
RValue Posted by The Insulator on March 2nd, 2001 05:37 PM In reply to Double Insulation? by Joe on March 2nd, 2001 10:05 AM [Go to top of thread]
RValue is a measurement ot resistance to conductive heat transfer / loss. It is the mathematical inverse of UValue, the measurement of actual conductive heat transfer, or loss (If you take 1 divided by the RValue, you get the UValue). The Uvalue shows the proportion of heat allowed to conduct through. When we consider the UValue, we get some rather interesting information (Well, interesting to an energy efficiency junky like myself). For example, take the 2 Rvalues discussed, R30 & R50. R30's Uvalue is U.033. R50's Uvalue is U.020. This means that R30 stops 96.6% of conductive heatloss & R50 stops 98% of conductive heatloss. A switch from R30 to R50 gives a net gain of only 1.3%. Unless you live in an extreme climate, it probably would not be cost effective to add the difference, unless you have an accessable attic & can blow more insulation on top (retro blowing in an open attic is realatively inexpensive. Retro adding a layer of foam & redoing the ceiling is not.). Also, if you have an R50 attic & double the insulation, the 2nd R50 also stops 98% of the heatloss, but only 98% of the remaining 2%. You will notice that the focus of Rvalue is on conductive heat transfer (the main heat transfer mechanisms are conduction, convection, radiation & evaporation/condensation). For many, this is the primary focus of energy efforts. However, conductive heat transfer is not the most important in homes. The most important is convection, the heat tranfered by air movement (you walk into a cold room. you notice 2 things, the walls are not insulated & the window is open. What do you solve first? Shut the window!). In fact, to accurately measure Rvalue, or conductive heat transfer, convection must be elliminated. Why do I bring this up? Simply put, If you have 2 apparently identical houses with radically different energy costs, the reason is not minor fifferences in Rvalues. Rather, the reason is one house is tighter than the other, thereby controlling convection better. The Insulator
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