Posted By: GotanewlifeSince 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.

Trouble is its difficult to give a worthwhile opinion on such a dynamic situation [trombe wall]. I think it is something which would need to be modelled using dynamic software to get any meaningful idea. Even then I think the results would be unique to a given situation/location.

Or where the foil is compressed at intersections of internal/external battens?

OK, thanks, I am very grateful....and I'll stop going on about it now....but at least you didn't say "no way"! I really don't see myself as an experimenter, but next autumn I'm going to do something about this and if it doesn't work at least my wife has agreed to paint the walls green, we'll debate the exact shade later . Nearer the time I'll ask for some advice on thermometers, placement, monitoring etc - may as well do a proper job on it. Cheers.

But as said above, overall temp diff is fixed so this doesn't tell us much/anything.

Need either to measure energy transfer or allow temp gradient to vary (measuring kWh of hot plate, melting ice or introducing known insulation like polystyrene tile).

Newspaper -- dont know why one thermocouple trace is odd

The passage of heat through the two systems looks identical to me.. Should we patent multilayer newspaper?

Could you try it with the mylar foil also replaced by newspaper?

To answer the question earlier in the thread about why a large gap either side of a multi foil is recommended even tho only one IR wavelength is required for it to work as a radiant barrier:
If you have the rad barrier with a small gap behind, say, plasterboard, then the plasterboard would feel the benefit of the IR bounced back from the foil, so it would keep warm. However, with a small air gap, that warmth would then be conducted through the small air gap back to the foil which would then conduct it back out the cold side. In other words the small gap set up would just convert radiant heat into conductive heat, which foil has no resistance to. Foil will only work if the radiant temperature is higher than the air temperature. As this is only ever likely to be a small number of degrees unless blocking out sunlight, any foil will probably be worthless as an insulator. The exception may be suspended flooring where the ratio of radiant to convective heat loss is greater. The ideal use of radiant barrier is if you have a foil with a block of a material that is transparent to IR but low conductivity, so the re-radiated heat would only warm the plaster board and not the material it is in contact with, while maintaining a differential between ambient and radiant temps. So what material is transparent to IR? Newspaper? Aerogel? polyester wadding?

This is also relevant to why Biff got a gradual(ish) decline in temperature across the insulation - the main mode of transfer was conductive not radiant so would behave as any other insulation.

Your are right, except that the recommended air gap for mf installation is the highest insulation value an air gap can have. Although thermal resistance per unit distance is higher at small distances, total resistance peaks around 2 cm. Convection effects are small below 2cm but will dominate above 2cm (or 2.5 or whatever, cant remember exact distance, and it varies depending on whether horizontal or vertical), and conduction will dominate below this number. As such a gap that is too small or too large will reduce the radiant insulation effects of a foil by transfering heat back to the foil non-radiantly. Air is pretty good at being IR transparent and not too bad at being low conductivity. Of course nothing like vacuum as you say. It would be possible to increase the conductive resistance with multiple layers of thin transparent plastic separated with air to create a good conduction barrier with a single metallised layer at the cold side for radiant resistance. A sort of pikey vac panel. Without the vac. It's thesis time for me so maybe i could test this idea.

Don't need to be transparent.

Multiple newspaper layers or even optical black layers, without wadding, not touching, 'any size' air gaps between, serially divide the delta-t temp difference between adjacent layers, hence the transfer rate of the radiant component from layer to layer to layer, compared with a single layer of same. That is transfer rate reduction purely by subdivision of the total temp gradient.

In addition, if you change the layers' material, from optical black to newspaper to optical mirror, then the actual radiant transfer that results from that delta-t, reduces sharply, by a ratio of 0.98 (optical black) to 0.5-ish (newspaper or typical building materials incl 'ordinary' insulations) or 0.98 to 0.05 (optical mirror or silver foil).

(For purpose of clarification, the above is only about the radiant component and leaves aside the conductive and convective components, which of course in reality apply also).

Wadding is optional, has (as we all agree) a contemptibly negligible benefit as a 'conventional' insulator, is there just to keep the foil layers apart (and to obstruct free convection). The wadding functions as an air-gap; on a micro scale the wadding is barely 'in contact' with the foil, except by tiny point contacts; almost entirely the foils 'see' the empty airspace between the wadding fibres.

Thinsulex in addition plays the absorbency trick (like Platinum EPS and aerogel); the wadding is carbon-blacked so as to absorb all 3 modes of heat transfer out of the airspaces and into the body of the wadding, where it gets baffled by very long conductive path-lengths.

To understand insulations, you have to imagine it at the micro-scale, where an interplay of conductive, convective and radiant transfers occur. This is complex enough under un-natural steady-state conditions, but almost unchartable under dynamically-varying temp conditions. It's a ferment that never settles down to equilibrium, although on the crude macro-scale of the hotbox test a rough approximation to equilibrium may appear.

The very idea of classing insulants' performance as 'conductivity' is ridiculous. Conductivity happens in homogeneous media, like copper, stone, water or air. When those media are interspersed with void spaces, convection and radiation play a large role; in the case of insulants the whole point is to minimise the conductive component in favour of convective and radiant. So to suggest that the composite nett effect of such an insulant will follow the laws of a pure conductor, is a fundamental flaw in the conventional 'science' of insulants.

Thermal transfer is just as chaos-ruled as weather systems, in fact just about all systems in now seems. 60yr old building industry 'science' ways of viewing thermal transfer are well outmoded, and misleading in cases now emerging, like multifoils, aerogels and other reflectivity- and absorbtivity-based insulants.

Keep thinking - and visualising

oops. attachment too big. If anyone wants a look then give me a whisper.

tony - yes i did mean transparent, but importantly transparent to far IR.

fostertom - still digesting your comments