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Community and Q&A

Insulating a crawlspace

MikeBlake | Posted in Green Building Techniques on

Hi, I have a 70 yr old home built on a post and pad foundation. It is quite leaky and I’m in the earlier stages of fixing it up. I’m currently in the process of removing and replacing the skirting that runs the perimeter as it is unisulated and has enough rot to warrant its replacement. We live in southern coastal British Columbia. Winters here are rainy and only drop below freezing for short periods of time. My plan is to frame between the posts and fill with sheets of 2″ xps topped with a rainscreen and cedar siding. The idea is to trap the heated air that escapes under the house. This will be enough to keep water pipes from freezing but probably wont keep our floors warm on cold mornings. In an addition to this, I am considering boarding the underside of the uninsulated floor joists with xps as well. The idea being that it would creat another pocket of trapped warm air directly underneath the floor. My concern is that it would also trap moisture around those joists. If I left small gaps between the sheets of xps would this solve a potential moisture issue or would the escaping warm air through those gaps defeat the purpose?
Any thoughts on this or suggestions on a better way is appreciated.
Keep in mind that the crawlspace has a dirt floor and the joists for the most part are 24″ above the ground so it’s a tight space

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  1. GBA Editor
    Martin Holladay | | #1

    1. Yes, you should insulate the floor assembly. Here is a link to an article that explains your options: How to Insulate a Cold Floor.

    2. It's essential that you install a vapor barrier -- for example, 6-mil polyethylene -- on the dirt floor of your crawl space. Hold it in place with rocks or bricks.

    3. To create a durable skirt, dig a shallow trench (about 3 inches deep) around the perimeter of your building. Locate the trench with a plumb bob. Use pressure-treated 2x4s for the bottom plate and studs of your skirt. Rigid foam is appropriate insulation, but it should be installed as a continuous layer on the exterior of the studs, not cut into small rectangles inserted between the studs. Tape the rigid foam seams with high-quality tape, and protect the rigid foam with a layer of pressure-treated plywood on the exterior side of the rigid foam. At every stage of the process, pay attention to airtightness. Minimizing air leakage is essential.

    Siding is optional.

  2. Dana1 | | #2

    In a coastal BC climate 2" of polyiso would outperform 2" XPS, and would be far greener due to the far less damaging blowing agents, and the lesser polymer per R. The only caveat is that the cut edge can't be in contact with the soil, since it can wick moisture and become less effective. Polyiso also comes in fire-rated grades (Dow Thermax), and unlike XPS, chars in place and never melts even when fully engulfed, rather than melting, dripping burning liquid polymer the way rigid polystyrene does. It's usually cheaper per R too.

  3. MikeBlake | | #3

    Thanks for the responses. I was planning on doing points 2 and 3 pretty much as you described. Martin, my only issue with those drawings is that they show the air barrier on the cold side of the floor. In a cold climate is this not an issue? I've always thought that the vapour barrier has to be on the warm side, and in this case that would be impossible. Initially I was going to install roxul between the joists and cover with rigid foam, but because of the difficulty of working under the house I thought I might be able to skip the Roxul. And by installing XPS under the joists I would essentially be do the same thing as insulating the skirting, trapping the warm air. Also, whoever rewired the house 20 yrs ago thought it would be a good idea to simply string the wires (and the water lines) underneath the joists so sealing off the cavities completely would be impossible without rerunning the wires which I'm not able to do at this time (their wiring job job could be a whole other discussion).
    So, what are your thoughts on
    1. skipping the air barrier on the underside of the joists
    2. aside from the lower R-value, leaving out the Roxul


  4. GBA Editor
    Martin Holladay | | #4

    It's OK to insulate your floor assembly with a continuous layer of rigid foam under your joists if you want. EPS or polyiso would be best, by XPS will work. Attention to airtightness is essential; if the continuous rigid foam layer is interrupted by pipes or wiring, you need to fill these interruptions with canned spray foam (or foam from a two-component spray foam kit). You also need to tape the rigid foam seams with high-quality tape.

    Your floor assembly definitely needs an air barrier; however, there is no need to worry about vapor diffusion or the presence or absence of a vapor barrier. (If you are worried, remember that either plywood subflooring or OSB subflooring is an adequate vapor retarder.)

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