On Fri, 25 Jul 2008 20:55:16 +0100, a particular chimpanzee, Andy Hall randomly hit the keyboard and produced:
Equivalent to a 40mm sheet, yes. Which means that, when installed properly with a 25mm airgap either side, it is thicker than PIR insulation.
I like that one.
Well OK. For this project, (i.e. not a controlled development),
40-50mm Celotex would be fine and it isn't really necessary for there to be an air gap at the front. Plus I believe that the foam is less expensive. It is certainly easier to fit and seal.
A piece of cling film with a 25 mm air gap either side would be as good. Its the air supplying the insulation not the foil.
You haven't seen the insulating foils, have you?
They're not a single layer, they're a quilted blanket that contains _multiple_ airgaps. That's why they have better performance than a single layer of clingfilm.
Only if they trap a significant amount of air, the ones I have seen don't!
There is no way they acheive the claimed insulation figures.. it isn't possible.
If you want to argue then use two layers of cling film with a 20 mm gap and beat the foil with ease.
They trap an _insignificant_ amount of air, that's their virtue (Assuming that "significant" is taken to mean "larger" than "insignificant", for the purpose of our discussions here).
Because they only trap small individual volumes, the temperature differences across the volume is too low (relative to the space) to set up an efficient convection cell. This is why they're more efficient that clingfilm (and a large convective cell) and why they need multiple layers (to reduce temperature differences across each cell).
So there is no air to provide insulation then? I will give you a hint.. it isn't the mineral wool in mineral wool that insulates, it isn't the cellulose in cellulose fibre that insulates it isn't the plastic in polyurethane foam that insulates and as you say there is an insignificant amount of air in foil insulations.
You get a static layer of air about 6 mm deep on any surface. You don't need to create cells to make it work. Cling film will have about 6 mm of static insulating air on each side just like double glazing does. Foils aren't thick enough to have this 6 mm layer of air so they conduct more heat. You have a basic problem with your physics ATM.
Dennis, you're the guy who thinks that hot lightbulbs cause condensation.
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.
So one layer of clingfilm gives you one stagnant layer, multi-foils give you multi-layer stagnancy - with proportionately better insulation.
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.
No they don't! Are you being thick on purpose? How does a thin foil sandwich provide many stagnant layers that are more than 6 mm thick? As I said in the first place they don't work, they are too thin to work as claimed.
BTW how can you tell me air doesn't insulate in one paragraph and then tell me it does in the next?
The whole thing is only a few mm thick!
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They aren't (indivudally), the overall sandwich is.
There's no need for an insulating air layer to be 6mm thick. That's one value for a _maximum_ thickness of a stagnation layer in these circumstances. Layers thinner than this will still have nearly as much thermal resistance.
The overall foil sandwich is much thicker than this. It has several layers, there are several air pockets.
How thin do you think these foils are, when correctly installed?
They will be proportional.. a 1 mm layer will be 1/6 as good as a 6 mm layer. A really thin layer will be useless.
The number of air pockets appears to confuse you.
tri-iso super 10 is about 30 mm so it is about twice as good as double glazing, that is, not vey good.
If heat transfer was by conduction, then it would proportional.
However air is a fluid. Heat transfer is predominantly by convection, not conduction. It's easily mobile, yet very low density. Even at these dimensions, convection predominates.
We could agree that as a definition of "really" thin. However these airgaps aren't that thin (unless you squash the foil). A mm or two is still useful.
The whole point is that this 30mm is not spent on a single gap, but rather multiple independent airgaps. That's why they work better.
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