[Greenbuilding] thermal bridging "interface term"

[Bruce Donelson]

Nick wrote -

> so a 1/16" air gap and mean temp Tm = 50 F and emittance
> E = 1 would make Hc = 2.75 and Hr = 0.91, ie R0.27, vs
> R5 for an inch of Styrofoam or R19 for 6" of fiberglass.

If we solve the above figures for a 1/16" distance then -
1/16" air gap = R0.27
1/16" Styrofoam = R0.3125
1/16" fiberglass = R0.1979

Rush wrote:
My shop has SIP walls that are 4.5" thick, made out of Urethane foam with a
R29 value (includes 15/32" OSB skins), so
1/16" SIP/Urethane = R0.4027

If we assume that the OSB has an R-value of about 1, then the 3.5" of foam
would have an R-value of 8. Seems unlikely. If I remeber correctly, urethane
foam has an R-value of about 6 per inch. Maybe the SIPs folks were selling
you an initial R-value. Urethane foam's performance degrades over time, so
generally it is calculated using an R-value that more closely approximates
its long-term perfomance. Or maybe they calculated an R-value including air
gaps, siding, sheetrock, and slow-moving air films near the surface. Not
that far off, though.

Bruce