That's not the insulative properties of glass, that's the insulative properties of air. Air is a great insulator. That's why our best insulating materials (fibreglass, foam, cellulose, etc.) are designed to contain large air voids. Glass by itself is a relatively poor insulator. You could sandwich 10 feet of air between any substance and obtain similar performance, it's not very practical though and it's an incredible waste of space.
As far as multiple pain glass system go (typical residential windows), even the most efficient are only about R-5 or so. A typical 4" exterior wall is about R-15. Windows are where the vast majority of energy is lost in the winter time, followed by walls and then the roof (due to the roof having a large amount of insulation present (R-30 or more)).
As a Mechanical Engineer, I have performed many heating and cooling calculations.
The numbers if you would like:
Thermal conductivity of glass...1.1 W/(mK)
Thermal conductivity of air...0.025 W/(mK)
For reference, the thermal conductivity of wood ranges from about 0.04-0.4 W/(mK).
The higher the number, the more efficiently the substance can transfer thermal energy.
So glass conducts heat 44 times better than air.
Last edited by Urbanomicon; 08-06-2010 at 03:44 PM.
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