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What R-value is required on the stem wall of a crawl space?

Keith Messick | Posted in Building Code Questions on

I am located in Zone 3b, Orange County CA.

I am building a new house and will be having a sealed crawl space. I have read the many, extremely, helpful articles on this site. My house is under the CEC 2008 code (2010 CRC). I have not been able to determine the appropriate R value required to meet code for the crawl space walls. The CRC clearly says how a sealed crawl space is to be constructed. But it refers to the CEC for the value of insulation required. The IRC and the DOE are very clear and easy to understand. Is there a simply answer to what the R value on the crawl space walls should be (to meet the CEC)? (The stem walls are 8″ poured in place concrete).

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Replies

  1. D Dorsett | | #1

    See section 150 on p.87 (p.98 in PDF pagination) part c "Walls".

    http://www.documents.dgs.ca.gov/bsc/Title_24/documents/2010/Part%206/2010-CA-Energy.pdf

    "Framed foundation walls
    of heated basements or heated crawl spaces shall be
    insulated above the adjacent outside ground line with
    insulation having an installed thermal resistance of at
    least R-13.

    Alternative to Section 150(c)1: Insulation which is
    not penetrated by framing members may meet an
    R-value equivalent to installing R-13 insulation
    between wood-framing members and accounting for
    the thermal effects of framing members."

    It looks like you only have to insulate the crawlspace wall down to the grade level, not more. A framed 2x4 / R13 wall with a 25% framing fraction comes in at around R9 after factoring in the framing, not counting interior gypsum or exterior sheathing & siding. The framing fraction of a wall with no windows will be less than that, but it won't be over R10. You can install 1.5-2" polyiso on the stem wall stopping at grade level, or build a mini-studwall to hold R13 batts for the section that is above grade. With fire-rated polyiso its pretty cheap & easy do to it with foam and cap-screws or strapped in place with furring through-screwed to the stem wall.

    See also tables also 151B, 151C, and 151D beginning on page 99 (p110 of PDF pagination):

    Find your relevant CEC climate zone on this map:

    http://www.energy.ca.gov/maps/renewable/building_climate_zones.html

    or on this ZIP code list:

    http://www.energy.ca.gov/maps/renewable/BuildingClimateZonesByZIPCode.pdf

  2. Keith Messick | | #2

    Thank you for the info.
    From the 2010 CRC: I am going to employ R408.3.2.1

    Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of crawlspace floor area, including an air pathway to the common area (Such as a duct or transfer grille).Crawl space perimeter walls shall be insulated in accordance with the minimum insu-lation requirements established in the California Energy Code. Crawl space insulation shall be permanently fastened to the wall and extend downward from the floor to the finished grade level and then vertically and/or horizontally for at least an additional 24 inches (610 mm).

    The CEC and the CRC are not quite in sync with each other. I am not sure I would call my crawl space heated. Maybe lightly conditioned. Do you still feel the above R values apply?

  3. GBA Editor
    Martin Holladay | | #3

    Keith,
    I'm not familiar enough with the California Energy Code to help you. But here's one thing I do know about building codes: it rarely matters how the code is written. The only thing that matters is how the code is interpreted by your local code official.

    So the best way to get an answer to your question is to contact your local building department -- the ones who will be inspecting your house and interpreting the code.

  4. Keith Messick | | #4

    Will do. Thank you for all the extremely valuable info you provide on your site.

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