so I could conduct a test using a sheet of cardboard, find that it's as good as 200mm of mineral wool and building control would have to accept it?

Maybe Martian can clarify for us?

For those with the appetite here's the latest from Actis

http://www.actis-isolation.com/fichenews.php?p=3&l=3&codenews=40&rub=5&vert=2&typenews=2

I guess the next thing is to see the guidance from CLG. If it says "it's up to each BCB how to interpret this ruling", then we are no nearer a solution than we were two years ago.

Meanwhile, what is happening with the European Technical Approval for multifoils?

So as a result of this ruling should I try and persuade my BCO to accept Tri-iso 10 rather than thinsulex? Should I put it above the rafter or below? Still favour above even if I need to put tyvec above it - I guess if nothing else the tyvec will help protect the shiny stuff and I have another waterproof layer between me and the weather.

Hi again PDobson, the interaction of foils and breather membranes is a sticky subject for me as various foils have diffent levels of air and vapour permeability. Some are stitched together and some are welded. My advice would be to choose a type which has some form of Certification. and follow the manufacturers technical data sheets to the letter.

If you've managed to read this entire thread you will know that most people here do not accept that multifoils work as effectively as is claimed. I myself am still on the fence but I would use a multifoil which is welded and therefore more airtight in use. Their airtightness alone makes them worth using in my opinion.

If your insulation is air tight, I would have thought that it is waterproof, so it would catch any water blown in, so it must terminate in the gutter to shed this. From what I have read foils need a 25mm air gap over them, so I reckon the total system is :- slates/tiles, battens, sarking cloth (into gutter), counter battens, foil, rafters infilled with foam, vapour proof membrane, ceiling material. WOW!
Frank

I'm fascinated by the multi-foil saga and the different points of view on this thread.

What I can't get my head around is the European assessment procedures that multi-foils will now be subjected to. As far as I can tell, there are three separate EU activities taking place:

i. Common Understanding of Assessment Procedure (CUAP) tied in with the European Organisation of Technical Approvals (EOTA). The reference is:
141. Thin reflective insulation product intended to be used as thermal insulation complement in the construction envelope
Final EOTA: March 2007
http://www.eota.be/html/etawhoutetag.htm

ii. CEN TC89 WG12 – Standardisation of the assessment of the thermal performance of products based upon or utilising reflective or low emissivity layers.

iii. CEN Workshop 36 - Evaluation of Thin Multilayer Reflective Insulation Products by in situ Testing.

The latter of these is, I believe, a workshop set up by the multi-foil manufacturers, the results of which could influence the decisions of the other two. However the workshop doesn't in itself have any binding regulatory effect.

In the business plan to CEN Workshop 36 there's a footnote that reads as follows:

'The Workshop will liaise with CEN/TC 89 “Thermal performance of buildings
and building components” and in particular its WG12, and the European
Organisation for Technical Approval (EOTA).

'Both CEN/TC 89 and EOTA have made studies on reflective insulation
products. However EOTA has decided to suspend its study of in situ testing and
to maintain reliance on the existing ”Hot Box” or “Hot Plate” test methods.

'CEN/TC 89/WG12 has embarked on a wide ranging study of insulation products having a reflective surface, or surfaces. Whilst WG 12 will include a review of in situ testing it has made no commitment to produce an EN on in situ testing of thin multilayer reflective insulation products. At the present the only test methods recommended by CEN/TC 89 are the existing “Hot Box” or “Hot Plate”.'

http://www.cen.eu/cenorm/businessdomains/technicalcommitteesworkshops/workshops/n002adoptedbusinessplancenws36.pdf

Am I reading this wrong, or doesn't this mean that the comparative in-situ testing preferred by the multi-foil manufacturers has already been dismissed by the EOTA and will probably be dismissed by CEN?

So in terms of the UK derogation period granted to multi-foils, are we waiting on the results of the workshop, run by the multi-foil manufacturers themselves? Even though the EOTA and CEN are likely to ignore the results?

I hope this posting makes sense, and people can follow my drift (told you I was confused). Any information / illumination would be great.

As I understand it the foil insulation relys in part on airgaps either side of the foil established during installation to reduce conducted losses. These still air gaps won't exist when the foil is tested in isolation in a heat box but would exist if tested in-situ. ..or have I missed something in this complex case?

The recommended gap is 25mm in most cases and can be achieved in a real building, providing the job is done properly, which is an issue regardless of insulation type. Also the spaces between the layers of foil are filled with something woolly. Actis make one with sheeps woll don't they? Others use polyesters or suchlike.

The air gap above the multifoil was, I had always assumed, to prevent the foil touching the breather membrane and causing wicking.

No. But there are graphs published that show a bell shape with U-value plotted against gap size. It's a rounded peak at about 17mm. There's not a lot of difference between 15 and 19mm but once convection sets in above around 20mm U-value drops off steeply. Hence the industry standard of 16mm gap, with wider gaps only used where noise reduction is the priority. That's for argon fill. For krypton the narrow gap side of the graph is steeper showing that this gas is more appropriate where narrow units have to be used. I've no idea who did the research or how good it is.

Regarding the gap above multifoil, I guess the 25mm came about because of the need to avoid physical contact with a good heat conductor that would negate the low emittance of the last foil layer. 25mm is probably arbitrary, owing more to being 'about an inch' than anything to do with physics, which requires only that the gap is more than one infra-red wavelengths.

The multifoil phenomenon continues apace.

I was in B&Q today (for my sins), and I saw an insulation material consisting of a foil layer, about 10mm of some sort of hemp (looked a bit like cheap carpet) and then another foil. Insert joke about what they did with the rest of the hemp here.

Amazingly, according to the manufacturers, this is equivalent to 200mm of mineral wool. It doesn't even have any of the magic inner layers!

I did not actually look at the price, sadly. However, I remain confident that it will have been ludicrously high.

If you want a reflective layer, why not use agricultural mylar? I'm sure this is cheap, particularly in whole-house quantities.
It certainly is in the US; I can't find a UK online supplier but farmers will know of one.

Wrap two layers with something (or nothing) in the middle. Easy, surely?

Incidentally, you can then use the off-cuts to grow your own hemp, thereby making a double saving!

It's just a thought, but surely external foil layers (like the B&Q stuff) are most susceptible to conduction sucking away the heat, not only though the material itself, but also along the plane of the material (ie. in an area surrounding the contact point at which conduction occurs)? I'm assuming that the foil is conductive to heat? In which case, any point of contact with the outermost foil layers will be worse than it would be for a non-conductive insulant?

I'm not trying to make a point for or against multifoil, but the B&Q stuff did make me think that surely it would be more effective to produce something with the reflective/conductive layer sandwiched between the insulating stuff, so that point contact is with an insulant rather than a conductor of heat? Or is it just a manufacturing convenience to have the insulation held together by a sheet of something stronger?

Aluminium coated mylar for agricultural and domestic use is available from,
www.apollo-energy.com/welcome/
It has BBA Certification