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Effectiveness of sheep’s wool as a radiant barrier

Woodland Dweller | Posted in Energy Efficiency and Durability on

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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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Woodland,
    I haven't ever seen any data on the emissivity of sheep's wool. But everything I know about fibrous insulation and emissivity leads me to believe that sheep's wool is not a radiant barrier or a low-e material.

  2. Expert Member
    Dana Dorsett | | #2

    The emissivity of hair & wool proteins in are fairly similar across species, about 0.80-0.95, varying somewhat with temperature. ( https://journals.uair.arizona.edu/index.php/jrm/article/view/6370/5980 )I can't imagine that ovine wool is dramatically different in this respect to llama, human or polar-bear.

    If you read the referenced article, the effecacy of penguin feathering RB effect is highly dependent upon the regular spacing between feather layers, despite a relatively high emissivity (about 0.80, according to the article) making it something of a special case. If you were able to reproduce this spacing with other materials you could achieve a similar effect. But the spacing and air-retardency of sheep's wool has a very difference character, bearing no structural similarity to feathering. All fiber insulation relies at least partially on the fiber emissiviy being less than 1, and the spacing between fibers. With the penquin feathers this is enhanced structurally by having less fiber-to-fiber conductive pathway than a more randomized structure.

    Polar bear fur has some unusual characteristics that make it less emissive in the infra-red, which helps it's overall thermal performance a bit:

    http://infrared.als.lbl.gov/pubs/PolarBearASME.pdf

    Even if you had a super low-E fiber, you still need the macroscopic air gap and a delta-T for it to have useful insulative properties. In contact with the ceiling the conductivity would swamp any low-E benefit. (An uncomfortable but effective demonstration of this would be to grab a bright aluminum pan after heating it on the stove.)

  3. Bill Dietze | | #3

    Dana you beat me to it!

    I found a value of 0.78 here: http://www.dtic.mil/dtic/tr/fulltext/u2/665647.pdf See Table 1.

    Bill

  4. Woodland Dweller | | #4

    Thanks for the replies folks! All very helpful.

    Now I don't see an alternative to aluminum foils or paints for a radiant barrier...

  5. GBA Editor
    Martin Holladay | | #5

    Woodland,
    No one has yet invented radiant barrier paint. While low-e paints exist (for example, white paint), no paint meets the technical definition of a radiant barrier.

  6. Expert Member
    Dana Dorsett | | #6

    Aluminum paint comes pretty close to radiant barrier definition, but since it's just a single low-E surface the performance isn't as good as multiple layers that are low-E on both sides, with at least 3/4" of air space between any low-E surface and the next layer in the stackup. Some of the hucksterism around nano-sphere paint additives seems well over the line into fraud territory, to be filed with the FTC.

    Rare is the application where radiant barrier (or even radiant barrier paint) is more effective (read "cost effective" than fiber insulation of reasonable density & air retardency. In buildings that have no insulation it's way better than nothing, but in most applications spending the r.b. money on more fiber or foam insulation is the better bang/buck.

    In the middle of Antarctica where fiber insulation in a human-occupied building is all but guaranteed to ice up and lose effectiveness over the long dark winter the complexity & expense of a multi-layered air tight foil or aluminized mylar r.b. approach can sometimes make sense, but most of us live in friendlier climates than that. (EPS works pretty well in those temperature extremes too, if you design it with enough mechanical tolerance for the thermal expansion/contraction it'll undergo.)

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