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.

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 That certificate expired in 2021 and the product was then withdrawn.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!

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.

Posted By: tonyI recon very controversial still

Can you expand at all?

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.

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.

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
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16804
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.

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'.

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.

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:

Positives
- 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

negatives
- 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.

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?

or maybe I just missed a return?

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.

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.

Thanks DJH. 2nd layer added but going under rafter again to keep the 50mm air gap to felt. My I value was wrong above.Isover ductwrap on its way.
I think second layer will be at 90 degrees to first. 1.2 m wide roll. 2.4 m rafters on 600mm centres.