and toasty now. 

Pete


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## irishthump (1 May 2021)

peterw3035 said:


> Under tacking with a single layer with taped joints creates the continuous vapour barrier. Just make sure you leave a ventilated air gap between insulation & decking, minimum 50mm.


Cool. 
Yes, as I said there will most likely be close to 100mm between the insulation and deck with air vents in front and rear soffits.


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