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Vented crawlspace: humidity levels and deciding to encapsulate

pdxcrawl | Posted in General Questions on

I live in Portland Or, zone 4C. I placed 2 humidity sensors in my crawl space the past week (9/13/19) and I see the Temp remains very constant at 65-66F and Humidity ranges from 65-72% while the outside humidity has been high 70-95% (outside temp is 54-68F). Is the crawl space humidity normal for this time of year in the northwest when the outdoor humidity is so high?

Back story:

Recently I had some work done to my vented crawl space, my house was build in 1994. Due to rodents and some mold on the joists and sub floor I had the floor joist insulation removed, existing flex duct removed, soda blasted, 24/7 mold inhibitor applied, and old vapor barrier removed. 

I installed new flex duct (fixing all the air leaks), and placed a temporary 6mil vapor barrier down (taped seams overlapping 6″-10″, and extending it up the wall 12″ (but not sealed against the walls, and not sealed around the cement pads for the piers). I also did a rodent extrusion so hopefully that will solve that issue.

I have not replaced the floor joist insulation yet as I have been considering  encapsulation. Due to expense, I am thinking about just putting down a good 15mil stegocrawl liner and fully sealing it to the walls and fully sealing around the cement pier pads. But I am concerned about the humidty level right now as I have read at 70% and above is when the mold will grow.

One of my main goals is improving the indoor air quality and I don’t like the fact that if I encapsulate, I have to have a register open between the house and crawl space to meet the code requirements.

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  1. john_heckendorn | | #1

    If the purpose of the register is to lower the humidity in the crawl space, is it possible that by venting the extremely dry exhaust from a heat pump water heater there -- if it's feasible for you to do -- that would mitigate the requirement for the register with your planning authorities? Assuming it wouldn't supercool your crawl space to hit dew point. No experience with this, so can't say it would work, but we're wondering if this idea would kill two birds with one stone on a renovation we're planning.

  2. Jon_R | | #2

    > I have to have a register open between the house and crawl space to meet the code

    Check your local code - there should be alternatives that don't create airflow from the crawlspace to the house.

  3. spudkensington | | #3


    If you chose to exhaust air from the crawlspace using an open register to the living space and an exhaust fan in the crawlspace, the negative pressure should keep most crawlspace contaminants away. I had the same concerns and I am chosing this method for the potential contamination aspects.

    On the othe hand, your crawlspace has been introducing contaminated.air into your living space this whole time. The stack effect will make sure of it, unless you have some sort of underlayment vapor barrier.


  4. Andrew_C | | #4

    Encapsulating the crawlspace is a good idea. Doing a good job sealing will reduce the humidity, along with any other ground-based funky smells.

    If you’re going to re-insulate, insulating the walls seems to be a better solution than insulating the floors, so combine the encapsulation and insulation jobs.

    My understanding is that one of the crawlspace options is to connect the crawlspace to the rest of the conditioned space, and then have a continuous low flow exhaust fan in the crawlspace. This should both keep crawlspace air out of the house, and also reduce the need to put a dehumidifier in the crawlspace. (You may need a dehumidifier in the shoulder seasons if you are the type to keep windows open.)
    Edit: this is what Matt Gates has done.

    We recently encapsulated an extremely damp crawlspace and installed a dehumidifier (hung high enough that we can drain it without a pump.) Now that the crawlspace has dried out, the dehumidifier runs very little. A good job of encapsulating made a huge difference in keeping the crawlspace much drier.

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