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Insulation under floor joists in a crawl space

pickaname | Posted in Energy Efficiency and Durability on

I have recently installed a new tile floor with electric radiant floor heating over a crawlspace and am looking for the most economical way to insulate the floor. If a radiant barrier blocks 97% of the radiant heat is it necessary to install foil faced EPS on the bottom of the joists for more insulation at three times the cost?

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  1. GBA Editor
    Martin Holladay | | #1

    What is your geographic location?

    The best way to insulate a crawl space is (a) to seal any vents, and (b) to insulate the crawl space walls. Here is a link to an article that explains what you need to know: Building an Unvented Crawl Space.

    While it's true that a radiant barrier can block most of the radiant heat transfer through a building assembly, you also have to worry about heat transfer by conduction and convection. In the application you are talking about, you need actual insulation -- a material with an R-value -- to slow down the heat transfer.

    A continuous layer of rigid foam insulation on the underside of your floor joists would be better than a radiant barrier in the application you are talking about.

    Depending on your climate zone, you may want to install insulation in the floor assembly (because you have radiant heat in the floor) as well as insulation on the crawl space walls.

  2. Expert Member
    Dana Dorsett | | #2

    The much reviled R19 fiberglass batt is your friend in this application. Whether the heating elements are above the subfloor or under the subfloor, cheap low-density fiber insulation snugged up to the subfloor (or heating elements) is both effect and cost-effective. While foil-faced R19s has a (very) slight performance advantage over unfaced or kraft faced, there is no point to using IR reflective materials in these low-temp applications.

    If it's a vented crawlspace, in most cooler climate cases code would require filling the joist bays with fiber insulation, R19 would be the minimum. (Even without the radiant floor.) Even if it's an insulated unvented crawlspace R19-R30 would still be financially rational when electricity is the energy source, depending on your electricity rates.

    See the floor insulation column in the IRC 2015 prescriptives:

  3. pickaname | | #3

    Thank you. I was not surprised with your answers, but hoping the less expensive radiant barrier on the bottom of the joists would be enough. By the way, our location is in the Colorado Mountains at elevation 7500 feet.

  4. Expert Member
    Dana Dorsett | | #4

    Got a ZIP code? (For climate data purposes.)

    How deep are the floor joists?

    With rock wool you can hit the R30 code-min with 2 x 8 joists, but the higher thermal bridging through only 7.25" of joist delivers a performance hit. If 2x8s are what you have, adding 1.5" wide strips of 1.5" rigid polyiso + 2x3 furring edge strips to the bottom of the joist edges and going with R30 fiberglass batts or blown cellulose for the cavity would be better. The 1.5" polyiso cuts the thermal bridging pf 2x8s by fully half. See:

    R30 fiberglass batts (or better yet, blown cellulose) in 2x10 joist framing is higher performance due to the marginally reduced thermal bridging.

    If you have 2 x12 joists, R38s or a full fill of cellulose is still going to be "worth it" for an electrically heated floor.

    Most 7500' locations in CO are in US climate zone 7, or the coldest edge of zone 6, but code-min for floors over a vented &/or uninsulated crawlspace is still only R30. But with heated floors more is better.

    An unvented sealed crawlspace that is insulated to at least R15 continuous on the exterior walls could do just fine with only R11-R13 batts under the radiant floor, and may be easier & better overall.

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