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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorShevek
    • CommentTimeDec 16th 2021 edited
    I know this one has been done to death in other threads but what are the key points we should consider before going with multi-foil insulation?

    We've specified 220 mm i-joist rafters fully filled with glass mineral wool (with 0.032 thermal conductivity) and an Intello Plus VCL to underside of rafters (with a further 25 mm batten/service zone + plasterboard)

    But there are currently very long lead times for the glass mineral wool (estimated delivery March 2022), so the contractor wants to look at alternatives. One of which is a multi-foil honey comb product called Hybris between rafters and a thin multi-foil VCL (HControl Hydbrid) to underside of rafters.

    HControl Hybrid + 170 mm Hybris is said to provide 0.13 u-value, and at roughly £26m2 considerably cheaper than mineral wool. And with a lead time of only 2-3 weeks. They also come with a suite of LABC certified details.

    So, if those figures are correct, the advantages are obvious. What are the other things to consider? I'm going to list them here as people post them:
    • CommentAuthorbhommels
    • CommentTimeDec 16th 2021
    From what I have learned from reading all those other threads, with multifoils, the catch is always the air gaps required. Looking at the information on their website, the HControl Hybrid is no different and requires 20mm air voids on both sides. It is unclear how the performance is influenced by air currents in these gaps. Probably not positively.
    Not sure what the Hybris needs, it says something about high performance air gaps in the data sheet but what that means needs clarification. Perhaps installation instructions for both products can tell you more on how exactly the quoted U value is achieved. It might be very difficult to replicate in a practical setting.
    • CommentAuthorGreenPaddy
    • CommentTimeDec 17th 2021
    I normally use Knauf wool, but I suspect that may be the one with the long lead times. There is also Superglass, based in Scotland, don;t know if you've tried that product for delivery times? It's not as nice to work with, but needs must.

    As to multi foils, I had a quick look at the hybris product previously, and it is not the same as the usual thin multifoils - often 15-20 mm thick, so very reliant on outer face conditions. This hybris looks more like a wool type insulation, in that it has considerable thickness, and so likely is more dependent upon air pockets in the 170mm thickness, meaning it shouldn't be squashed. That's much the same for wools.

    Could you not go for 210mm of the hybris, so all insulation is between the rafters, as you had planned with the original wool. That keeps your design detailing the same, just a like for like replacement (Hybris shows lambda of 0.033 W/mK). No concerns then with thin multifoil of the Hcontrol additional surface layer?

    Your Uvalvue will be better than that calc'd on the Hybris website, as your timber I beams will give significantly better thermal values. On that point, your insulation will need to be a bit wider, as your rafter thickness will be 9mm not the usual 45. Check that the Hybris can still expand to fill the 590mm gap, rather than the usual 555mm gap - it could be slack, and so not prevent air by-pass at the rafter interface?
    • CommentAuthorShevek
    • CommentTimeDec 17th 2021
    Excellent point about width of joists, thanks. But they do come in 1200 x 1145, so can be cut along the 1200 length.
    • CommentAuthorShevek
    • CommentTimeDec 17th 2021
    What about the breathability of (or lack of) the Hybris?

    The Intello Plus VCL allows for radiative summer back drying, providing extra protection to that the top layer of OSB, but might the Hybris insulation layer interfere with this process?
    • CommentTimeDec 17th 2021
    Posted By: Shevekextra protection to that the top layer of OSB

    Sorry, I can guess but perhaps it would be better if you explained your buildup since this is the first mention of OSB?
    I thought that to hit building regs that you couldn't use multifoils on their own, but had to combine them with other forms of insulation such as rockwool or polystyrene?

    We used a multifoil that is combined with a waterproof breather membrane (TLX Gold) on the upper (tile-facing) side of out roof, with Celotex PIR between and under the rafters.

    It would be interesting if multifoils on their own are now OK for building control.
    • CommentTimeDec 17th 2021
    The Hybris isn't a [pure] multifoil. It's PE foam, plus aircavities, plus foils. So much more plausible than many previous products.
    • CommentAuthorHollyBush
    • CommentTimeDec 19th 2021
    We found cutting foil was difficult, and in the end purchased some Fiskars Shears.

    Recommended (by supplier) Tape to stick the thing together was poor at sticking - bought a different tape in the end.
    • CommentAuthorShevek
    • CommentTimeDec 20th 2021 edited
    This is the proposed build up. Not compliant with BS 5250 because 5250 assumes the 18 OSB layer to be high resistance and requires ventilation between it and the breather membrane. But we have Glaser and Wufi calcs that demonstrate this build up to work, helped in part by taping of all membrane joints + the Intello Plus to the warm side, which allows for summer time back drying.

    My concern would be that the introduction of a non-breathable insulation like Hybris would interfere with that summer back drying process, slowing down the roof structure's ability to get rid of moisture.

    - Plain clay roof tiles
    - 25 mm softwood tile battens
    - 11 mm softwood counter battens
    - Pro Clima Solitex Plus breather membrane
    - 18 mm OSB/3
    - 220 mm Metsä Wood Finnjoists i-joists at 600 centres, with Knauf Rafter Roll 32 in between (3x 75 mm)
    - Pro Clima Intello Plus vapour control layer
    -25 mm softwood batten/service zone
    -12.5 mm plasterboard
    • CommentAuthorGreenPaddy
    • CommentTimeDec 21st 2021
    did you try "superglass 32" alternative for availability? You'll likely be getting a lot of rolls, so several pallets may even be sent from manufacturer directly, without touching the merchant (though he'll still take his cut)? Eliminates any concern over breathability.
    • CommentAuthorShevek
    • CommentTimeDec 21st 2021
    Hey David, yes Superclass 32 has same lead time problems. We had a meeting about it and decided to avoid the Hybris (but reconsider it on future projects). The contractor managed to source some Omnifit 35, which will lower the u-value slightly but we have some wiggle room on that front.
    • CommentTimeDec 30th 2021
    There's an excellent example of the proper use of multi-foil in the news at the moment. :bigsmile:

    The James Webb space telescope uses a 5-layer multi-foil for its sunscreen and achieves a 300 K difference between front and back. The key of course is high vacuum on both surfaces and between all the foils :devil:
    • CommentAuthorJonti
    • CommentTimeDec 31st 2021
    Posted By: djhThere's an excellent example of the proper use of multi-foil in the news at the moment.http:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt=":bigsmile:" title=":bigsmile:" >

    The James Webb space telescope uses a 5-layer multi-foil for its sunscreen and achieves a 300 K difference between front and back. The key of course is high vacuum on both surfaces and between all the foilshttp:///newforum/extensions/Vanillacons/smilies/standard/devil.gif" alt=":devil:" title=":devil:" >

    With each layer thinner than a human hair can you image the state it would be in after the builders had installed it :bigsmile:
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