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Encapsulated crawlspace conditioning with necessity for isolation

1869farmhouse | Posted in General Questions on

The final task I have before moving in is to encapsulate my crawl space.  I’ve read all the usual GBA articles, but in my situation there are some additional factors to consider.

1869 house with a low crawlspace.  Was full of asbestos, but was abated.  The area is so cramped, it’s almost guaranteed it wasn’t 100%.  In addition to asbestos, it’s full of general nastiness I want to keep out of my house.

My plan was to use a combination of polyiso and rock wool to insulate and seal below the main level floor.  Then I’d hope to spread 6mil poly sheet for a vapor barrier and seal to rim joist area (balloon framing) with spray foam.

Now I need to prevent stack effect from sucking up into living area.  I assume my best bet is vent to outside from rim joist area with 50-70cfm fan and cut an access grate in main level floor?

Opening the floor to connect to crawlspace is not overly appealing, even if air will hopefully be negative pressure down below.  I seriously considered going old school vented… I have a newborn baby so any help appreciated.

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  1. 1869farmhouse | | #1

    I forgot to mention, there are no mechanicals or ductwork of any kind in this crawlspace.

    Is there any “right” way to create a vented crawlspace? It hasn’t had a vent of any kind since it was built in 1869, and is in surprisingly good shape structurally. Surely plenty of “venting” via leaks though.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi A Gouch,

    I know less about vented crawls than I do about unvented. I'm not sure if the exhaust fan will mitigate the stack effect. It might help, but more importantly would be for the floor and rim to be air sealed so there is no way for air to get pulled into the conditioned space above.

    That said, I wonder if the best option for you is an unvented crawlspace with a continuous exhaust fan, one of the two options for conditioning a crawlspace. I assume you have read this: Building an Unvented Crawlspace, but wanted to be sure you had.

    Also, where is the house located?

    I look forward to others' knowledge on this one.

    1. 1869farmhouse | | #3

      I have read it, I’m in Kansas, near Kansas City. That article says that with an unvented crawlspace, if a continuously operated fan is used, I must cut a hole between the living space and crawlspace for air correct? That’s the part that worries me, even if air should only be flowing INto the crawlspace rather than the other way

      1. GBA Editor
        Brian Pontolilo | | #4

        Got it. Then back to the air sealing. Do a super diligent job with it. And hopefully someone else will be able to answer your question about whether it makes sense to mechanically vent a vented crawl.

        1. 1869farmhouse | | #5

          It might be an unwarranted concern. There’s clearly no perfect answer here, and I’m only an amateur on green building type construction, so hopefully we can figure out the best solution.

          The guys I work with are still all stuck on, “a house needs to breathe!” and think I’m crazy for air sealing obsessively and relying on mini splits instead of a monster furnace, haha.

  3. Expert Member
    Akos | | #6

    Generally the simplest way to deal with nasties is to encapsulate it. Spraying the whole crawlspace with a coat of latex paint should do the job. The bonus is that it will make it much brighter to work in there.

    With an unvented crawlspace the insulation should always be on the floor of the crawlspace, not under the joists. You might be in warm enough climate that insulation there might not do much. It is also better to use a beefier poly for the VB, the 6mil is for walls.

    If you want to air seal the crawl space from the floor above, a layer of OSB under the joists with taped seams would do the job. If you are carefull, you can probably do a decent job with well detailed house warp. House wrap would be easier to handle in a tight space.

    Make sure to block the bays of the balloon framing, you definitely want those well sealed.

    In terms of conditioning the crawlspace, if you don't want to connect it to the house, you can install a small electric space heater and a stand alone dehumidifier.

    1. 1869farmhouse | | #8

      I appreciate the info! Insulating the floor of the crawl space would be nearly impossible. It’s extremely uneven and the house was built on top of large rocks, not to mention is probably only 18-24” tall. It’s going to be a blast...

      But the idea of complete encapsulation with a dehumidier could be a good one.

  4. JC72 | | #7
    1. 1869farmhouse | | #9

      Very interesting. I don’t have any mold right now, so the last thing I want to do is make it worse. Somehow it lasted over 100 years unvented without problems, but the picture is exactly what I had in mind originally. I used a lot of foil faced poly iso elsewear and really like it.

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