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Community and Q&A

Vented crawl in the PNW

joenorm | Posted in General Questions on


I have read and re-read every article on conditioned crawls and why they are better than those vented to the exterior.

What I notice is a lot of the moisture concerned building science comes from the East Coast where temperature and humidity are much different than in my Zone 4 marine climate. The standard around here by a long shot is still the vented crawl space(if not a slab on grade).

Could it be that a vented crawl is just fine here and doesn’t warrant the condemnation it receives here on GBA?

I ask because I am planning a build and I wonder if it is worth pushing for a conditioned crawl while most builders around here are not familiar with them at all. And while the articles here lay out how they should be built, there still seems to be lots of debate about them.

I have poked around in many crawl spaces around here and I do not notice the problems that many associate with vented crawls. No mold growth if the water has been managed at the site. 

That say, it does seem crazy to be living under a cold cave, hence my interest in building a conditioned crawl. Thanks for your thoughts.

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  1. Expert Member
    Dana Dorsett | | #1

    In zone 4C vented crawlspaces work just fine from a moisture management point of view.

    But from an air sealing perspective it's FAR easier to make the foundation wall & crawlspace floor air tight than to air seal the subfloor and all plumbing/electrical penetrations thereof.

    From a critter-condominium management point of view it's a lot easier to go with a sealed conditioned crawlspace too.

  2. joenorm | | #2

    thank you! What would be the simplest approach to building a sealed crawl in this climate that would not confuse concrete guys used to doing things one way.

    Just omit the vents, insulate the exterior, and ventilate the space with a low cfm fan?

    Is it that simple?

    1. Expert Member
      Dana Dorsett | | #5

      Most contractors unfamiliar with the concept usually get confused by the insulation on the exterior, often stopping the insulation at grade, leaving the above-grade portion of the foundation exposed. An exterior insulation approach requires Z-flashing between the housewrap and exterior side of the foam, and a coating over the foam to protect it from UV light degradation and other damage.

      There seems to be less confusion if the insulation is installed on the interior, all the way up to the top of the foundation wall, with any ledge of concrete between the foundation sill and interior filled in by 1.5" foam board (as thick as the 2x lumber of the sill) filling in with R23 rock wool on the band joists.

      With either approach there needs to be a heavy ground vapor retarder (best if protected by a rat-slab poured over it), well sealed to the foundation.

      1. joenorm | | #6

        So there is no debate that putting foam on the interior is OK? This seems like a much easier approach because you don't have to worry about the foam degrading in the elements. And you don't have to address the interface of exterior wall sheathing to the foam.

        1. Expert Member
          Dana Dorsett | | #8

          >"So there is no debate that putting foam on the interior is OK? "

          None. Interior side foam is a standard approach suitable for all climates.

          In termite zones it's useful to leave a 3-4" inspection strip between the bottom of the wll foam and the floor , filling it in with rock wool that can be temporarily removed to be able to look for termite tunnels.

  3. frasca | | #3

    Joe - I live over an unvented crawlspace in the PNW too. You probably already read Martin’s comments on crawlspaces in dry climates here: I’m deciding whether to seal and insulate mine or put something under the subfloor as I currently don’t have any insulation of any kind :( .

  4. user-723121 | | #4

    When I added a room to my parents house in western MN in 1998 the crawlspace was unvented. Footings were frost depth and I used R-10 rigid insulation on the exterior of the foundation wall covered with prefinished coil stock above grade. I also sealed the rim joist area in the crawlspace and added another R-10 to the warm side of the foundation wall. It has stayed very dry in the crawlspace over the years. The copper supply lines feeding the hydronic baseboard give off enough heat to keep the crawlspace at room temperature.

    I did pour a concrete floor in the crawlspace for storage and ease of getting around to run computer wires and the like.

  5. GBA Editor
    Martin Holladay | | #7

    In my article on crawl spaces, here is what I wrote:

    "While vented crawl spaces often perform poorly in the humid states of the Southeast, they perform well in most Western states. According to an article in Home Energy magazine, “In the drier regions of the West, and even — surprisingly — in the marine climates of the Northwest, vented crawl spaces work acceptably most of the time. The hot-dry conditions in summer and the cold-moist conditions in winter do not cause the same problems that hot-humid conditions cause in the rest of the country. … The Washington State University Extension Energy Program (WSU-EEP), as part of its work for Building America, monitored four test houses in Vancouver and Moses Lake, Washington, for over a year and found that the vented crawls rarely, if ever, reached dew point and that they remained above 80% RH only for brief periods of time.”"

  6. joenorm | | #9

    Thank you Martin,

    I do remember reading this in your article. I think I took the term "acceptably" to mean OK but not great. But perhaps if done right they can be effective around here. I have to choose my battles on this project so maybe this is one I can afford to concede, and I can focus on and direct more funding to wall and roof assemblies.

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