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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorlineweight
    • CommentTimeApr 20th 2022
    The last time I looked at multifoil insulation was maybe 10 years ago and there seemed to be sufficiently many people sceptical about it that I decided not to pursue it further.

    Increasingly I'm seeing it suggested as a preferred (and cheaper) option by builders, in place of more conventional methods.

    I know there are some very long threads on here on the subject - but not much activity on them in the past five years or so.

    My question is; has thinking changed significantly in the past ten years? Has additional testing proved their worth, or has technology improved such that they are now more effective?

    Is it an option that I should remove from the "probably snake oil" category?
    • CommentTimeApr 20th 2022
    Posted By: lineweightMy question is; has thinking changed significantly in the past ten years? Has additional testing proved their worth, or has technology improved such that they are now more effective?
    I am by no means an expert but here's my top of the head view - I don't think the thinking has changed, but I do think the products have changed. They seem to be not so much 'multifoil' now as multilayered conventional insulation making use of reflective surfaces within the layering. How well they work I do not know, neither how cost effective they are, nor how sustainable.
    • CommentAuthorlineweight
    • CommentTimeApr 20th 2022
    There's a BBA cert for "Superfoil" in roof & wall installations. It's shown installed inside of a conventional PUR type insulation layer. Extract below.

    I haven't worked it out in detail but it doesn't immediately look like it saves you a lot of buildup thickness.

    For a pitched roof - 2 layers of 100mm conventional insulation (one external to the rafters) can get you close to U=0.10.

    According to the BBA cert, you can reach U=0.13 with SF60 and 75mm of conventional insulation.

    SF60 is 100mm thick but you need to ensure 13mm gap each side so really it's more like 125mm thick.

    So you end up with a similar overall thickness but a worse U value. (And cold bridges at each rafter?)
      Screenshot 2022-04-20 at 11.26.38.jpg
    The controversial product had a certificate that was obtained using an unusual test method. That certificate expired in 2021 and the product was then withdrawn. Other products dont make the same claims and are not so controversial!
    • CommentAuthorlineweight
    • CommentTimeApr 20th 2022
    Posted By: WillInAberdeenThe controversial product had a certificate that was obtained using an unusual test method. That certificate expired in 2021 and the product was then withdrawn. Other products dont make the same claims and are not so controversial!

    Thanks - useful knowledge!
    • CommentAuthorrevor
    • CommentTimeApr 20th 2022
    Reminds me of a build I visited about 10 years to see liquid screed being poured as I was considering it for our screed. I noticed the roof insulation had been in a multifoil and I had previously discussed the use of such material with my BCO who told me that it was controversial and to avoid it as it did not perform as claimed. When I mentioned this to the house owner (self build) that LABC may not like it he answered that he was not using LABC but an independent company who had okayed it.
    I then mentioned that he may want to top it up to keep the bills down, and he answered that it would not be his problem as he was building it to let, and he then went on to boast how cheap he had managed to build it for. So looks like some self builders, certainly this one, are no better than the volume builders.
    • CommentAuthortony
    • CommentTimeApr 22nd 2022
    I recon very controversial still
    • CommentAuthorlineweight
    • CommentTimeApr 22nd 2022
    Posted By: tonyI recon very controversial still

    Can you expand at all?
    • CommentAuthortony
    • CommentTimeApr 23rd 2022 edited
    Have a look at the ‘multifoil insulation’ thread 🙂

    The million-page thread was back in the day when a certain thin multifoil product was claiming equivalence to 200mm of mineral wool.

    Those claims have been dropped, the product has been scrapped, and the new products claim R values which are only slightly better than the same thickness of mineral wool.

    So while the claims about performance are no longer controversial (afaics), I don't see what advantages they offer over mineral wool.
    • CommentAuthorlineweight
    • CommentTimeApr 25th 2022 edited
    The last post on that other thread mentions an Austrian product "Lupotherm" which seems to be making the controversial claims made by the (now withdrawn?) UK products.


    The post is from 2016, so I looked to see if the website still existed - it does - but unclear whether they are still active.

    However - I see they have uploaded several videos to youtube just in the last week or so.


