Tue, 19 Dec 2023 06:24:55 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300765#Comment_300765 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300765#Comment_300765 Thu, 16 Mar 2023 12:45:51 +0000 greenfinger
Back again with another insulation question. I've came across a couple of examples of people pushing PIR further into the void between rafters (but not so far as to impede the airgap behind for ventilation) rather than flush with the rafter face. Looking into it, I found this was done on purpose to create a gap between those between rafter pieces of insulation and the perpendicular insulation installed beneath to the underside of the rafters. These airgaps apparently create reflective radiation and increase the effectiveness.

Just wondered what people think of this principle and whether it is right or not?

And if the idea does have merit, what would the minimum amount of air gap needed be to have any worthwhile effect? Would 5mm/10mm do anything?

Many thanks]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300767#Comment_300767 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300767#Comment_300767 Thu, 16 Mar 2023 13:15:30 +0000 WillInAberdeen
In practice, if you leave a gap between your 2 layers of PIR, it's likely that a cold draught will find its way in there and completely bypass the outer layer of insulation. Also the shiny surfaces will get dusty after a few years, and maybe some mice will move in!

Commercial products that do this, use many layers of foil stacked a mm or so apart, to get the cumulative effect of many lots of shiny surface resistances. They use a wool filler to stop air moving between. A good few years ago, some extravagant claims were made about how well this worked for a particular product, which might have given people the idea of doing the same with PIR, but that product is withdrawn now. The current shiny foil products have insulation about the same as the same thickness of wool, IE not as good as the same thickness of PIR.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300768#Comment_300768 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300768#Comment_300768 Thu, 16 Mar 2023 15:12:23 +0000 fostertom Posted By: WillInAberdeenthat product is withdrawn nowWhich product is that - not multifoils in general?

greenfinger describes the function that multifoils remain good for - resisting radiative transfer, especially where it's 'cat on a hot tin roof' summer overheating under a roof slope. In that case, MF can
a) resist the powerful radiation, driven by a delta-t that's times greater than any winter heat-loss differential;
b) form an air barrier against the powerful convective transfer that the same delta-t creates;
c) also form an cooling air-channel that can be ventilated eave to ridge;
d) is available in airtight but vapour-open grade.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300769#Comment_300769 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300769#Comment_300769 Thu, 16 Mar 2023 17:11:29 +0000 WillInAberdeen
There are plenty of other multifoil products available, some by the same manufacturer, which have well-documented performance and do all the good things you mentioned.

Two sheets of PIR spaced 5mm apart will not do all the good things you mentioned!]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300770#Comment_300770 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300770#Comment_300770 Thu, 16 Mar 2023 17:48:00 +0000 fostertom http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300772#Comment_300772 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300772#Comment_300772 Thu, 16 Mar 2023 20:34:23 +0000 djh ]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300774#Comment_300774 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300774#Comment_300774 Fri, 17 Mar 2023 08:33:29 +0000 fostertom Posted By: djhdepended on it being in a hard vacuumYou mean, in a hard vacuum, blocking radiation is the only thing that would work - that's a bit different. Doesn't mean that radiation blocking won't also work (for the radiant component) just as well in air.]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300776#Comment_300776 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300776#Comment_300776 Fri, 17 Mar 2023 09:12:59 +0000 greenfinger
Having thought about it again, I think I'm best off not leaving the airgap. Because space is tight the best I could really do is a 5mm-10mm airgap between the top/bottom PIR boards. But that would eat into the ventilation gap behind the PIR, so on reflection I think it would be best to keep the vent gap at 50mm and just butt up the two layers of PIR.

And if that wasn't enough of a reason, the mention of mice was the nail in the coffin!

Cheers]]> http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300806#Comment_300806 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300806#Comment_300806 Sun, 19 Mar 2023 06:11:24 +0000 cjard http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300807#Comment_300807 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17781&Focus=300807#Comment_300807 Sun, 19 Mar 2023 06:11:59 +0000 cjard