Sawdust Insulation for a Wood Drying Kiln?

      Theoretical cautions on the possible drawbacks, but a real-life success story. April 29, 2011

Does anybody know about the R-value per inch of dry sawdust or wood chips? I would like to use it in the attic of a kiln.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Sawdust is not suggested, as it has a high fire risk, insect risk, rodent risk, compaction risk, decay risk and will lose insulation value when it gets wet (as a ceiling is virtually impossible to build with vapor leaks). The insulation value of fairly dry oak sawdust is about 0.5 Btu inch / hr / (ft x ft) / degree F. The value changes with MC and species, chip size, compaction, etc.

The R-value is about 1.4 per inch of thickness. A cubic foot of dry sawdust (R = 5) would weigh about 30 pounds, so the ceiling would have to be extra strong. Seems that a moisture vapor resistant insulation would be cheaper in the long run and would provide more insulation without the risks mentioned.

From contributor R:
An article I read says R 2.2 per inch dry pine sawdust. And there might be a flame retardant for sawdust a person could use. I might be wrong too.

From Professor Gene Wengert, forum technical advisor:
For pine, it seems that the value of 2.2 might be a bit high, but it is close. It does depend on the species, size of the particle and the compaction, plus other items, as mentioned.

From contributor A:
My whole kiln is insulated with sawdust. Most of it cedar and pine. The walls are 6 inches thick and there is about 8 inches on the ceiling. The inside walls are just pine boards with 30lb felt behind them. The ceiling is pine boards with .6 mil plastic. The kiln has been in use for 3 years with no problems. The building is balloon framed and sawdust is added in the attic and pushed to the walls to keep the thing filled.

Now some things I have learned. When green lumber is added, the walls absorb a lot of the moisture when the kiln is coming up to temp. Then it lets it back out as the wood dries. This seems to keep case hardening down to almost nil. With woods like hickory and maple it also dries down quicker to prevent sticker stain. The Nyle unit has worked just fine and other than one blower motor, I have not had any problems. It runs all the time with just long enough time to load/unload. There has to be something to the thermo mass of the sawdust, as when we opened up the kiln it was 125 degrees in there. We removed1,500 bdft of walnut and put in almost 2,000 bdft of oak and it took about 2 1/2 hours to do this. Day time temp was around 74 degrees and when I closed it up and turned on the unit, the temp in the chamber was 92 degrees and within 4 hours was up to 125 (that was when I went by to check). The chamber is 12' w x 20' l x 10' h and sits on 6 inches of concrete that is sitting on 2 inches of foam. I need to get around and paint the outside someday.

From the original questioner:
Thanks for the info. Talking to some researchers recently and it seems that the insulation value is around r 2.2. For the insect prevention borax or Portland cement could be mixed with the sawdust. Balloon wall structure is a good suggestion.

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