PIR Insulation Foam Board - Size Query

[isaact4569]

Hi,

I am looking into installing 25mm thick PIR Insulation Foam Boards (Celotex) in my loft. Its a normal standard pitched loft (triangle shaped). Currently my loft side vertical walls going in traiangular shape has battens which hold the roof. Behind the battens is felt. The battens appear to be 4 x 2 inch C24s. The PIR boards will fit inside the frame (battens) as friction fit. The battens are 57cm width between each other (inside measurement between the battens). My question is what size width would I ideally need to cut the PIR boards. I have read somewhere online to add extra half inch (13mm / 1.3cm) to your measurements and this will friction fit perfectly between battens but that isn't correct because it won't fit. I know another option is to cut it short and use expansion foam but this is a messy job and we have carpets in loft. I was wondering whether anyone knows how much extra width should be added to existing width in order to friction fit the boards perfectly. I tried cutting 57cm exactly but the boards are not stable, too loose.

Thanks.

Isaac

 

[23vc]

It’s not an exact science, but it helps to cut them at a slight angle to encourage them to wedge in tight. If I had to say how much I’d add on a typical 400mm panel, I’d probably add about 10mm ish

I assume you mean rafters when you say battens

 

[noseall]

Hi,

I am looking into installing 25mm thick PIR Insulation Foam Boards (Celotex) in my loft. Its a normal standard pitched loft (triangle shaped). Currently my loft side vertical walls going in traiangular shape has battens which hold the roof. Behind the battens is felt. The battens appear to be 4 x 2 inch C24s. The PIR boards will fit inside the frame (battens) as friction fit. The battens are 57cm width between each other (inside measurement between the battens). My question is what size width would I ideally need to cut the PIR boards. I have read somewhere online to add extra half inch (13mm / 1.3cm) to your measurements and this will friction fit perfectly between battens but that isn't correct because it won't fit. I know another option is to cut it short and use expansion foam but this is a messy job and we have carpets in loft. I was wondering whether anyone knows how much extra width should be added to existing width in order to friction fit the boards perfectly. I tried cutting 57cm exactly but the boards are not stable, too loose.

Thanks.

Isaac

Friction fitting will work for about 80% of the fit. The others will fall out - usually where there is some discrepancy in the rafter spacing. It's impossible to friction fit on a one-size-fits-all basis. Wood structures are nowhere near obedient enough. We like to use foil tape (very sticky) and ex-foam, when fitting PIR.

Foil1

• noseall
• 2 Apr 2019

Foil2

• noseall
• 2 Apr 2019

 

[robinbanks]

25mm thick

I you're looking to make the loft habitable and reduce heat loss appreciably, you're going to need a good deal more than that

 

[^woody^]

Yes, a complete waste of time, effort and money for 25mm of insulation.