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Community and Q&A

Closed Crawl Spaces/For Ted Clifton

FrankFulton | Posted in General Questions on

I have read with interest Ted Clifton’s comments on closed crawl spaces, in which he advocates insulating the walls AND the floor (eg, comment #9 here: here). But, I’m unclear on teh recommended R value for floors.

What R value insulation should we install in the crawl space ceiling?

We are in CZ4 and have just closed our crawl space, including a 20mm polyethylene vapor barrier on cement floor and 2″ RMax (R13) on walls. We are considering following Ted’s suggested approach, for comfort in bedroom above (we have R3 stone walls on first floor of house). Code for floors here is R30, but the nominal cost for R38 is negligible. So all things being equal I would lean toward R38. R38C batts will fit in our 2×10 joists with minimal compression.

Thank you.

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  1. User avatar
    Michael Maines | | #1

    Emerson, are you in climate zone 4A or 4B? In other words, dry or humid? Ted Clifton's approach may work in a dry climate, but it goes against standard best practice when there are periods of high humidity. The safest approach is to fully condition the crawl space, including insulation on the floor of the crawlspace if possible. (It needs a fire-resistant covering of some sort and is not required by code, so it's often eliminated.)

    If you want to add insulation in the first floor joist bays, I can't think of any reason why R-30 would be better than R-38, if cost and space requirements are not an issue.

  2. FrankFulton | | #2

    We’re in CZ4 humid.
    The newly conditioned crawl is attached to a conditioned/finished basement via 2x3’ open grate. Weve insulated the crawspace walls, as I noted, but the floor is concrete and uninsulated. The crawl is notably cooler than the basement, and our bedroom above is also chilly. So, we were thinking of double insulating - eg, the crawl ceiling also, to keep heat in our bedroom. Note there is a heat pump hot water heater on the far side of the furnished basement. To out knowledge, no mositure issues in past many years, although we are new to the house. Thanks.

  3. User avatar
    Michael Maines | | #3

    Emerson, thanks for the additional information. I've detailed a couple of similar crawlspaces recently, using a Fantech FR 100 inline fan and off-the-shelf ducting to push conditioned air from the main level into the far reaches of the crawlspace, which can then return to the basement or living area via openings similar to yours.

    The IRC code has specific requirements for ventilating crawlspaces continuously, but the inspectors I've worked with have been ok with putting the fan on a timer so the equivalent amount of air is being circulated on a daily basis. The FR 100 is very efficient to operate, and since your crawlspace is part of your conditioned space, you are losing little if any net heat energy.

    You could add a similar fan and also insulate your bedroom floor for comfort reasons.

  4. FrankFulton | | #4

    Appreciate the counsel
    To be clear, there is currently “open air” circulating from the finished basement into the adjacent conditioned crawl.
    1. Do I still need the fan?
    2. If I hang fiberglass batts in the joist bays, may they be uncovered/exposed?

    BTW I pasted a photo in the ballon framing thread, if of interest. Where are you locate

  5. User avatar
    Michael Maines | | #5

    Emerson, there are many possible answers to your questions, but I'm happy to provide what has worked for me. I design a lot of additions and used to just leave an opening and assume the air would circulate. What I believe happens is that the relatively cool, damp air in the crawlspace is at a similar vapor pressure to the warmer, drier air in the basement or living space, so it doesn't circulate readily. By pushing warm air into the crawlspace it moves the air around and raises the temperature in the crawlspace. If the crawlspace were to cool off again it would be a bad thing, because the relative humidity could rise to mold-friendly levels, but as long as there is insulation and a regular supply of heat I think it works fine. I have also used exhaust-only fans, which meet code, but keep the space cool and waste energy.

    Fiberglass batts perform best when in an airtight cavity, so I might use kraft facing just to keep things neat-looking and to block some air movement. The kraft paper can face down; even in a vented crawlspace there isn't a lot of vapor movement into a floor system, and kraft paper is a variable-permeance membrane, so it doesn't really matter much whether you use it or not, or which way it faces.

