MJNewton

PIR Insulation Installation Tips?

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I am about to be fitting 100mm PIR insulation between studs (600mm centres) and am wondering what approach to take. I am mindful that my previous experiences with PIR boards have been mixed (generally okay) in terms of accuracy of fitting and that has always been with thinner, easier to handle/cut/coerce, sizes and so I am assuming it’s going to be even more difficult with this stuff.

 

I am toying between two methods: 1) Attempting to cut neatly for a perfect friction fit or 2) Intentionally cutting small (say, 5-10mm gap each side?) and then foaming the gap. For both I’ll be taping for vapour control and possibly adding a barrier sheet on top.

 

The first method requires more skill, which would usually lead me to believe it is the better option if achievable, but then I can’t help but think that actually the second might be better in all respects – not only easier (no corrective cutting/trimming, no bashing the boards in to fit, no concerns regarding less-than-perfect timbers etc) but perhaps also benefitting from a better full length seal along the edges thanks to the foam filling in all the voids (throughout the depth of the board-timber interface) whilst also providing a more secure fitting due to the adhesive properties of the foam.

 

Any thoughts?

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Cut them 10mm narrower, fit the whole lot, then foam them all in. Next day, trim surplus foam and tape joins. Tape to the studs as well. Wear a dust mask and goggles. There is no shame in using the rips up, if you foam them in and tape them.

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Posted (edited)

That sounds positive; thanks. I was hoping to get votes for the foaming approach as it just felt like it was going to be a whole lot easier. That said, even though I've got a controllable gun, whenever I use foam I *always* underestimate the expansion and end up coming back to something of a monster. At best it's a waste but in this case I don't want too much going out of the back where I can't see/trim it. I suppose I could just be extra conservative, even if it means going back to top up any shortfalls. 

Edited by MJNewton

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Use low expansion foam, such as "window and door fitting foam" to avoid the over expansion

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44 minutes ago, Mr Punter said:

Cut them 10mm narrower, fit the whole lot, then foam them all in. Next day, trim surplus foam and tape joins. Tape to the studs as well. Wear a dust mask and goggles. There is no shame in using the rips up, if you foam them in and tape them.

 

Taking that one stage further....

 

I usually cut the rips into wedges, and then use them to hold the big bits in place and foam between. Just ram the wedges in and snap off the excess. Toolstation 100mm foil tape is very good too and not too expensive

 

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Another vote for loose and foam. And a mask

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33 minutes ago, bassanclan said:

Use low expansion foam, such as "window and door fitting foam" to avoid the over expansion

 

I did consider that, but given the ubiquitous availability of 'normal' expanding foam I figured I probably ought to just learn to go easy on it and trust the small(er) amount to expand sufficiently! I've also been confused by the reviews of many of these low-expansion foams as often they've said they expand just as much!

 

Quote

Toolstation 100mm foil tape is very good too and not too expensive

 

Yes, and great stuff it is too - aside from the functional properties it also makes even the roughest of installs look so much neater! Hides a multitude of sins you might say. Reassuringly sticky too.

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If you foam you will also find that the boards will feel really solid, probably more so then if you cut and fit accurately.

 

Also I presume you have one but you really want an gun for the job rather than a nozzle from the can.

 

We are also going to be fitting a lot of PIR insulation, but for our rafters over the next few weeks. 

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Okay, that's good to hear regarding it being more solid. And yes, I've got a gun - great things aren't they. I think I just need to better exploit it. To be fair, all my usage with it so far has been to seal up gaps overhead and so I've been having to use a 300mm pipe extension on it (just something I lashed up; not something that came with it) which makes it rather more tricky to use neatly as the foam keeps expanding out of the end even once the trigger has been closed.

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I found when cutting anything above 50mm thick to get an accurate cut , ie 45 degree or whatever I use a cordless circular saw and then finish it of by hand saw. Also agree with cutting, wedging and foaming. 

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Also a tip for overhead or eaves. Bang a couple of nails half into side of the rafters or joists. When you push the pir up it snags into these and holds flush while foam gets it

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