    In this one they do an "ice test".


    You don't need to understand German to see what they are doing. There are a few questions I would have. They are not brave enough to have comments turned on, on any of their videos though!
    • CommentAuthorJonti
    • CommentTimeApr 26th 2022
    I am not sure having the comments turned off is anything other than a sensible option to avoid the usual trolls who always appear.

    I would however ask why they did not use the same tape for both samples as this must have an influence with one being reflective the other transparent and so of different thermal qualities. Secondly why did the mineral wool gain 7g in weight during the test where as the foil remained the same weight. I would have expected a small amount of weight loss due to evaporation with both samples though this is based more on common sense than actual knowledge.
    • CommentAuthorlineweight
    • CommentTimeApr 26th 2022
    We also didn't get to see what the data loggers logged.

    And the foil version was essentially made airtight while the mineral wool one wasn't.

    We don't get to know what the air temperature is, outside the boxes. So it is possible that most of the heat melting the ice is coming in the form of direct radiant heat from the sun, rather than a temperature gradient from inside to out? In which case it doesn't tell us so much about what would happen in a typical building insulation scenario with a big difference in air temperature inside to out.
    • CommentTimeMay 3rd 2022 edited
    Posted By: WillInAberdeenI don't see what advantages they offer
    Having been a staunch defender of multifoils' potential, via an understanding of how they work which no one seemed to agree (but was confirmed by the tech boffin of one of the firms (don't remember now) who was leading their drive for compatible test methods until politically defeated) ... where was I, oh yes, I'm currently specifying MF in a special situation of a Listed Building, where east-slope slates get too hot to touch and having no underlay felt over titchy 60x50 rafters, blast the room-in-roof interior with unbearable summer heat, despite extg gappy 50 Celotex between. After GBF brains-trust
    MF between the rafters will sequentially resist that radiant heat, will be well ventilated under the slates from bottom to top up its outer face, will be airtight against convective transfer of same, and can be had in 'breatheable' grade. 75 phenolic across the rafters' undersides will do some good in winter, leaving the purlins just visible.
    Just to check, it's going in a 60mm gap, between the slates and a 75mm phenolic board?

    What thickness of multifoil can you fit in there? How much improvement does it make, compared to the U value of the phenolic and an empty air gap?
    • CommentTimeMay 4th 2022
    80mm - 60 rafter height + 3/4" slating battens.

    A variety of choice @ about 40 nom thickness, so nom 10 air gap to its inner and outer faces.

    Improvement to U value = 'some' - but as I'm saying the 'advantage' of MF that's used here is its undisputed resistance to radiant transfer, as well as airtightness with water vapour 'breatheability'.
    Mmm, but accepting the phenolic layer is unbreathable, and as you're ok to encroach in the usual ventilation gap, then wouldn't you get the same results by part-filling that space with the same thickness of wool, which does have a tangible insulation value? Or part-fill with woodfibre (breathable and decrement delay) or with more phenolic (better insulation value and a shiny surface initially)? Any opaque material is also opaque to thermal radiation.

    Sounds like you are using it more like a shiny breather membrane, but how is a foil product certified to be both breathable and airtight? Have they needle-punched lots of little holes in it, do you need to tape round the edges?

    How should it be fixed between the rafter faces, to achieve the 10mm air gap inside and out?

    Sorry for all the q's!
    • CommentTimeMay 4th 2022
    Its prime purpose is as best-poss radiation barrier, at which MF is supreme, having multiple reflect surfaces.

    The radiant heat up there (bedroom and work room) on a midsummer clear morning is phenomenal - long elevation with clear low view 20o N of E to catch everything from sunrise, so by mid morning it's cooking; the sun moves round to the smaller S hip to keep it topped up; by the time it's full on the W it's sinking; but still hot beyond bedtime.

    Extg shiny Cellotex completely ineffective, partly because gappy, so powerful convection (temp differentials far greater than winter) drives hot air inward.

    No amount of decrement would smooth out this temp curve!

