PIR detail? Under rafters where PIR meets the wall.

[connick159]

Hi,

 

tried searching but not sure of the terms so coming up blank.

 

I've just installed PIR 100m between rafters on a pitched roof. Now about to fix 50mm PIR under the rafters and have a question about what do do to the PIR boards where they will be butting up against the wall plates. Do I need to cut them at an angle so they lay flush with the wall. Alternate is to leave as is but then assuming I'd have to fill the gap which is left at the wall with other wedges of insulation or possibly spray in foam.

 

Can anyone give pointers or even detail pics of the recommended way to do this please?

 

thanks in advance.

[nod]

Yes cut them at an angle so they fit flat against the wall plate

[connick159]

thanks!

 

[nod]

13 minutes ago, connick159 said:

thanks!

 

?

[tonyshouse]

You need to seamlessly join roof insulation to wall insulation, mine goes through the wallplate zone at 400mm thick - leave no gaps, cracks or as is more common complete misses

[Cpd]

11 hours ago, connick159 said:

Do I need to cut them at an angle so they lay flush with the wall. Alternate is to leave as is but then assuming I'd have to fill the gap which is left at the wall with other wedges of insulation or possibly spray in foam.

 

how are you securing the 100mm insulation between the rafters ?  I check to make sure that there is not any variation in insulation thickness or discrepancies in the inside face of the rafters and if it’s all good I then attach roofing batten to the inside of the rafters at the desired depth so that when you push the insulation in it buts up nice and solid against the battens.  Would then foam in all the 100mm insulation, cut the foam back and then tape over the joints and right across the front of the rafters so that it’s a continuous silver foil run from one side to the other. Depending on your wall build up I would then be putting any inside wall insulation on that so that I overlaps the bottom of the rafter insulation (foam and tape) and then I would be putting my 50mm sheet insulation over the rafter ends to eliminate and cold bridges. I  find that the only way to get all the 45 degree cuts  (or whatever your roof angle is) perfect is to use a hand held circular saw set to the desired angle, if the insulation is thicker than your saw cutting depths then just finish of with a hand saw - it’s easy to follow the angled cut that you made with the circular saw. 

the pic is just to give you an idea - this is how I have just insulated a room in in the roof of my shed but your design may be different - and I may be wrong... 

however this system has worked well for me and gives me 100% airtightness  and would take a lot of shrinking and timber movement to cause failure. 

 

 

 

 

[connick159]

Thanks for the drawing, unfortunately i did not do the ones between the rafters like that. I left them square at the ends and sat them on the wall plate. They are now all fitted, except between two rafters. They were held in pace temporarily and then used the illbruk foam around perimeters to hold them in place. Was then going to comeback over them with the next layer and screw them in with drywall crews and washers.

 

I do have 170mm wool insulation closing the cavity but undoubtedly there are now gaps in between wall plat and in rafter PIR.

 

I dont have internal wall insulation but other than that the drawing is what i'm doing. thanks.

 

 

 

Edited April 8, 2021 by connick159

[connick159]

should i cut them out and start again?

 

messy job but only around 10 lengths between rafters at 2.4m long. 

[Mr Punter]

It is worth filling in the little bits at the top and bottom to keep the insulation fitted without gaps.  You can always stuff in some mineral wool if the PIR is too fiddly.

[Cpd]

2 hours ago, connick159 said:

should i cut them out and start again?

You only need to cut out a small section top and bottom, just jam in a wood saw or bread knife and cut down the side of the foam, you will need to put the new bits in in two sections as I don’t think you will get the new insert in in one bit...  put the filler triangle in first (1) and foam it in and then put the last bit (2)in between the remaining insulation (3) and the triangle insert. Foam and tape. I used 70mm over the top on my last project and made my own washers out of 5mm ply - bandsaw and pillar drill and 10 minutes later I have 100 washers.... away as I was not having a service void in the roof section I drove the washers in below the surface of the insulation by 5+ mm and then foam over the top, stick a bit of tape on and then glue plasterboard over the top.... obviously if your having a service void then you can leave the washers proud. 

[connick159]

thanks everyone for the replies. Especially CPD for the pics. They've been a big help. Thanks again.