Will adding rigid foam insulation to the inside and outside of my d-log house cause mold?
D-Log Renovation
| Posted in Energy Efficiency and Durability on
I am planning to add rigid foam insulation to the outside of my old leaky d-log house and adding douglas fir slab siding with chink. My electrician would like for us to fir out the inside to avoid having to router out chases for new wiring. If I add foam insulation (open or cloased cell) to the inside will I risk mold on the logs? The house is located in the upper Arkansas valley in central Colorado, zone 6 I believe.
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Replies
Nope!
If anything putting continuous rigid foam on the exterior reduces the mold risk by raising the average temperature of the logs. (Warmer= drier.)
Judith,
The answer is, "It depends." If you install a very thin layer of exterior rigid foam, and your wall is leaky, it's conceivable that warm interior air might contact the inside face of your exterior rigid foam, and that the moisture in the air could condense.
The thicker the layer of exterior rigid foam, and the more airtight your interior air layer, the safer the wall assembly will be. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Thanks, very helpful answers.
One more question, is drywall an air tight layer in this application?
The vapor permeance of solid logs is quite low- about the same as closed cell foam of equal thickness. The vapor permeance of the chinking is high, but the cross-sectional area of the chinking also quite low. Vapor diffusion & air leakage through the chinking will dominate the moisture migration from the interior into the space between the logs and foam.
Drywall can be air tight or not, it depends on how you install it. The seasonal expansion & contractions of logs are big enough that you can't count on drywall to remain air tight over the long term. It's worthwhile to use a flexible material such as housewrap over either the interior or exterior of the logs detailed as an air barrier to keep it air-tight over the long term. If you are going to finish the interior with wallboard, putting a flexible smart vapor retarder such as Certainteed MemBrain or Intello Plus as the interior side air barrier is cheap insurance, to limit vapor diffusion from the conditioned space into the wall without limiting the drying rate very much, should moisture get in.