Friday, December 01, 2006

Role of internal foil layers in Multifoil

In summary the internal foil layers of a multifoil, defined as those between the inner and outer surfaces, are to a large extent redundant. This was demonstrated in an earlier experiment when 2 layers of foil either side of 12 layers of newspaper performed equally well in insulation terms as a typical multifoil. (see previous posting). Although it seems obvious to me why they have little of no effect, I attach a schematic diagram of what I think happens, without recourse to complex heat transfer calculations.

Aluminium foil is an excellent reflector of radiation with upwards of 90% efficiency in reflectivity often quoted. For this example I choose a conservative figure of 90% to illustrate my point, higher figures will make the argument even stronger.

So, assuming the foil reflects at least 90% of incident radiative energy then the remaining 10% of the energy will be absorbed by the 1st foil layer, 1% by the 2nd layer, 0.1% by the 3rd and so on(assuming all energy is transferred by radiation). With this argument alone is is clear that the second and subsequent layers have only a very small role to play. However, any radiation which is adsorbed by the first layer can be dissipated by any of the 3 heat transfer processes (convection, conduction and radiation). Now aluminium is an excellent conductor and will rapidly conduct the heat energy throughout the layer and most importantly to any cold bridge - eg a hole, a joint or any conductive bridge to the exterior. This will lead to even less of radiation energy penetrating the first insulating layer.

Nevertheless lets assume that some radiative energy is transmitted by the first foil layer into the interior of the multifoil, this will be reflected by the next layer with the same effeciency as the previous one (90%) and because aluminium is a good reflector this radiation will continue to be reflected back and forth with the same efficiency until it is eventually adsorbed and conducted away.

Therefore I conclude that radiative heat transfer though the interior of the multifoil cannot occur to any significant degree and conduction and convection must be the main heat transfer processes through the bulk of the material.

Note: The internal foil layers however do have a negative effect on the overall heat transfer process, because they exacerbate heat loss via conduction to the nearest cold bridge. This probably counterbalances their only known positive effect, namely that of reducing convective heat transfer between two insulating layers. However this could be achieved much more effectively, and no dount cheaper, by using a non-conducting film such as plastic.