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Crawl Space Vapor Question

[email protected] | Posted in Energy Efficiency and Durability on

I’m currently rehabbing an older property and I have some questions about the crawl space that the place came with. I had to tear out all the existing floor joists because they rotted, likely due to batt insulation placed in the cavities.

I have the new floor almost finished and have put 10mil poly on the dirt floor beneath it. I would like to insulate the new joists to R30, but I’m worried that there will still be too much water vapor and it may condense on the bottom of the joists. The foundation is made of soft brick and is quite porous – I fixed the bulk water issues the house originally had, but I’m still cautious that water is entering the building from the brick.

I can vent the foundation to the outside if need be. I plan on using mineral wool insulation. One potential fix I thought of was to insulate and cover the bottom of the assembly completely with spray foam. I’m open to other thoughts. The property is located in climate zone 6A.

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  1. mr_reference_Hugh | | #1


    I thought I would share this GBA article by Allison A. Bailes III, PhD.
    "Five Ways to Deal with Crawlspace Air"

    I am not a fan of spray foam or XPB but we need to take a balanced approach. If it is humid in the crawl space, that would logically mean that the floor joist and sub-floor would be more humid than normal - even if they are new. That could result in the spray foam peeling off the wood if they are too "wet" at the time of install, which is a known problem. Also, spray foam can be installed to be air tight but two issues can arise: 1. the spray foam can peal off the wood if it the wood was humid at install, 2. there are also known issues where the spray foam has small holes that go unnoticed at installation and later cause issues.

    I would think that installing a class 1 vapour retarder on the underside and sealing the under side from the crawl space would be better than spray foam. One would expect the floor to be able to dry to the inside. If you don't install a rigid foam under the floor joist, you would have the thermal bridging from the floor joists. Using rigid foam like XPS (bad for environment and bud less water vapor issues) could get you an air barrier and a vapour retarder as long as it is well sealed. XPS may require fire protection, depends on your code.

    The article I shared provides a few strategies to deal with humidity, which I think is your major concern. There are other articles to read if you want:

    1. [email protected] | | #2

      Thanks for the reply - that article has many of the thoughts that I've mulled over. I think you're right that I can achieve the same results with a vapor barrier on the bottom of the joists as I would spraying foam on them.

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