Mitigation for potentially shrinking PIR board

[Beau]

Hi all.

 

I just read something on here about PIR boards shrinking. This has got me a little concerned as I'm in the process of insulating between the rafters in a low bedroom. I dont have the hight to use lots of mineral wool on the sloping parts of the ceiling so how can I best mitigate any potential shrinkage? 

 

Thanks

Edited February 11 by Beau

[MikeGrahamT21]

There are products designed exactly for this, though quite expensive https://gapogroup.com

 

or you can opt for what most do on here, as I have done, get it as tight as you can and inject some flexible expanding foam around the perimeter and joins, which will move with any shrinkage.

 

it’s hard to say just how much PIR will shrink over time, seen some photos of crazy shrinkage, I’ve installed quite a bit in my place over the years, closed stuff off and gone back to do other work and it looked the same as it did when I put it in, 10 years down the line. It’s possible that there have been many bad batches

[Onoff]

Cut the boards with an angle either side so they "fit" but there's a gap either side closest to the face of the joists. Ditto where joints butt up.

 

Then spray the gaps lightly with water in a plant sprayer and fill with Illbrück FM330 expanding foam which is about the best you can get and super flexible as well as airtight. 

 

Cut the set foam flush and repair the foil faces with silver tape. If you foil over the joist faces too that can act as a vcl. 

 

You need ideally a 50mm ventilation gap between the top of the boards and underside of the "roofing felt".

 

Don't block any cross ventilation. 

[Beau]

10 hours ago, MikeGrahamT21 said:

There are products designed exactly for this, though quite expensive https://gapogroup.com

 

or you can opt for what most do on here, as I have done, get it as tight as you can and inject some flexible expanding foam around the perimeter and joins, which will move with any shrinkage.

 

it’s hard to say just how much PIR will shrink over time, seen some photos of crazy shrinkage, I’ve installed quite a bit in my place over the years, closed stuff off and gone back to do other work and it looked the same as it did when I put it in, 10 years down the line. It’s possible that there have been many bad batches

Thanks

 

I did see that tape at a homebuilding show. Nice idea but as you say a tad expensive 🫣

 

"get it as tight as you can and inject some flexible expanding foam" is what I've been doing but I wasn't aware of this flexible foam so will track some down before doing the next part. Mind you if you only have a hairline crack its very difficult to get any in and looking at the foam it talks about flexing 25%. 25% of a nabby grabby bit is diddly squat! 

 

Might try Onoffs suggestion of a deliberate taper to leave room to properly get foam in if I can work out a neat way to cut a taper and hit my width marks

 

With any luck problems there may have been with shrinking boards should have been ironed out but better safe than sorry.

 

 

 

[Beau]

9 hours ago, Onoff said:

Cut the boards with an angle either side so they "fit" but there's a gap either side closest to the face of the joists. Ditto where joints butt up.

 

Then spray the gaps lightly with water in a plant sprayer and fill with Illbrück FM330 expanding foam which is about the best you can get and super flexible as well as airtight. 

 

Cut the set foam flush and repair the foil faces with silver tape. If you foil over the joist faces too that can act as a vcl. 

 

You need ideally a 50mm ventilation gap between the top of the boards and underside of the "roofing felt".

 

Don't block any cross ventilation. 

Thanks Onoff

 

I like you taper the boards idea. Do you make a guide or just freehand cut your angle?

 

I was intending to fully tape the joints I've done and if I get a 100mm wide roll I can bridge over the 50mm rafters completely eliminating any air leakage on the joints. No idea how well the foil tape holds up over time though if the boards shrink but it will be sandwiched between the next layer of PIR boards below the rafters. I always spray some water over surfaces before foaming 

 

Yes I was working on a 50mm air gap. No problems with ventilation as the roofing felt joints are pretty sloppy and we are in an exposed windy spot.

Edited February 12 by Beau