Cut tight or foam?

[gc100]

So I'm about to embark on putting the PIR inbetween the studs, I have a debate happening with the builders about the best way we are going to do this. Basically the option is either cut to size as best as possible and fit in hoping we get it tight everywhere and the cuts are straight (depth wise) as possible. Or we cut 5mm short and foam all around.

 

The advantage of using the foam is we would be guaranteed an air tight fit. However u value / W/mk of foam is about double of PIR so you lose there.

 

Any thoughts? 

Edited March 11, 2020 by gc100

[ArtyWoodworker]

Hi, Having a tight fit is very doable by competent and patient builders - foam should be reserved for really hard parts e.g. when a joist is close to an uneven brick wall like in old Victorian houses, for example.

 

I feel like the approach should be to aim for cut-to-fit, and use the foam in the (hopefully) rare occasions you get gaps. Just my opinion based on what I have seen getting done in the past. 

 

Edit: I remembered this video I saw recently, showing how getting a tight fit is possible 

 

Edited March 11, 2020 by ArtyWoodworker

[gc100]

26 minutes ago, ArtyWoodworker said:

I feel like the approach should be to aim for cut-to-fit, and use the foam in the (hopefully) rare occasions you get gaps

 

Yes thats my feeling as well, but wanted to get any other opinions on this.

Thanks

[Onoff]

Cutting square is the key imo. A bfo table saw with dust extraction is good.

 

 

[Russell griffiths]

If you are worried about u value of foam, then buy better foam

i have just started using products by ILLBRUCK, 10 times better than the normal stuff. It’s got a tighter cell so drys to form a far more compact blob. 

[gc100]

26 minutes ago, Russell griffiths said:

If you are worried about u value of foam, then buy better foam

i have just started using products by ILLBRUCK, 10 times better than the normal stuff. It’s got a tighter cell so drys to form a far more compact blob. 

the foams I was looking at are equivalent mW/mk as ILLBRUCK

[Roundtuit]

What thickness?  I think up to 100mm is relatively easy to cut accurately to get a tight fit (then just foam any 'errors').  I did mine with a hand saw; after the first few cuts, you get pretty good pretty quick.  Anything thicker, and I think I'd have struggled keeping the saw square and there's a lot of friction on the sides of the saw.

[gc100]

I’ve got 120mm for the stud walls.

[Vijay]

If you have loads to do, make a hot wire cutter?

[Onoff]

20 minutes ago, Vijay said:

If you have loads to do, make a hot wire cutter?

 

The one I made with kanthal/nichrome wire worked well enough / got hot enough to burn through the foil either side as well as the pir. Had issues with it keep breaking though. It's much harder to cut thru than eps. Thin wire offers lower mechanical resistance but obviously less strength. The psu which was just a wall watt crapped out in the end.

[MikeGrahamT21]

I spotted this in the latest toolstation catalog, looks interesting: https://www.toolstation.com/smart-trade-insulation-blade/p62092

 

Smooth sided 100mm long blade for a oscillator, not used one, but considering getting one to try for £9

 

Probably beats what I do at the minute, disposable stanley knife with blade at full stretch, doesn't quite get through 100mm, and can veer off course a bit!

Edited March 12, 2020 by MikeGrahamT21

[Onoff]

16 minutes ago, MikeGrahamT21 said:

I spotted this in the latest toolstation catalog, looks interesting: https://www.toolstation.com/smart-trade-insulation-blade/p62092

 

Smooth sided 100mm long blade for a oscillator, not used one, but considering getting one to try for £9

 

Probably beats what I do at the minute, disposable stanley knife with blade at full stretch, doesn't quite get through 100mm, and can veer off course a bit!

 

I reckon for squareness you might be better with a Bosch or Milwaukee "wave" blade in a jigsaw. Holding a multitool square over 100mm deep cut...good luck! Anything "powered" will also create more dust I imagine. 

 

I still like the pink Celotex saw held against a length of square timber. Gives a pretty square cut tbh. 

 

[Jenni]

We tried jigsaw blades. The slower you take it the straighter the cut, very clean but they blunt very quickly then stink as they struggle. 

In the end went back to normal toothed hand saw, fast and straight, sadly very messy.