Trw144 Posted October 12 Share Posted October 12 Hi all, With my project slowly progressing, I've come across the chance of a deal on some PIR insulation. I'm trying to see if I can make it work for me and save me a good few thousand in the process, the issue is the the cavity insulation available is 75mm so not enough for what I am trying to achieve. Has anyone/is it possible to double up the insuation or would it become a complete faff to install? thanks Link to comment Share on other sites More sharing options...
Iceverge Posted October 12 Share Posted October 12 Pir in cavity wall. Nope. Use mineral wool batts instead Link to comment Share on other sites More sharing options...
SteamyTea Posted October 12 Share Posted October 12 You can fit PIR into a cavity, but it is a pain to do properly, you end up with too many air paths that are impossible to seal. 2 Link to comment Share on other sites More sharing options...
Trw144 Posted October 12 Author Share Posted October 12 Was my original plan - why is PIR not seen as good on the forum? Link to comment Share on other sites More sharing options...
SteamyTea Posted October 12 Share Posted October 12 (edited) 2 minutes ago, Trw144 said: why is PIR not seen as good on the forum Not that it is not good, thermally it is brilliant. Just most people dislike working with it. If you are factory/shed building panels, then it is easy to the quality needed, fitting onsite between brick/block, a nightmare. Having said that, you could use it to IWI/EWI Edited October 12 by SteamyTea Link to comment Share on other sites More sharing options...
saveasteading Posted October 12 Share Posted October 12 4 minutes ago, SteamyTea said: too many air paths that are impossible to seal. What is your situation? Walls, floors, cobstruction? Our solution was 1. 2 x thin skins of pir can be cut more accurately between studs, then a second layer which can slso be staggered to cover joints. On the floor two layers staggered too. 2. on walls then use mineral wool bevause it pushes in tight. Link to comment Share on other sites More sharing options...
Russell griffiths Posted October 12 Share Posted October 12 75mm keep it for the floors, double it up to 150, stagger the joins. 2 Link to comment Share on other sites More sharing options...
FarmerN Posted October 12 Share Posted October 12 We did 115mm PIR in block/ brick external wall. The block wall was perfect with cavity side beatifully pointed but still impossible to get a really tight fit between PIR and blocks. Even very small air gaps alows for a chimney effect, leading to thermal bypass and a massive reduction from the paper u value. Come January I'll find out what it really does to our heating bill. We were supposed to have a 10mm cavity between PIR and brick but this is also near impossible and gets too much mortar in while the bricks are being laid. The builder did use the correct tape to tape all pir joints, think the tape alone came to almost £1000 ! 1 Link to comment Share on other sites More sharing options...
Trw144 Posted October 12 Author Share Posted October 12 2 hours ago, Russell griffiths said: 75mm keep it for the floors, double it up to 150, stagger the joins. Yes, had thought about doing that as an option - wasn't sure how well 450mm x 1200 sheets would "sit" down to lay and then put UFH Link to comment Share on other sites More sharing options...
saveasteading Posted October 12 Share Posted October 12 18 minutes ago, Trw144 said: how well 450mm x 1200 sheets would "sit" down to lay and then put UFH No problem. Are you screeding over it? Sticky tape on the joints, and then the weight sorts everything out. Link to comment Share on other sites More sharing options...
Trw144 Posted October 12 Author Share Posted October 12 10 minutes ago, saveasteading said: No problem. Are you screeding over it? Sticky tape on the joints, and then the weight sorts everything out. Yes will be screeded over. Link to comment Share on other sites More sharing options...
saveasteading Posted October 12 Share Posted October 12 1 minute ago, Trw144 said: Yes will be screeded over. There was a similar discussion last week if you want to look for it. google Buildhub PIR screed or similar. The screed is quite heavy, and plenty to bend down any wobbles. Plus when you are walking on it to fix the ufh pipe, use a board to spread your load enough to protect it but it will also press down any wobbles. 1 Link to comment Share on other sites More sharing options...
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