PIR insulation between rafters

[Lincolnshire Ian]

Hi folks

I'm going to fit 150mm PIR insulation between the rafters of my vaulted ceiling. Can I simply cut the insulation slightly too large and squeeze it into the gap and the minimal expansion of the foam will hold it in place, or do I need to fix it in place. 

After the 150mm PIR is fitted I will be adding another 50mm PIR across the whole ceiling which will cover the rafters and 150mm PIR. 

Many thanks 

[AartWessels]

150mm can be quite a job to get in if it's tight. But yeah in principle that's what you do. 

Measure top and bottom space, might not be exactly the same. Measure each gap by itself, they might not be exactly the same. 

 

If you cut the plate wider, I wouldn't do more than 1mm wider. It's a b##ch to squeeze it in between the rafters if it's too wide. And it's even worse to get it out again if you want to reposition it. 

[saveasteading]

55 minutes ago, Lincolnshire Ian said:

Can I simply cut the insulation slightly too large and squeeze

No. It doesn't squeeze. Especially when thick, it won't fit.

The size of the gaps will be +/-  and the rafters may not be vertical. This has has been discussed on here before, and the consensus / easiest seemed to be to cut it a lot smaller, and inject expanding foam in to close the gap.

I don't like expanding foam as it goes where it shouldn't, and may shrink.

 

On our project where the timbers are 100 years old, it was  distorted in all directions:  we tried with 2 layers of 75mm which allows easier fitting but it still left too may gaps.

So what we did was 75mm of pir board, cut as well as poss, but with inevitable gaps., then 75mm of rochwool slab which can be squeezed in to the space.

Or 50 + 100... I can't remember the detail.

 

 

 

[Mattg4321]

Or use gapotape. Expensive though 

[nod]

You want the opposite 

Cut the insulation 10 to small and tack a nail either side and foam up The reason being is that the joists are rarely square 

Use a circular saw to do your cuts and finish off with a handsaw 

This way you can keep your cutting square and it’s much faster 

[JohnMo]

I would just do mineral wool something Rockwool Flexi or Frametherm 32. Then under-draw in 60-70mm PIR. Easier life, as good U value on paper and likely quite a bit better in real life. Better decrimant delay.

[saveasteading]

Insulation-wise pir is twice as good as rockwool, but gaps defeat the object.

 

Are you pushing it up to a  roofing felt or to exposed tiles/slates?

[JohnMo]

34 minutes ago, saveasteading said:

Insulation-wise pir is twice as good as rockwool, but gaps defeat the object.

Not really always correct - Frametherm 32 has an R value of 0.032 and PIR 0.022W/mK so 50% difference not 100%. There is no shrinking with mineral wool, and you are not bridging with a lower R value expanding foam.

[scottishjohn]

4 minutes ago, JohnMo said:

Not really always correct - Frametherm 32 has an R value of 0.032 and PIR 0.022W/mK so 50% difference not 100%. There is no shrinking with mineral wool, and you are not bridging with a lower R value expanding foam.

still means you need half as much PIR to get same value --or twice as much mineral wool

 I like NOD idea  cut on small side --nail to hold inpklace then some foam to fill in 

[JohnMo]

44 minutes ago, scottishjohn said:

still means you need half as much PIR to get same value --or twice as much mineral wool

 I like NOD idea  cut on small side --nail to hold inpklace then some foam to fill in 

Here are some numbers comparing PIR and 0.032 mineral wool. 

 

 

Ca	vity insulation	Underdraw (PIR)	Rafter spacing (mm)	U-value (W/m²K)
A	150 mm PIR + 10 mm foam infill	50 mm	400	0.113
A	150 mm PIR + 10 mm foam infill	50 mm	600	0.110
B	150 mm mineral wool	60 mm	400	0.144
B	150 mm mineral wool	60 mm	600	0.136
B	150 mm mineral wool	70 mm	400	0.134
B	150 mm mineral wool	70 mm	600	0.128

Edited Saturday at 09:11 by JohnMo

[scottishjohn]

13 hours ago, JohnMo said:

Here are some numbers comparing PIR and 0.032 mineral wool. 

pir =0.022

std mineral wool 0.044

you quote nigh spec at 0.032

 but now expesnive is that compared to std ?

 and even then  its still neraly 50% worse

 and if is a  non scottish roof with no sarking --then you will definately get some sort of air flow through the mineral wool 

 how much is not easily deefinable

why anyone these days would not sheet a roof and use double batons is beyond me  plenty of air gap and no where for crap to gather up  and then become a moisture trap and cause rot in20 years time

I have a cold roof  with sarking boards 

and insulation is in loft on top of ceiling,which is mineral wool -300mm, along with some slate vents as there are no battons --slates nailed direct to sarking 

 

[Lincolnshire Ian]

21 hours ago, saveasteading said:

Insulation-wise pir is twice as good as rockwool, but gaps defeat the object.

 

Are you pushing it up to a  roofing felt or to exposed tiles/slates?

I am pushing it into a 220mm rafter leaving at least a 50mm void between the OSB sarking on the rafter and the insulation apparently the gap is important as this is a warm roof (vaulted ceiling). 

[JohnMo]

7 hours ago, scottishjohn said:

but now expesnive is that compared to std ?

 similar price to PIR.

[Crofter]

7 hours ago, Lincolnshire Ian said:

I am pushing it into a 220mm rafter leaving at least a 50mm void between the OSB sarking on the rafter and the insulation apparently the gap is important as this is a warm roof (vaulted ceiling). 

I would use blocks of PIR offcuts to get that 50mm standoff, and cut your slabs approx 5-10mm undersize, then use low expansion foam applied with a proper gun, to fill the gaps and hold in place. 

 

I don't see how you could guarantee the 50mm gap if you were filing with any sort of wool. And the u value would be a lot worse. 

[DownSouth]

On 11/10/2025 at 08:03, saveasteading said:

It doesn't squeeze.

+1

 

cut it slightly smaller and then use a couple of nails to hold it in place. It definitely doesn’t squash in.

we found the Spear & Jackson predator insulation saw was great for cutting slithers off where there were brackets.