Reflective foil (roof) insulation

[david2]

Does anyone have any experience of using reflective foil insulation in attics. I am trying to find something to insulate my 17 Century attic. Preferably insulating between the rafters to expose them. I do not have much space to do this as I need to leave some air flow to prevent condensation. I have been told that this foil stuff might do the trick.

Thanks

 

[biffvernon]

This is stuff steeped in controversy. Multifoils do not seem to perform as well as the adverts would imply when tested in a laboratory hot box experiment, but the makers hold that such tests do not adequately reflect real conditions in a building. The amount of air leakage provided by carefully installed multifoils may lead to performance gains over some traditional materials which are sometimes installed in such a way that the insulation is compromised by excessive air movement.

Earlier this year I conducted a little kitchen experiment described Here. I reported the results on the Green Building Forum and that led to a very long thread called Multifoil Insulation, which got quite heated at times and to which directors of the leading industrial protagonists contributed. There have been 146 posts to date, so make sure you're sitting comfortably.

There is at present significant doubt as to whether the level of certification that multifoils have obtained is sufficient to allow Buildings Control Officers to accept them in Part L calculations.

Certainly, when space in an old building is limited, multifoils are better than nothing. Whether they work as well as the manufacturers claim is open to doubt and is strongly disputed by manufacturers of other insulation products.

 

[MattJ]

I used a multifoil in my attic conversion. The rafters are relatively modern (1905 according to the carpenters signature) and 75mm deep, while the purlins are older, reused from other buildings in some cases and more interesting (i.e. lots of tree bark still on them). Using a conventional insulation would have meant covering up both the rafters and the purlins, whereas with the multifoil at least an inch of each purlin is exposed.

Do they work? Well the room was warm through the winter, doesn't appear to run the radiators much more than other rooms, but that proves nothing.

The foil has to be 50mm from the roof to allow air circulation, so you'd need deep rafters if you want to only have the foil between the rafters and you'd probably have to beef up the insulation to make up for the cold bridging by having the rafters exposed.

MattJ

 

[Me!]

to make up for the cold bridging by having the rafters exposed.

I'd always assumed that dry timber must be quite a good thermal insulator, can anyone quantify how good it is (in non scientific terms suitable for the hard of thinking please!) for example inch for inch compared with rockwool?
I've been to northern Finland in the past, (well within the arctic circle) in the middle of winter with daytime temperatures not going above -20C and been very warm in a log cabin that was built with no extra insulation in the walls other than the wood itself. They were a bit thicker than the average rafter though to be honest!

 

[biffvernon]

Thermal conductivity of wood is around three or four times as great (i.e.worse) as mineral fibre insulation.

The Finnish hut, as you say, has thick walls and when burried in snow will perform even better as dry snow has a lower thermal conductivity than many insulation materials. In'The Ecology of Building Materials'by the Norwegiann Bjorn Berge, there is a description of the six ways to retain snow on a building.

 

[NT]

Foil as inuslation strikes me as one of those cases of people being willing to believe what doesnt make much sense due to the pedestal effect.


NT

 

[biffvernon]

Copied from Green Building Forum

C'mon guys, the multifoil story's not over, in fact it holds stupendous potential. There's a mega battle of vested interests going on right now and frankly, leading correspondents on this thread seem to be so keen to hang onto their conventional understanding.... If any of them want to talk to a real techie insider on the CMM (Confed of Multifoil Manufs) side, then I can pass on a name and number. Already, as a result of CMM's representations, the insulation industry's comfortable technical assumptions are in tatters. As far as I can see, the result will be a radical fresh approach to energy calculations, that involves lifecycle, local microclimate, heat gain as well as loss, workmanship etc. - all stuff that wasn't practicable when Uvalue calcs were done manually but is perfectly feasible now that it's all computerised under the Building Regs. We all know that as-built insulation rarely comes close to the calculated performance, for the reasons I've mentioned above. Well, if the CMM has its way realistic factors for all of them will become legal and we'll all benefit from some energy truth. Meanwhile, we could maybe find a bit of enthusiasm for some really exciting and radical technical stuff that's struggling to be born, and has already transformed and liberated my way of designing and building and has given me, so far, one very happy customer, with two more projects under construction now.

 

[Mark]

So, would it be fair to say that we should think in terms of thermal barriers rather than insulation?

The point being that different products act in different ways depending on how they interact with heat flow in the real world in a building.

I can see that the foil would be good against radiation - keeping attics cool in summer. But Im less convinced about them keeping heat in the property in winter. Metals conduct heat dont they?

I started to read the loooooooong thread but theres too much vested interest and not enough clarity. (where are the certified results which might be real world applicable)

Confused (with a v.hot attic)

 

[NT]

quote Mark

>I can see that the foil would be good against radiation - keeping attics cool in summer.

you appear to be implying that radiation is how summer heat gets into the loft. I dont think it is. It is how the tiles get hot, but the tiles dont get anywhere near the temperature of the sun, (not on my house anyway) so the means of heat flow from the tiles is not the same.


> But Im less convinced about them keeping heat in the property in winter. Metals conduct heat dont they?

aye. Air trapped between metals doesnt, just as air trapped between glass fibres or polystyrene doesnt.


NT

 

[thiambal]

From what I can make out your best bet is to fill your attic with dry snow. Then all you need is a high powered refrigeration system to keep the snow from melting and your house should be as warm as toast! :lol:

 

[Paul Hopper]

Thiambal


welcome to hell (sorry PPUK) if you read posts by such as Middi, Penners and several others I think you will find there area many here with a similar sense of humour to you.You will finish your house.. promise,, but when? ah-ha.. that is the question.

Good luck

Paul

 

[biffvernon]

This Document from the BRE may be instructive.

 

[Penners]

A builder told me the other day that he's heard that the Building Regs authorities are going to withdraw their approval of multifoil insulation. I can't vouch for the veracity of this, but the builder in question is a reliable chap.

 

[Johneds]

Obviously different local authorities (and different inspectors come to that) have different views on this stuff, but my particularly authority has not allowed it for about 6 months now.

The big problem as I see it is that there is no way to make calculations for this material that compare with conventional products. Until these figures are available I can't see how you can input the values into approved carbon index software - and therefore there is no way (and I mean no way) to show complience with current building regulations.

The material has also become very popular with the green lobby and I'm sure it gets used (due to its must have status) in situations where conventional materials would be very much more efficient.

One thing I can tell you is that it is very, very popular with roofers. The reason is simple, it is much quicker and less hassle to fix.

 

[Gervase]

It's also ridiculously popular with architects, to the extent that in these parts it's almost universally specified on plans. Makes me wonder if there isn't some sort of scam going on to match what happens with pharmaceutical companies and GPs. Or maybe I'm just a cynic and this stuff really is the dog's danglers...