dbsg (01/06/83)
An R-value expressed in scientific units is hour * feet * feet * degrees F / BTU I found this out last weekend at our local library (thus answering my own query). It was remarkably hard to find (not the library, the answer!). Several people were kind enough to supply information in response to my query. - R = 1/U ( a U is unit of thermal conductivity rather than insulation) - R's are equivalent to inches of wood (this of course varies with the wood used, but according to the table I found it's remarkably close) - Bill Brown (cbosgd!cbosg!cwb) sent the unit equivalence, thus independently confirming what I had found. Thanks Bill! Thanks also for all of you that indicated interest in this. Chris Scussel ihuxq!dbsg (312)-979-7728
heliotis (01/07/83)
It seems strange that degrees FARENHEIT would be used, since this is not and "absolute zero" scale. Is it possibly a degree of the same magnitude, but where room temperature is above 400?
djb (01/08/83)
I assume that the FAHRENHEIT value in the equation is actually a temperature difference, in degrees fahrenheit, between the "inside" and "outside." As I remember, thermal difference is the driving force (analagous to voltage difference) in heat transfer problems. It's in fahrenheit in order to be consistent with BTUs (good old English units). David Bryant Bell Labs Columbus, OH (614) 860-4516 (cbosg!djb)
karn (01/08/83)
The Farenheit values used in specifying insulation effectiveness refers to temperature DIFFERENCES between the two sides of the insulation, so it doesn't really matter that Farenheit is not an absolute scale. It would have been better, of course, to derive a metric unit specifying the temperature gradient in degrees Celsius (or Kelvin, same thing) to cause a 1 calorie/hour/square meter heat flow. I'm sure the rest of the world already uses such a unit. Phil Karn