Effectiveness of sheep’s wool as a radiant barrier
In short, my question is: what is the emissivity (or reflectance) of sheep’s wool insulation?
This article gives an R-value of 3.5 to 3.8 per inch for wool: http://en.wikipedia.org/wiki/Wool_insulation
This article gives an R-value of 3.7 per inch for cotton: http://en.wikipedia.org/wiki/Building_insulation_materials
Thus, the R-value (measure of thermal conduction) is about the same for wool and cotton. But my experience with blankets and clothing is that wool is much more effective than cotton at reducing heat loss (for the same thickness/weave and wind/convection conditions). Since conduction and convection are ruled out, this leaves radiation transfer as the main difference between wool and cotton in heat transfer.
Another thing that makes me suspect that wool is a good radiant barrier is that scientific studies into other natural animal insulating materials, such as penguin feathers, turn out to be excellent radiant barriers. See http://biomimetic.pbworks.com/f/Heat%2BTransfer%2Bthrough%2BPenguin%2BFeathersDawson.pdf
I have searched a lot of emissivity tables on the web, but I have been unable to find any data on sheep’s wool emissivity. All the info I found on radiant barriers is for aluminum ones, and I have several reasons for avoiding them. It would be so simple if the wool insulation would serve double duty as a radiant barrier also (in both directions). Therefore, does anybody have any links for scientific studies or tests for the emissivity (or reflectance) of sheep’s wool insulation? Best would be specifically for ceiling insulation, considering both summer heat gain and winter heat loss.
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