On 28 Jul, 15:05, "dennis@home" wrote:

So there is no air to provide insulation then?

Air doesn't insulate, it transfers the heat. It's a worse transfer
medium than typical solids, so in the sense of that comparison it's an
"insulator", but it's still worse than a vacuum. In particular,
circulating air is a fairly large means of heat transfer. So to apply
the idea of "a blanket of air is an insulator", then this must be a
blanket of _still_, stagnant air, not just any old thickness of
unconstrained air.

A thicker layer of wood or plasterboard is a better insulator. A
thicker layer of air, particularly on a small scale, is a _worse_
insulator.

If you have a thick layer of air with a heat differential across it,
you'll get a convection current starting. If you can make the layer of
air thinner, still at the same temperature gradient (i.e. slicing it
up with multi-foils) then convection becomes less efficient and the
overall insulation goes up. It goes up non-linearly, depending more on
layer count than thicknesses owing to these boundary layer effects,
until the cells are no more than a few mm thick.

You get a static layer of air about 6 mm deep on any surface.

So one layer of clingfilm gives you one stagnant layer, multi-foils
give you multi-layer stagnancy - with proportionately better
insulation.

Foils aren't thick enough to have this 6 mm layer of air so they conduct
more heat.

That's why it's important not to press on their surfaces, or to pull
them too taut. When correctly installed, they do contain multiple
cells, each of a few mm thickness, and stagnant.