PIR Insulation Where Joists & Rafters Meet

[phillonahill]

Hi There,

I'm looking for some help. I was advised to use 100m Celotex to insulate a single story lean-to roof, however, I'm a little confused as to how I'm going to fill the area where the rafter has been fixed to the side of the joist.

I've tried to draw the issue and include a photo from when the roof was being repaired so hopefully you can see the problem. I'm assuming I shouldn't reduce the depth of the insulation to fit.

Please feel free to ask any questions - all help very much appreciated.

All the best
Phill

 

[23vc]

So it’s not a vaulted ceiling, are you sure you should be celotexing and not using fibre wool loft insulation?

 

[phillonahill]

Hi,

Many thanks for your response. No, not at all - I'm going on the advice of a friend. I'm happy to be told that I've got it wrong - I haven't made a cut into the Celotex yet.

Thanks again.
Phill

 

[23vc]

The (more expensive and time consuming) cutting and fixing of celotex between rafters and underneath, is for when you want a vaulted ceiling inside. In your case, it’s more of a “normal roof” loft space type thing, assuming you’re keeping your flat ceiling.
Maybe check it with your BCO.

 

[^woody^]

Reference the sketch, you need to allow a 25mm gap on the underside of the felt/membrane and then whatever distance is left is the thickness of the PIR - so might be more like 60 or 70mm of PIR.

Normally you would use quilt insualtion to the ceiling level and tuck that down over the wall plate, and use honeycomb tray to keep it off the felt at that location to provide the air gap. Dont insulate the rafters all the way up, insulate the ceiling, and rigid board is not the correct type for that.

 

[datarebal]

I'm happy to be told that I've got it wrong

Not wrong, There are easier options as above

 

[garyo]

Given the shallow depth of the roof I can see some sense in using PIR at ceiling level rather than failing to achieve 300mm of rock wool. It's an all-round fiddler job, though.