Richard Marin Posted October 1, 2022 Share Posted October 1, 2022 Hi all, How do I calculate the 'R' value of air gap in a cavity wall? I have attached my 'R' value and 'U' value calculation. I know that the air gap should be part of the calculation but just can't find any information online to assist me. Can anyone with more experience than me please help? Many thanks in advance. Link to comment Share on other sites More sharing options...
SteamyTea Posted October 1, 2022 Share Posted October 1, 2022 May help, even if it is Scotch.6.B+-+Worked+examples+of+U-value+calculations+using+the+combined+method++.pdf 1 Link to comment Share on other sites More sharing options...
PeterW Posted October 1, 2022 Share Posted October 1, 2022 Something wrong with that calc as brick/insulation/block doesn’t allow for a 3mm cavity - you need to leave 10mm. Using standard 90mm PIR you will get 0.18W/m2k but that’s just enough to scrape building regs Link to comment Share on other sites More sharing options...
Radian Posted October 1, 2022 Share Posted October 1, 2022 1 minute ago, PeterW said: Something wrong with that calc as brick/insulation/block doesn’t allow for a 3mm cavity - you need to leave 10mm. Using standard 90mm PIR you will get 0.18W/m2k but that’s just enough to scrape building regs I might be wrong, but it sounds to me like it's 100mm rigid PIR packed in tight as 97mm doesn't strike me as being a manufactured item. Is there a bigger story behind this? 1 Link to comment Share on other sites More sharing options...
Richard Marin Posted October 1, 2022 Author Share Posted October 1, 2022 Hi all, Thank you for your prompt responses. The below spec was recommended and it's a 97mm thick. https://www.insulationshop.co/cf5097_celotex_97mm_cf500_full_fill_cavity.html It just may be easier to change the specification to a 100mm to avoid challenges with building control. Link to comment Share on other sites More sharing options...
Richard Marin Posted October 1, 2022 Author Share Posted October 1, 2022 3 hours ago, SteamyTea said: May help, even if it is Scotch.6.B+-+Worked+examples+of+U-value+calculations+using+the+combined+method++.pdf This very useful document thank you. Someone also referred me to the attached BS document which seems to be useful and talks alot about air gaps in cavity etc i.s.eniso6946-2017.pdf Link to comment Share on other sites More sharing options...
PeterW Posted October 1, 2022 Share Posted October 1, 2022 18 minutes ago, Richard Marin said: The below spec was recommended and it's a 97mm thick They don’t make it anymore, and it’s tolerance was +/-3mm which caused issues with aligning skins. Much easier to go partial with a gap - 100mm cavity boards with a 20mm gap doesn’t lose you much but your brickies will be much happier. 1 Link to comment Share on other sites More sharing options...
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