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Heating and cooling: Reflective Radiation - Install PIR with air gaps to increase effectiveness?

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    Hi

    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
  2.  
    In theory, BRE recognise an air gap with reflective surfaces as giving 2 sets of surface heat transfer resistance, so improving insulation value. Although the insulation value of the air gap will not be as good as filling it with the same thickness of PIR.

    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worked for a particular product, which might have given people the idea of doing the same with PIR, but that product is withdrawn now.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.
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      CommentAuthorfostertom
    • CommentTimeMar 16th 2023 edited
     
    Posted By: WillInAberdeenthat product is withdrawn now
    Which 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.
  4.  
    The particular product that made outrageous performance claims, which caused the 10000-post multifoil discussion thread that is still legendary among long time members of GBF (but before my time)!

    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!
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      CommentAuthorfostertom
    • CommentTimeMar 16th 2023 edited
     
    I was v active in that legendary 10,000! But I don't remember any particular product making exceptional claims, beyond the others. There was one manuf, don't remember name now, which had the best boffin-dept and was leading the multifoil R&D charge, and which painted ambitious pictures of poss future development of the genre (all such development now ceased) and of the figures which should be obtained currently if only the industry's standard hot-box test procedure could be adjusted/augmented so as to not systematically minimise simulation of the interstitial-radiant fraction of buildings' heat loss (which MFs are particularly good at). Work did indeed begin on development of a few such rigs, but that got scotched by the bad-science influence of the conventional insulant incumbents. So the MF industry was defeated and remains an undeveloped niche to this day.
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      CommentAuthordjh
    • CommentTimeMar 16th 2023
     
    Multifoil insulation was developed for spacecraft IIRC and its much vaunted properties depended on it being in a hard vacuum. But please lets not go there again :bigsmile: :devil: :cry:
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      CommentAuthorfostertom
    • CommentTimeMar 17th 2023 edited
     
    Posted By: djhdepended on it being in a hard vacuum
    You 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.
  8.  
    Many thanks for the replies and thoughts. Interesting stuff.

    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
    • CommentAuthorcjard
    • CommentTimeMar 19th 2023
     
    If space is at a premium you can look At phenolic, aerogels or VIPs
    • CommentAuthorcjard
    • CommentTimeMar 19th 2023
     
    (You'll pay for it though!)

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