# For Jay J - Moderator

Posted by Dodgeman on December 21st, 2000 01:13 PM
In reply to plexiglass by Art Bishop on December 20th, 2000 11:37 AM [Go to top of thread]

11 of 11 people found this post helpful

The kitchen tile is still not down, but here goes anyway:
I am strictly speaking of the properties of thermal conductivity, or rather,the resistance to thermal conductivity (R value) in my comments. Please note, for the purpose of this discussion, I am ignoring the other means of heat transfer, radiant and convection. I am also ignoring the boundary layer effects that all surfaces have.
So:
Glass has an R value of approximately 0.14 per inch thickness
Celluloid (Lexan, Plexiglass?) has an R value of approximately 0.7 per inch thickness
Air has an R value of approximately 6.0 per inch of "thickness"
So let's take a window with an 1/8 inch plexiglass pane on the outside, 1 inch air gap, and 1/8 inch glass pane on the inside. The total R value (again ignoring radiation, convection, and boundary layer effects) would be .09 + 6.0 + .02, for a total of 6.11. My point is the air gap does most of the insulating, and the R value difference between the plexiglass and the glass is insignificant, even though the plexiglass has 5 times the insulating value of glass.
As to the windows facing North or South, the warmer the air gap, the poorer the insulating value. The R = 6.0 for the 1 inch air gap above is at 32 deg f. At 200 deg f. the R value drops to 4.6 for the same 1 inch air gap.
By the way, Argon has an insulating value of approximately 9.0 per inch.
Reference: Marks' Handbook For Mechanical Engineers, 8th Edition