    I live in Maine and mostly work here, but also do design work for other locations. My most recent project that involved a crawlspace was in Cleveland, and I am about to start another one in Virginia.

    I have to run (Eastover Fool's Day dinner) but will take a look at your balloon framing thread when I have a chance.

  6. Kevin Spellman | | #6

    Also an option in some areas is to close it up completely, not ventilate at all, and just use a dehumidifier. That's what I will be doing in my sealed crawl. I am insulating only the floor and setting a dehumidifier.

  7. User avatar
    Jon R | | #7

    Emerson, air sealing the bedroom floor will probably have much more effect on the floor temperature than insulation (between the two conditioned spaces). I'd try it first.

    I don't know just how cool the crawlspace is, but insulation might warm the floor by 2F. R38 vs R30 is probably an insignificant .1F difference.

    Ted C's comments were about using foam insulation and moisture issues - not cold season comfort or batts.

    Edit: noticed you have R3 walls. So you have a significant problem with cold air sliding down the walls and accumulating at the floor. Consider radiant floor heat for the bedroom.

  8. FrankFulton | | #8

    Kevin Spellman, Thanks for the article. What is your dehumidifier setup?

  9. FrankFulton | | #9

    Jon R,
    1. How would you air seal wooden floors? Note I did air seal behind the quarter round this weekend - took well over 2 tubes of caulk to seal a 3/8" gap along 30 or so linear feet! and the space is much less drafty.
    2. You can read about our wall setup at this link here.. How does that background impact your thinking?
    Thanks v much.

  10. User avatar
    Jon R | | #10

    Lots of options, but I'd probably attach foil faced polyiso under the floor joists and tape it with 3M 8067. Provides air sealing and insulation in one step.

    If you slightly overheat (vs underheat) your crawlspace using whatever heating system you have, you will have a radiant and convection heated floor without a need for floor insulation or air sealing.

  11. Kevin Spellman | | #11

    Emerson--I was told you can use the expensive Santa Fe type models, but even the cheap stuff you can buy at Home Depot works as long as you monitor humidity in your crawl.

  12. FrankFulton | | #12

    Michael Maines,

    You said: "so I might use kraft facing just to keep things neat-looking and to block some air movement."

    We're going to insulate the crawl floor next weekend. In such a retrofit instance, with kraft paper facing down into the closed crawlspace, are you concerned with fire risk/code?

    Thank you.

  13. User avatar GBA Editor
    Martin Holladay | | #13

    As far as I know, leaving kraft paper exposed is a fire hazard and a code violation. (Kraft paper is basically asphalt-impregnated paper. It burns.)

    Either use unfaced batts, or cover the kraft paper with taped drywall for fire safety.

  14. FrankFulton | | #14

    Thanks Martin. Would you be concerned about "windwashing" into the fiberglass in a closed crawlspace? There isn't much air moving, but 2 posters above mentioned air sealing the floor.

    (In this project we are creating a 2x thermal barrier, so the closed crawlspace walls are R13 and now we are insulating the crawlspace ceiling. Covering the fiberglass, eg with Thermax, would involve substantially more effort than simply stapling batts.)

  15. User avatar GBA Editor
    Martin Holladay | | #15

    I never advocate installing fiberglass batts in a crawl space ceiling, because it's hard to keep the batts in place. Eventually they slump and fall to the floor, or slip from their intended location.

    You probably don't need to insulate your crawl space ceiling. But if you want to insulate the ceiling, the best approach is to install a continuous layer of 2-inch-thick foil-faced polyiso on the underside of the floor joists. Before implementing this advice, however, check with your local code authority to find out whether your local code requires rigid foam in a crawl space to be protected with a layer of gypsum drywall.

  16. FrankFulton | | #16

    Martin, Thank you. A load calc on our bedroom indicated we should strive for >R30. Our plan has been to hold the batts in place by stapling non-stretch supporting twine every 18" or so. Is your recommendation still to go w 2" polyiso?

  17. User avatar
    Michael Maines | | #17

    Good points regarding fire safety, and an embarrassing oversight on my part. I agree with Martin, of course.

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