    So radiant reflection first, continuous airtighness second extended as far as possible at edges, third any conventional insulation a bonus

    And copious valley-gutter to ridge convective ventilation above the MF/under the slates (incl crossflow via the slating batten spaces).
    https://www.nicholsonsts.com/products/airtrak is a great, robust product range.

    Breatheability - yes the inboard phenolic isn't but I still like the outer to be breatheable anyway. If that turns out to be by needle-punching then we won't have that.

    We have a good way to accurately centre and tension the MF in the cavity so the min 10mm gap (deeper alongside the rafters) inboard and out is real - and it runs continuous (crushed) over the rafters'/hips' under-face incl a wangle where that's interupted where running 'above' the purlins.
    That's interesting, so you are using it as a reflective airtight breather membrane?

    One of the product websites said they do make theirs breathable by punching little holes through the foil layers, but then they laminate a layer of 'proper' breather membrane into the multi layer sandwich, so indeed it should be breathable and airtight overall. They seemed to have a few layers of foil on each side, and a thick core of mineral wool in the middle. They sell tape to stick over joins between sheets.

    There used to be a neat product available which was a roll of mineral wool encapsulated in a thin plastic 'sock', which made it easy to handle and resistant to wind washing, and strong enough to pull it through under-floor gaps with a piece of string. I haven't been able to get it recently but possibly the 'thick' multifoils would work the same?

    They were selling some multifoil in Aldi a while back but when it's gone it's gone...
    • CommentAuthorHollyBush
    • CommentTimeMay 5th 2022
    I am using a "duo" method breathable outer (SF65 BB?) and an inner (SF19+ I think - sorry recollection not perfect on the numbering) and we increased this where thickness not an issue.

    We were converting the loft space to a room, changed the ceiling joists and needed to retile. I was happy
    that we had specified foil coated boards cut between rafters (50mm?), and slightly thicker on top. Builder that originally agreed got injured (not on site!) and didn't want to work on the roof after.

    I couldn't get a builder or roofer to finish the work. In the end we found a really good roofer that we had used previously, had a discussion and agreed on the foil. Less than a quarter of the roof needed to be re-done. Whilst I still need to put in place some of the under rafter layer, so far I've found:

    - to Tom's point, loft room was far cooler last summer than previously, though I don't have objective data
    - no draughts appear to be getting through the outer layer (not something I was confident on the boards)
    - it is breathable - no need for additional membrane
    - easier and quicker to put in place
    - roofers found the rafters much easier when counter battening
    - lower overall thickness to roof
    - excellent support from the technical team at Superfoil
    - no issues with building control
    - room doesn't have any additional heating, and probably won't need it

    - supplied foil not very good, and bought additional foil tape
    - care needs to be taken screwing through it as insulation can get wrapped and dragged through - nailing is easier, but with care screwing is OK
    - cutting the foil is far easier with shears - to the extent I would say don't attempt any other way - big special scissors are the way to go.

    Hope this helps.
    • CommentAuthorEd Davies
    • CommentTimeMay 6th 2022
    Posted By: HollyBush- it is breathable - no need for additional membrane
    If it wasn't breathable how would an additional membrane help?
    Fostertom - did / do you have any way to quantify the performance against radiant heat, compared to foil faced phenolic between and under rafters?

    If you used board with both faces foil faced, would that not effectively give you 4 layers of foil?

    If the existing board was installed with gaps, surely it's not a fair comparison to the results with MF?

    How do you know how much of the improvement is down to the MF rather than the additional phenolic board, and the elimination of air gaps?
    • CommentAuthorHollyBush
    • CommentTimeMay 6th 2022
    Posted By: Ed Davies
    Posted By: HollyBush- it is breathable - no need for additional membrane
    If it wasn't breathable how would an additional membrane help?

    :) yes that was badly written...

    The configuration may need to change, but I am not an expert - I guess what I should have said is that it also functions as a breathable membrane on the outer layer. You don't need to have another membrane (breathable or not).

