Vapor detail when fitting PIR board to roof

[garyo]

I'm just starting to fit PIR board to a lean to roof. 100mm of board between the 150mm deep raters, and 50mm underneath.

I've read the Kingspan technical document and they recommend paying close attention to the detail where the roof meets walls and windows. Fair enough.

I have 100mm wide foil tape that I was going to use to form a continuous barrier between insulation boards on the final 50mm layer, but I'm wondering if I should also use it over the top of the first 100mm layer to seal the rafters in too?

Where the roof meets the wall, what should I do - Gripfill, silicone, or foil tape the PIR board on to the wall plate?

It's a tiny roof, but fairly complicated having a hip and two velux's in it. nearly every board has a triangular aspect to it so the initial 100mm layer isn't going in especially gap free.

Thanks

Gary

 

[catlad]

You will have to do the best you can with cutting the insulation to size, and then use expanding foam to fill any gaps and then trim flush and tape over the joists.

 

[excited]

Gary can I ask, How are you fixing the board underneath the rafter.

Thanks

 

[garyo]

Cheers catlad.

Excited - I've not got that far yet. I intend to buy the 50mm and plasterboard layer on Saturday morning. If I buy the 50mm foam backed plasterboard then I've heard of people using extra long drywall screws but there are obvious thermal bridge issues with that, and I don't know what that means for the condensation, because it'll mean I can't use my foil tape to seal them together.

Would it be okay to make the 100mm under layer condensation proof with foil tape, and then use 50mm/12.5mm foam backed plasterboard for the underlayer?

 

[catlad]

If you are using insulated plasterboard it has a built in vapour barrier, you will still have to vent the space to be sure, and if your rafters are 6" or more I would think the thermal issues of the screws is minimal. You can buy drywall screws up to 120mm

 

[garyo]

Excellent. What kind of joint tape do you seal the plasterboard with, or is it not necessary? Just scrim and skim?

 

[catlad]

I think there is a mastic type sealant to join the boards together but better to check with celotex tec dept. But if you have taped all the insulation together underneath and it is ventilated I would not bother.