It muddies the water (or air) making it harder to rubbish the product.

I agree saint - if air gaps are important in any product for overall performance, then the calculations should include them.

I have always held the opinion that the tests conducted by the MF manufacturers show multifoil and mineral wool as having similar heat losses mostly because of the vastly improved airtightness a multifoil will achieve. The problem is with the way they convert their results to u-values, which is wholly wrong, as u-values do not consider air losses either.

One of the problems with any lab tesing [such as the hot plate] is that it gives no consideration to air tightness, hence, the better k-values published.

Presumably your testing does not allow for air tightness either? so it is not surprising to me that you have the results you report.

I have yet to hear anyone prove or disprove the argument for airgaps or the radiant heat transfer part of the eqation.

Not wishing to hyjack the thread but I have also conducted comparative simulation testing with Tas with results which reinforce the MF manufacturers claims. The trouble is that I cannot explain why the results are as they are -and the research has come to a grinding halt. I need someone with a greater understanding of the alogrithms and physics involved- anyone want to take a look at the results? I posted the results on the MF thread some time ago. To date no-one has conducted any thermal modelling which questions my methodology or the results obtained.

Posted By: biffvernonAnd nobody has been able to explain how the internal foils do anything other than boost the profits of the manufacturers.

Agreed

The no.1 point here:

Posted By: biffvernonYou are measuring the temperature gradient from a warm place to a cold place, but not measuring the rate of energy flow along that gradient

Correct. So far, this v interesting experiment says nothing, either comparatively or absolutely, about the rate of flow of heat in any of the variants. It does throw light on the temp gradient's distribution from layer to layer in the different variants, and it throws light on Biff's contention that the inner layers of foil do next to nothing:

Posted By: biffvernonSuggest you keep foils 1 and 6 and just replace the internal foils, 2-4, with paper. It's the added value of the internal foils that are the real issue. The outer ones probably are worth the money

because if the inner layers do next to nothing then the temp difference between any two of them wd be small to zero. On these results, that's not so - temp differences between all layers are of the same order, though irregular.

I'm all set to say more, but need some clarification, Tony.

Posted By: tonyPut my temperature probes in between the layers

but your diagrams seem to show the temps of the foils, not of the gaps between. Are you assuming that the temp between a foil and a wadding is same as the foil's temp? Not so. It wd be interesting to put sensors into every gap, not just alternate ones - then you'd see whether the foil or the wadding was creating most of the drop between adjacent layers.

In the first two cases, one outer foil seems to be free, at different temp to the cold source (room), but the other outer foil is at same temp as the hot source - in gapless contact with the hot plate? Or if gap between hot plate and foil, did you put a sensor between and assume that was the foil's temp? Can't be - both the hot-side outer foil and even more the gap between it and first wadding must be lower than the hot plate. The effect of this experimental error is to diminish the %age of the total temp drop that's apparently contributed by the inner layers - but despite this in all the variant cases the inner layers clearly do approx their full share pro rata.

In the third case - re-run of the second, it looks like replacing the wadding with newspaper has now not impared, but greatly improved the inner foil layers' ability to resist heat transfer, relative to the two outer layers'! Which one (second or third) to believe? And which one to take as comparison base to the fourth case?

In the fourth case, there's one too many readings, compared to previous, plus a contradiction - says 'hot plate 74' but first reading is 72.

Apart from that, great, very instructive. I'll say again, my suggestion is to put sensors into every gap of standard multifoil, not just alternate ones - then you'd see whether the foil or the wadding was creating most of the drop between adjacent layers. Seems to me you can't measure the temps of the layers, only of the gaps between *all* layers, plus room temp, plus hot plate temp but not the gap between hot plate and first foil - get clear about that and we'll get some useful results.

What, perpendicular to the direction of the temp gradient? Only fraction of millimeter thick? Made of plastic not aluminium?

"Made of plastic not aluminium? "

If it's plastic, sounds sensible Tom. I don't know what the shiny stuff is either but assumed it was metallic: Is that not so?

Even a fraction of a millimetre could conduct heat very efficiently.

I thought of using something like Celotex (in layers) for a rather specialist application I'm developing but the surface conductivity was too high (from the readings I got). Would be interesting to know what the 'multifoil' material is

The shiny stuff is alluminium. It's a thin layer on the plastic substrate, thin enough to be not quite light proof (oops, infra-red proof?). Yes it does affect phone signals. Even the tinted windows on trains block phone signals. That's a much thinner metalic layer yet they have to install antennae and transmitters within trains so your phone works. Tom, when you said "temp differences between all layers are of the same order, though irregular." he's just measuring the temperature gradient (with a pretty rough measuring technique) and one shouldn't expect a great deal of difference whatever the layers are made of. And yes, djh, I couldn't be arsed to argue whether its lambda or lambda/2 or lambda/4 or whatever either :)

Posted By: biffvernonThe shiny stuff is alluminium

Well, blow me. What's mylar then - surely not metallic? If metallic, does multifoil in fact block mobile signals? This wd have significant health implications, prob for both good and bad.

Posted By: biffvernonThe shiny stuff is alluminium.

How do you know Biff?

Posted By: fostertomWell, blow me. What's mylar then - surely not metallic? If metallic, does multifoil in fact block mobile signals? This wd have significant health implications, prob for both good and bad.

It would have no health implications as there is no demonstrable link between electromagnetic radiation and health (psychosomatic effects notwithstanding and obviously not including such radiation as X-rays, microwave in the oven frequency etc.). Mylar is the trade name for a polyester film. It's a good substrate for applying a metal coating (like in those survival blankets you see).

See http://en.wikipedia.org/wiki/PET_film_(biaxially_oriented)

Paul in Montreal.

Paul, some of the foils over here are definitaly a polythene type material. You can rub the foil between finger and thumb and the coating will come off leaving the transparent [tough] plasticy film. I have been told that whether the coating comes off or not seems to be a reasonable measure of quality.

Should say that some I have tried do not lose their coating at all so may be a different material altogether.

I have tried to find out the exact composion of multifoils several times- manufacturers are not forthcoming.

Interesting jon - on a micro scale looks like the foil (if as conductive as it wd be if metallic) wd act as a ready lateral redistributor of heat. Cd be just the thing to amplify any turbulence within the nett heat flow, exagerating small transient inequalities in any tendency towards steady-state - which by my theory would be re-normalised predominantly by radiant rather than convective/conductive activity.

However on a macro scale I don't see why heat would want to flow laterally, in the plane of the foil, when the temp gradient is perp to it.

We have celotex in our walls and roof. The foil does a really good job of blocking our mobile phone signal. Outdoors we have 5 bars, indoors 1 if we are lucky. I thought about getting a repeater but they are illegal.

Since you guys seem to have paid more attention in physicis than I. So if one side of this foil or mylar were black it would absorb direct sun light and get very hot, right (Q1)? Then because it is thin and metal, or metalised, it would conduct heat really well, right (Q2)? So if it was touching something, that something would get hot too, right (Q3)? but if that 'something' were massive it wouldn't get much more than warm slowly (Q4) and then because the foil was reflective on one side that massive thing wouldn't be able to radiate (infra-red) that heat when the foil became cold, right (Q5)? So the foil is acting like some one way energy gate, changing light energy to heat, 'convecting it' across the film but then because the Dt is less the other way when the sun goes down, convection is less back the other way and infra-red negliable - bound to be wrong, right (Q4)?

I know, I know, this is my Trombe wall question, not very well disguised, but above you are jumping all over my issue without quite stepping on it - and I could really use your opinions.