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Community and Q&A

Central California remodel with concrete crawl space recommendations

caliberger | Posted in General Questions on

We are located next to the ocean with an average low of 40 in the winter and high of 75 in the summer
The average humidity is 70-80% throughout the year
The house was built in 1974 and has a vented crawl space with a concrete pad of unknown thickness which insects and rodents love to use as their home
I don’t know if a vapor barrier was used
We are planning to remodel (the footprint cannot change much) and would like to know the best approach to deal with this design feature in our particular climate
The goals in relative order of importance
1. Keep animals out
2. Use materials unfriendly to termites
3. Improve energy efficiency
4. Achieve good value for the money

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  1. Expert Member
    Dana Dorsett | | #1

    Got a ZIP code? (First three digits are enough, if you're too shy)

  2. caliberger | | #2


  3. GBA Editor
    Martin Holladay | | #3

    If you have insects and rodents, you need to investigate how these creatures are entering, and you need to seal up the entry points. In most cases, it's easier to keep critters out of a sealed crawlspace than a vented crawlspace.

    For information on sealed crawlspaces, see "Building an Unvented Crawl Space."

  4. Expert Member
    Dana Dorsett | | #4

    Subsoil temps in your area are north of 60F, and the dew point of the outdoor air is under 60F 90% of the time even during the most humid months. This is a very dry-air type of climate, which means moisture risks in a crawlspace (vented or unvented) are very low.

    Seal all cracks in the slab as well as the perimeter seam with the foundation wall with a purpose-made self-leveling polyurethane caulk formulated for concrete. If this is just a 1" rat-slab that's crumbling in places, dig out the broken concrete at least 2" and patch it, using a bonding agent in the mix.

    Wood boring insects rely upon gut flora protozoans to tolerate & digest cellulosic fibers (wood). A strip of copper flashing & or a copper-clad plastic sill gasket can provide a lot of deterance, since copper is toxic to that gut flora.

    IRC code minimum foundation wall insulation for zone 3C is R5 continuous insulation, but it's not insane to go with up to R10-R12 in a cool coastal climate. See Table 2 p.10 of this document:

    The simplest way to go may be to install 1.5-2" of fire rated foil-faced rigid polyiso (eg Dow Thermax) cap-screwed to the foundation, stopping at your copper (flashing), then carefully sculpt R15 rock wool batts to insulate the top of the foundation, foundation sill & band joist. Caulk the foundation sill to the top of the of foundation, and all framing seams that will be covered with the rock wool, and caulk the facer of the polyiso to the flashing. Stop the polyiso 2" above the slab as a permanent inspection strip for termites, and to keep the polyiso from potentially wicking ground moisture from the concrete. Tape the seams of the polyiso with a decent quality aluminum foil tape.

    With only sculpted batts insulating the wood portions it will be easy to inspect and treat any affected areas if any termite tunnels or ant/bee infestations are discovered on either the interior or exterior. There is no need for any kind of vapor or air barrier on the fiber in your climate- rock wool batts are dense enough to prevent performance loss to convection through the batts, and have far fewer issues with friable fiber becoming airborne if left exposed.

  5. caliberger | | #5

    Thank you for the link Martin and Dana for the detailed response
    Can you confirm the following?
    1. Stop the Thermax two inches above the rat slab
    2. No insulation on the rat slab/floor of the crawl space

  6. GBA Editor
    Martin Holladay | | #6

    Yes, that approach will work.

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