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Community and Q&A

Insulating our crawlspace (with limitations)

barnabaas | Posted in General Questions on

Hi all.  So our home was built in 1952 out in the pacific northwest – I have no exterior wall insulation, no floor insulation and a vented crawlspace.  Only insulation we have is in the attic which was recently air sealed and then we had blown in cellulose which brought that up to code.  For now (we have renovation plans which might remedy the exterior wall insulation issue later) we’d like to insulate the floors / crawlspace.  I hate to say it but I think spray foam might be out of the question and I think encapsulating is also out of the question.  Due to $$$.  Curious what my options are with that or what suggestions y’all might have.  Bonus if it’s DIY’able and I can tackle this myself.

Pics of the crawlspace here

The only thing else to note here I think is that the plumbing is under the house but I think you can see that from the photos.  Thanks

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  1. Expert Member
    Peter Engle | | #1


    Installing insulation in the kneewalls in the crawlspace won't do any good so long as the crawl space is vented to the exterior. Venting is just like leaving your living room windows open in winter - there's no point in adding insulation to the walls if the windows are open.

    So, you need to close the vents and seal them up airtight. But you also need to perform air sealing on all of the joints in the crawl space - foundation to sill, sill to kneewall, etc.

    Most important, you need a vapor barrier on top of the soil, at a minimum. A vapor barrier over a few inches of insulation would be better. The vapor barrier must also be sealed airtight. Seal the seams to each other. Run the vapor barrier up the crawl walls.

    Finally - first, really - is to figure out why that water enters the crawl during heavy rains. It is probably coming from the soil surface outside. If water pools against the foundation, it's going to find a way in. Make sure downspouts are clear and that water is carried far from the foundations. Make sure that the soil outside is graded away from the foundation. You want a positive slope for 10 from the house if possible.

    There are plenty of articles on this site for all of this, and most of it is DIY-able. Not much fun, but certainly doable.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Brett.

    If you are handy and patient, sealing up and insulating your crawlspace may be something you can tackle on your own. To see what is involved, I suggest you read this: Building an Unvented Crawlspace.

  3. Expert Member
    Dana Dorsett | | #3

    A heavy polyethylene or EPDM vapor barrier on the crawlspace floor sealed to the foundation walls would allow you to safely close the vents to the outdoors.

    Using RECLAIMED roofing foam to insulate the foundation walls can be cheaper per R than batt insulation.

    The Pacific Northwest covers a lot of territory and three IECC climate zones. Got a ZIP code?

  4. barnabaas | | #4

    Thanks all for the replies. Dana - 94523 (Northern California) Sounds like at bare minimum I need a vapor barrier - any recommendations on a place to purchase from if I DIY?

    I guess the worry I had with closing off the venting and adding a vapor barrier was that I was going to need a sump pump (maybe I do?). Is it safe to add a vapor barrier and close off the vents considering we have plumbing under the house??

    Brian - thanks for that link. I've read that article a few times and always enjoy the refresher.

    Peter - what do you mean when you said "A vapor barrier over a few inches of insulation would be better." Thanks again for the comment

  5. bennettg | | #5

    Here are some of the links beyond the articles here I've collected while researching closing my own crawlspace in the southeast. Disclaimer: I have no affiliation or direct experience with these vendors or their products.

    Design and general info:

    Vapor barriers & materials: I saw this or very similar in a friend's closed crawl. It's tough stuff.

    I'll note that in examining my friend's crawl, the tape was coming loose in spots. I think, as with taping in general, the devil is in the application. Going over the tape with a roller, which strikes me as difficult over the rough surface of a crawl, is important to getting tape to adhere well. Mastic with fiberglass mesh tape would likely work better. Not much adheres to polyethylene well.

    Re sump pump: Better if you can drain to daylight from the lowest point of the crawl. Remember to block rodents.

    Re vapor barrier with plumbing: You'll install a drain through the low point of the vapor barrier.

  6. Expert Member
    Peter Engle | | #6


    I was suggesting installing some foam insulation on the floor under the vapor barrier. Even in your relatively moderate climate, the soil will be naturally cooler than room temperature. This increases the risk of condensation on the vapor barrier if outdoor air still finds its way into the crawl. By insulating the floor, the surface temperature of the vapor barrier will equalize to nearly room temperature. This saves some heating energy and decreases condensation risk.

    If there is any chance that the crawl will flood, this won't work because insulation floats.

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