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Community and Q&A

Insulating a crawl space

Joel Katz | Posted in General Questions on

I understand the value of a sealed crawl space.

We are currently remodeling a home that has an existing vented crawl space.
The space is very tight under the home and the exterior skirting is not in great shape.
It would be very difficult, and not in the budget, to insulate the exterior skirting.
We have just enough access to foam the underside of the floor and not enough access
to apply poly on all of the ground and up the skirting.

The City inspector is “happy” with whatever improvement we can make.

Is this a mistake just to foam the underside of the floor and keep the space vented?
If we are able to cover most of the ground with the poly, would that be acceptable?

Any suggest would be greatly appreciated.

Thank you

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  1. user-669103 | | #1

    It is not just covering the ground that is important it is insulating and air sealing the walls of the crawl space. The idea is to "defend" the minimum surface area from thermal losses and the walls of the crawl space have a smaller surface area than the floor. Gluing 2" of XPS (pink or blue board) to the crawl space walls and taping joints (with aluminum HVAC tape) important, plus foam sealing rim is good.

    As for whether not being able to put poly over the whole floor, it is hard to imagine how the underside of the floor above the crawl space can be accessible (for foam) but the dirt floor not accessible. To some extent having a gap in the poly is like leaving a door open and having R40 walls. However, this is an exaggeration. If you have very dry ground year round then maybe a 5% gap will only make the rest of the poly 50% effective, just as water will find a weak spot to leak in so will water vapor. If you have anything except very dry dirt year round than anything less than 99.9% poly over the dirt will be ineffective.

    Maybe you need to find a smaller worker to get into the tight space (no child labor please).

    However, I do defer to the professionals in this forum (I'm not one), but I have done my crawl space.

  2. GBA Editor
    Martin Holladay | | #2

    It's time to get out the shovels and buckets. Crawl spaces can be lowered. The work isn't complicated, but it isn't much fun. Remember, it's important that you don't undermine the existing footings (if any). Your description of "skirting" leads me to think you have a mobile-home-style crawl space. If shovels with full-length handles are awkward, get a folding shovel or an entrenching tool from a military surplus store.

  3. Riversong | | #3

    You don't indicate where you live (what kind of climate) and whether your site is dry or wet. You also don't tell us what kind of foundation you have - is the house on piers or on a mobile frame (as Martin suggests)?

    If you're in a mild and not very wet climate (low summer humidity), then keeping the crawlspace vented and insulating the floor above should be fine. But make sure to completely bury the floor joists in foam to decouple them from the radiant temperature of the ground and to protect them from moisture.

    If you do have a damp soil condition, then I would suggest looking first to gutters and site grading and drainage.

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