EcoRenovator - View Single Post - AC Insulates Under Floor of 120 Year Old Crawlspace

Snail · 09-27-14, 11:18 PM

Originally Posted by AC_Hacker

I'm pretty sure that you need an air gap between the radiating surface and the reflective material. I've seen recommendations of 1/2 inch to 1 inch. But the wavelength of IR is very short (< 1 mm), so I think that the gap can be small.

-AC

You certainly do need an air gap. However, although my physics is very rusty, I can’t for the life of me think what the IR wavelength has to do with anything! (Around 10microns at these temperatures). I’m sure that I would have remembered something if it did.

For double glazing ½ to 1 inch is recommended as an air gap. This is an optimal compromise between the benefit of minimising conduction through the air, for which thicker is better, and damping down convention currents, which requires a thin air gap. However, under the floor is a completely different situation. Since the air will be hottest at the top of the air gap, convection will be supressed. Furthermore, the top of the joists and the underside of the floor will be fairly uniform in temperature, whilst the foil and the joist where the foil meets it will also be at a uniform temperature. Where then is the driving temperature difference for convection cells to develop?

My guess is that the bigger the air gap the more “free” insulation you will get.

PS How do you quote properly on this blog? Is there a page anywhere with this type of info?

09-27-14, 11:18 PM	#43
Snail Lurking Renovator Join Date: Jul 2011 Location: Palmerston North Posts: 20 Thanks: 3 Thanked 9 Times in 7 Posts	Air Gap over foil Originally Posted by AC_Hacker I'm pretty sure that you need an air gap between the radiating surface and the reflective material. I've seen recommendations of 1/2 inch to 1 inch. But the wavelength of IR is very short (< 1 mm), so I think that the gap can be small. -AC You certainly do need an air gap. However, although my physics is very rusty, I can’t for the life of me think what the IR wavelength has to do with anything! (Around 10microns at these temperatures). I’m sure that I would have remembered something if it did. For double glazing ½ to 1 inch is recommended as an air gap. This is an optimal compromise between the benefit of minimising conduction through the air, for which thicker is better, and damping down convention currents, which requires a thin air gap. However, under the floor is a completely different situation. Since the air will be hottest at the top of the air gap, convection will be supressed. Furthermore, the top of the joists and the underside of the floor will be fairly uniform in temperature, whilst the foil and the joist where the foil meets it will also be at a uniform temperature. Where then is the driving temperature difference for convection cells to develop? My guess is that the bigger the air gap the more “free” insulation you will get. PS How do you quote properly on this blog? Is there a page anywhere with this type of info?