    Hope that helps
    • CommentAuthorHollyBush
    • CommentTimeMay 6th 2022
    or maybe I just missed a return?
    Posted By: Ed DaviesIf it wasn't breathable how would an additional membrane help?
    Maybe it means if it wasn't airtight and water-shedding, you'd need to put a breather membrane underneath the tiles? Or if it wasn't breathable, you'd need an additional VCL on the inside?
    Edit: post crossed with HollyBush

    (The internal layer that HB mentioned is not breathable, and is described in the catalogue as performing like a VCL - the two layers are described as performing as the VCL and BM respectively).

    The two products HB mentioned have a combined thickness (per their BBAs and including two air gaps) of 45+120+2*13 = 192 mm and thermal resistance (including two air gaps) of 1.63+4.42 = 6.05 m²K/W, so a conductance of 1/6.05 = 0.17 W/m²K .

    The U value will be pretty close to this, plus/minus bridges and surfaces. That's good for a loft conversion.

    The conductivity is equivalent to 0.17*0.192 = 0.032 W/mK which is similar to the very best mineral wools. It's not as good as PIR/phenolic etc, but you might just use a thicker layer of multifoil between the rafters, as HB has done.

    The fire class E is not very good, that might/mightnot be a factor, they do offer a non-combustible alternative.

    Edit: the BBAs are based on the air gaps being sealed/unventilated. If there's free air movement under the slates/tiles as Tom and HB intend, the performance will be slightly poorer than the numbers above.
    • CommentTimeMay 6th 2022
    Posted By: lineweightIf the existing board was installed with gaps, surely it's not a fair comparison to the results with MF?
    True, but it's easier to get to 'no gaps' with new MF, than by faffing with extg Celotex, along with the other benefits of MF, which Cellotex can't achieve - like multiple reflect surfaces, membrane-continuity rather than multi-multi foamed edges, and breatheability (maybe). The 50 Cellotex wd have to be carefully positioned in the 60 rafter space to not touch above or below. The underlining isn't planned as foil-faced, anyway only its outer face would work as reflective, the pbd being fixed to its inner face - dodgy in itself.

    Posted By: lineweightHow do you know how much of the improvement is down to the MF rather than the additional phenolic board, and the elimination of air gaps?
    Phenolic can't offer resistance to radiation, and certainly it is partly thanks to elimination of air gaps.
    Assuming we just get rid of the existing badly-fitted Celotex (because that's what the MF approach does), then the options are to replace it with well-fitted new Celotex, or MF.

    Ignoring radiaton to start with, and just looking at "conventional" U values I don't see how the MF can do better. I get it that making it continuous across the undersides of rafters has benefit for airtightness - but because of the way I assume it bunches, there must be wedge-shaped voids next to each rafter where there's not really any insulation. And for the un-bunched middle portions, as far as I can tell, the same thickness of MF can only do worse than phenolic board, insulation-wise.

    If there's going to be insulation under the rafters anyway, then I'd feel that this gives you pretty good airtightness anyway, obviously as long as joints don't line up with the rafters/board joints in the layer above.

    So it all comes down to the question of radiated heat, and you may well be right that the MF performs better in this regard, but I'd want to see something quantifiable to convince me, especially seeing as it seems a trade-off against poorer insulation which will perform worse in cold weather.

    As per previous comment, if each layer of celotex had foil each side, then you've got 4 layers of foil. Plasterboard can then be fixed independently.
    • CommentAuthorLF
    • CommentTime3 hours ago
    Looked at this type of stuff for sloping roof but fire rating is poor - it was for unheated workshop/utility area attached to main house. Roof that has nothing but felt and tiles but reasonable floor and walls. No plasterboard.

    I set fire and the polythene and foils burns readily and "drips out flames."
    I tried expensive multilayer (MF)and thin economy bubble wrap type samples. All kindling.
    When you set fire to PIR, harder to light and when flame removed it goes out. This this stuff did not.
    Many people must be stapling the plastic and foil stuff to loft rafters, which if it found an ignition source would make fire go faster.

    There are non combustible multifoils but at many times the unit cost.

    I am now going with 2 inch thick 1.2 m wide rolls of mineral wool with foil layer. I will staple to underside of 65 mm deep sloping afters to make an inner tent. U value maybe 0.44 overall but it is not adding kindling layer in roof.
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