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Crawlspace in the Pacific Northwest

tnav21 | Posted in General Questions on

My question is about whether or not I should air seal and insulate my crawlspace walls or just put down a vapor barrier and leave the vents.
I live in Issaquah, WA 18 miles east of Seattle.  The house is a 1300 sq. ft. 1961 ranch house at the base of Tiger Mountain, one of many built in the area for the families of the loggers who worked the hillside.  When we bought the house it was heated with baseboard heaters which we have since removed.  It is now heated with a ductless heat pump with a ceiling cassette in the main living area supplemented with a small electric resistance heaters in each of the 2 bedrooms.  The crawl space is 2 feet high with no vapor barrier and no mechanicals except plumbing.  The crawlspace seems mostly dry except for one corner with some grading issues on the exterior of the house that I am currently working on.  I haven’t checked a humidity level, but I do know that we run a dehumidifier in the living area of the house and have to empty a gallon per day or so.
I’ve listened to many of your podcasts and read many articles about it and it seems like sealing the crawlspace is desirable, but I read Martin Holladay’s article Building an Unvented Crawlspace in which he cites a couple studies that in the said in the Northwest vented crawlspaces work acceptably most of the time.
So back to my original question, should I build an unvented crawlspace, or should I keep the vents? I know I’m going to put down vapor barrier at the very least.  If the answer is I should build the unvented crawlspace how should I condition the crawlspace?  I don’ t have any heating ducts I can tap into.  If I put some registers in the floor to connect the crawlspace to the living area I’m worried that my current heating set up won’t condition the crawl space we’ll enough. It also think it would be very hard to convince my wife to do that. 
Thanks for your consideration. 

Tony in Issaquah

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  1. exeric | | #1

    I understand that Washington State is a different climate zone from the Eastern tier states that have high humidity in Summer. That seems to be the qualifier that determines the need for an unvented crawl space. I think it might be wise to install a wireless humidity monitor in the crawlspace. In my home I had very high humidity in the crawlspace in winter and it persevered sometimes until midsummer. It would reach maximums of 80 to 90 percent RH down there. Not good.

    I installed poly sheeting down on the wet ground and it completely eliminated the humidity problem down there. It never gets above 50-60 % RH even in the worst of the winter rainy season and it quickly goes down to 40% after storms pass. Although your climate is somewhat different than mine I think it has enough similarities that poly sheeting down there may be all you need. Just make sure to always shingle the poly so that moisture always stays under the poly. Also run it up part ways up any perimeter foundation walls.

    There is nothing stopping you from trying this and seeing what the results are. If humidity sensor indicate that you still need to encapsulate the entire crawlspace that can be done and won't interfere with poly you've already put down.

  2. Expert Member


    Vented crawlspaces generally work alright in the PNW from a moisture perspective, but they are still energy hogs unless you have an awful lot of insulation in the floor above, and air-seal it well.

    The easiest way to condition it is to put a small exhaust fan in the wall, and a couple of vents in the floor above to drawn warm air down into the space. That presupposes you have insulated and air-sealed the walls.

  3. tnav21 | | #3

    Thanks for your replies.
    We don’t have any insulation under the floor. I’m thinking it will probably be about the same amount of work to air seal and insulate the walls of the crawl space as it would be to insulate the floor between all the joists.
    I put a humidity sensor down there out of curiosity without any vapor barrier down yet and it’s sitting at 83%. No bueno.
    How many registers would I need to put in the floor? Could input them in a closet?
    Thanks again for your replies. I really appreciate it!

  4. Expert Member
    Dana Dorsett | | #4

    >"We don’t have any insulation under the floor. I’m thinking it will probably be about the same amount of work to air seal and insulate the walls of the crawl space as it would be to insulate the floor between all the joists."

    It's a far easier to make the foundation walls air tight than the subfloor, so insulating the walls would be the right choice, and it would protect any plumbing from freeze-up issues (not that you have experienced that problem).

    In your area it's not a good idea to use fiberglass for insulating foundation walls. With a vapor barrier sealed to the foundation wall with caulk & mastic, 3" of reclaimed roofing polyiso strapped to the wall with 1x4 furring though-screwed to the masonry would work just fine.

    >"I put a humidity sensor down there out of curiosity without any vapor barrier down yet and it’s sitting at 83%. No bueno."

    The relative humidity number is pretty meaningless without the temperature to which it is relative, since the temperature is likely to be changing seasonally, sometimes hourly, and in a vented crawlspace the outdoor air will change the moisture levels fairly rapidly too. Measuring the moisture content of the joists and subfloor with a 2 pronged moisture meter would give you a better idea of the relative mold & rot risk than any spot checks of air moisture.

  5. bliljequist | | #5

    Hi Tony,

    I am in Seattle and looking at the exact same question. Playing around with Manual J calcs it is clear that there is a lot to be gained by enclosing the crawl space, esp. in my case where we have 60% of the house ducts - both from an envelope and duct air leakage and heat loss through the duct standpoint.

    There was an Oak Ridge National Lab study that folks at the WSU energy program did and they did highlight that every enclosed crawlspace they studies had elevated radon levels. I'm getting a meter to chat this out, and if the levels are low I'm going to go for it.

    If you have moisture in the house now, you definitely need to get a vapor barrier down in the crawl, that is really critical in our area, no matter what.

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