Are you all still against multi foil or has anything changed?

[Roz]

The guy doing our SAP calcs pointed me in the direction of Superfoil. Space is at a premium, this actually doesn't save us any space due to the batten voids, compared to just using kingspan,  but it looks a bit easier to lay, and I think it might be cheaper per metre but I need to work that out. I previously thought you couldn't use it on its own and it had to go with another form of insulation, but that doesn't seem to be the case according to the superfoil website.

 

I also like that it's breathable and apparently means there won't be condensation in our old 500mm thick walled granite barn. 

 

However, I have read on here before that many of you think it's 'snake oil'. Is that still the case or has anything changed? Why is it that this opinion is held? Does anyone disagree?

[nod]

I used it on my fibreglass roof 

Brilliant stuff 

From memory the 18 layer foil is equal to about 300 mil of rock wool 

[PeterW]

It works as an air tightness layer but past that it is very suspect - it also has a few supporters and I’ve seen it used in conjunction with standard methods but I’d be hard pressed to recommend it. 

[A_L]

Here is the BBA Cert for the product. According to this the SF19 variant is 42mm thick and has the insulating effect of about 55mm of ordinary loft roll. It is also to be considered a VCL with a vapour resistance of 1200Mn.s.g (Polythene is normally about 300 IIRC)

 

 

Superfoil BBA Agrement Certificates Roof and Wall.pdf

[Temp]

Discuss with your BCO as he may have a view. These multi layer products have changed over time.

[Ed Davies]

Not sure where the 42mm thickness comes from, that BBA cert says 40mm.

 

Something else I'm not sure about is whether the quoted resistances include the surface resistance with the low emissivity. I'd assume they do though I wonder how as that tends to depend on the orientation of the surface because of air movement. Maybe they assume the cavity each side is narrow enough to not have significant air movement. Anyway, you need to include the 13mm gap each side to assess the stuff fairly for equivalents so 40+26 = 66mm thick. With a thermal resistance of 1.24 m²·K/W that comes out to an equivalent resistivity k = 0.053 W/m·K which is not going to beat anything but the cheapest fluffiest light loft insulation.

 

The thicker SF60 which has R=3.58 m²·K/W and 100mm thickness so 126mm with the gaps comes out to an equivalent k = 0.028 W/m·K which is quite respectable, better than most (all?) mineral wool and getting down towards PUR values. Maybe equivalent to EPS though I'd need to check the numbers.

 

That's all quite reasonable. The controversies with the multi-foils came when people were claiming that this sort thickness was equivalent to 200mm or more of mineral wool (maybe 4 or 5 times the resistances given in that BBA cert) because of some sort of hand-wavy dynamic effects. Many had doubts.

[scottishjohn]

I have  sf60  in my loft over mineral wool in gable ends  and under my suspended ground floor as insulation,stapled up and all sealed together with correct tape  as my ufh is set in floor boards 

 

 seems works very well is all  I can say ,whether it is the sealing up of all the underfloor or the insulation that is doing most I cannot say ,

but when I have had to pull some down to do things to wring + plumbing behind it --then you can  feel it is a barrier to heat loss +air movement,as the air trapped in between joists is  very much warmer than the cold draft under the house 

[A_L]

 

11 hours ago, Ed Davies said:

Not sure where the 42mm thickness comes from, that BBA cert says 40mm.

 

from section 6 of BBA cert,  '1.24 m2·K·W–1 R value for SuperFoil SF19 (41.9 mm thick) with no air gaps either side '

 

wiith air gaps the thermal resistance will vary with orientation, as suggested here

 

https://www.roofingsuperstore.co.uk/product/superfoil-sf19-multi-layer-foil-insulation-15m-x-10m-roll-15m2-1.html?gclid=EAIaIQobChMIs5GD97OR4AIV6Z3tCh16IQxeEAQYAyABEgJHufD_BwE

 

although I do not know where the R values come from