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Looking for details on sealing a crawlspace

nateflanigan | Posted in General Questions on

Hello – I just purchased a 1920’s craftsman in southern NJ (climate zone 4a) there is a lot of air sealing and insulation work to be done but I’m most concerned with treating two crawlspaces. The original house has a full basement but sometime in the 90s an addition was put on as well as the front porch being converted to enclosed space. Both crawlspaces are at grade, have no air/vapor barrier and batt insulation in the joists. They’re not in bad shape but they still freak me out and I’d like to get them handled asap. I’ve been reading through the relevant resources on this site but still have a few questions about details that I haven’t seen addressed.

1) How much rigid foam insulation should I put on the foundation walls?
2) Should I insulate the “party walls” shared by the crawlspace and basement?
4) When laying down the plastic barrier that flashes up the walls how are the corners dealt with?
4) When crawlspaces have access doors how does that interface with the plastic barrier?
5) I’m assuming that access door should be insulated and weather stripped.

I don’t currently have a forced air HVAC system in the house (I have hydronic radiators). I understand the concept of the passthrough grills, rather than a low cfm fan venting the conditioned air to the outside would it be a good idea to return that air to the basement?

Thanks all

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Replies

  1. Patrick OSullivan | | #1

    > How much rigid foam insulation should I put on the foundation walls?

    The code requirement would be same as for a basement which in 4A is R-10 of continuous insulation. Reminder: there are some fire protection requirements for foam in crawlspaces.

    > Should I insulate the “party walls” shared by the crawlspace and basement?

    This is a funny situation, because once the exterior crawlspace walls are insulated, only the below grade portion of the 'party' wall is a heat sink. Ideally you should insulate them on the basement side, along with the rest of the basement. This will also help control vapor drive into the basement (and therefore the whole house).

    > When laying down the plastic barrier that flashes up the walls how are the corners dealt with?

    Practice with a scrap. You can do it a few ways. You can gather and fold a triangle shape to one side of the corner, you can cut out a square out and you'll have the edge inside the corner to seal, you can cut a slit and lap and tape that exposed edge. It doesn't need to be pretty, but it should be durable.

    > When crawlspaces have access doors how does that interface with the plastic barrier?

    An exterior access door? I would suggest abandoning it and sealing if there is sufficient access from interior.

    > I’m assuming that access door should be insulated and weather stripped.

    Abandoning it sounds better, right? :-)

    > I don’t currently have a forced air HVAC system in the house (I have hydronic radiators). I understand the concept of the passthrough grills, rather than a low cfm fan venting the conditioned air to the outside would it be a good idea to return that air to the basement?

    I know what the code says in this area but I personally would not purposely facilitate the movement of air from a below grade space to the rest of living space unless the entirety of the below grade space (these crawl spaces plus the full basement) are known to be detailed well all around and there is no concern about radon.

    1. nateflanigan | | #2

      Thanks for the reply -

      Regarding access There really is not sufficient access from the below grade basement to the at/above grade crawlspaces. In the back addition there is a small window, big enough to crawl through if your life depended on it, big enough to eventually run some duct work through. I don't plan on storing things there (I have plenty of other space for that) but I'd like to maintain sensible access for maintenance. While tossing and turning last night I came up with a few ideas for sealing that.

      >I know what the code says in this area but I personally would not purposely facilitate the movement of air from a below grade space to the rest of living space unless the entirety of the below grade space (these crawl spaces plus the full basement) are known to be detailed well all around and there is no concern about radon.

      A good point - two follow up questions
      1) Is pulling conditioned air from inside through the crawlspace and venting to the outside not an energy loss concern? Maybe it's less air than I realize.
      2) We just had the radon tests done as part of buying the house, is that not sufficient to alleviate concern? I don't know enough about radon and the limitations of standard testing to have any insights here.

      Thanks again

  2. Walter Ahlgrim | | #3

    To my ear the crawlspace sounds like it was designed to be a vented space given the insulated floor above.

    If your plan is to convert it to a conditioned space I say great idea but do not stop half way. Part of the plan must be to remove the old insulation connect the crawlspace to the house so that its temp and humidity are almost the same as the rest of the house.

    If you seal the vents and insulate the exterior wall but fail to remove the old insulation parts of the crawlspace will often be below the dew point of the air in the crawlspace. Those spots may get and stay wet long enough to mold and rot.

    All too often people look for the free lunch and call there space “encapsulated “ I say vent it or condition it anything in between is Russian Roulette.

    If you can bring yourself to call it a “conditioned crawlspace” it shows you will have accepted the bitter pill of heating and cooling the space.

    Walta

  3. nateflanigan | | #4

    Walta > If your plan is to convert it to a conditioned space I say great idea but do not stop half way. Part of the plan must be to remove the old insulation connect the crawlspace to the house so that its temp and humidity are almost the same as the rest of the house.

    Yes - that would be part of the plan

    > If you can bring yourself to call it a “conditioned crawlspace” it shows you will have accepted the bitter pill of heating and cooling the space.

    I like the semantics of this. To that end do you have thoughts on bringing that conditioned air back into the basement?

    I should clarify a few things -
    1) the addition crawlspace is about 360 sq ft, the former porch crawlspace is 168
    2) I plan on bringing ductwork through the addition space at some point, thus the preference for sealing and conditioning. Otherwise I'd just insulate better.

  4. nateflanigan | | #5

    Another question -

    To leave a termite inspection strip or not?

    This fine homebuilding article specs them -
    https://www.finehomebuilding.com/membership/pdf/459754/021299046.pdf

    and this buildingscience.com article shows continuous insulation from the rim joist down the foundation walls

    https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-0401_Conditioned_Crawlspace_Construction.pdf

    I think it would easiest to just spray the whole thing with closed cell vs rigid foam board and not worry about getting all those transitions right or adding a drywall layer. I mean, drywalling a crawlspace does not sound fun.

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