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Community and Q&A

Re-Using and Venting an Existing Foundation

rshuman | Posted in General Questions on

I am building a 26’x36′ house at a location that is currently occupied by a 16’x36′ structure. That structure sits on a dirt-floor basement, the walls of which consist of 4′ of poured concrete that reaches to approximately ground level and ~3′ of concrete blocks on top. The site is located in the midcoast region of ME, CZ 6.

I would like to make use of the existing foundation by doing the following:

1.) Remove the concrete block, leaving the 4′ walls of poured concrete

2.) Remove the front wall of the existing foundation, extend the 16′ side walls by 10′ to achieve the 26′ depth of the new building, and pour a new 4′ tall front wall.

3.) Pour a concrete floor over the entirety of the new 26’x36′ foundation.

The 4′ tall crawl space walls would be insulated with 2″ of foam on the exterior and interior sides, 2-4″ of foam and poly would be placed under the slab. Access to the crawl space would occur via a door or trap door from the first floor and a short door (or bulkhead) into the space from the exterior of the house.

The advantages of this approach include a dry insulated storage area, access for floor plumbing and electrical work , and space for a pressure tank and water heater, while saving considerable money on the earthwork, demo, and concrete work needed to build a full basement.  The lower profile of the house is also a plus IMO.

I have read about options for venting a sealed crawlspace on GBA but in many cases those articles deal with spaces with dirt floor, rubble walls, and the like (i.e. situations with high moisture potential). Insofar as I am, in essence, building a short basement, I am wondering what venting requirements apply. I have lived in various houses that were essentially sealed, save for an egress door, that did not have active ventilation. Do I need anything more in this case?

Thanks for any advice you can offer.

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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Rob,

    I’m giving your question a bump. While looking for thoughts related to your question, I found this one from Expert Member “Akos”: “Ventilation is something you design for the whole house, not just the basement.” He recommended this article: Revisiting Ventilation.

  2. Expert Member
    NICK KEENAN | | #2

    This is the definitive article:

    Basically you have to decide if the crawlspace is inside or outside of the building envelope. If it's inside, you insulate it just like a mini basement. If it's outside you insulate between it and the building envelope, and vent it. From your description it sounds like you want it inside.

  3. walta100 | | #3

    I have to ask is it really worth the effort to reuse the old foundation?

    In my mind the risk Vs reward equation does not work in favor of the old work.

    From a financial point of view I doubt the numbers work unless you are doing the labor yourself.

    From a design point of view my guess is the old wall is putting limits on where on your property you will locate the new building and its size and shape.

    From a drainage point of view installing new foundation drains close enough to the old work without damaging it will not be easy and tying into an existing drain is risky in that it may or may not have been installed correctly and could easily be clogged.

    From a structural point of view your joints to the old wall will never be as strong as a one piece foundation and will require conservable amounts of time and effort.

    The only upside I see is you may keep two fewer truck loads of new concrete from being produced.


  4. Expert Member
    BILL WICHERS | | #4

    It sounds to me like you're planning to build a sort of cut-down basement, fully insulated on all sides and under the slab. You would NOT vent a "basement" like this, or a crawlspace, since you're preparing it to be INSIDE the building envelope. You don't typically vent basements, and you only vent crawlspaces that are OUTSIDE the building envelope (which typically means the floor under the living space is insulated and not the walls or floor of the crawlspace.

    You'll want to be sure to air seal the perimeter of those foundation walls too, so that they tie into the air barrier of the walls of the living space. The floor under the living space isn't important in terms of air sealing or insulating since it's entirely within the building envelope.


  5. plumb_bob | | #5

    Designing the slab/floor with an air barrier is a good idea to mitigate soil gas (radon) and for very little effort and $ you can install a passive radon venting system while you are at it.

    You are in climate zone 6, do I understand correctly that the foundation walls are 4' below grade for frost protection?

  6. Expert Member
    Dana Dorsett | | #6

    In a mid-coast Maine location the slab temps would be WELL below the midsummer outdoor air dew points, leading to high moisture content developing in anything resting on the slab.

    Mid-coast Maine 99% heating outside design temps are in low single digits too, making an uninsulated vented crawl/storage space VERY lossy for the house as a whole even when not actively heating it.

    Insulate the full perimeter of the foundation walls where it's easy, and do NOT vent it to the outdoors. You are in climate zone 6A. In the "A" climate zones vented crawlspaces are a both mold hazard in summer and a heat leak in winter. A vented crawlspace can work sort-of OK in the "B" & "C" zones of the western US where the summertime dew points rarely exceed the mid-50S F, assuming you don't care about the energy loss or how it makes air infiltration into the whole house

  7. rshuman | | #7


    Yes, I am proposing a short or mini-basement, inside the building envelope. Thanks for confirmation of my approach. I will take advantage of the opportunity and deal with radon venting.

    Walta, I will sharpen my pencil and see if the savings are great enough to adopt the hybrid approach I have discussed here or if it makes sense to just start from the ground up with a fully new foundation.

    Again, thanks everyone.

  8. Expert Member

    There seems to be some confusion in the replies between venting and providing ventilation. I took your question to be whether you needed to supply active ventilation to the space, as you would any other area within the conditioned area.

    Whether it's a short basement, or tall crawlspace, it needs some strategy to manage humidity. That can be a few things:

    - Connect the basement to the house above with vents in the floor.
    - Run an exhaust fan outdoors from the basement, and supply air via a grill in the floor above.
    - Install a dehumidifier.
    - Incorporate the basement into your main ventilation system with supplies and returns.

    While any of these would work, probably the best place to start would be which meets the requirements for crawlspaces under whatever building code you are subject to.

    1. rshuman | | #9


      Your reply gets at some of my confusion. I live in a 5 year old, pretty tight house that has a partially finished basement. The only communication between the main floor and basement is an open stairway. There are no floor grills that provide supply air for basement exhaust fans, there is no ventilation system in the basement and I do not run a dehumidifier. Humidities are acceptable year-round. In other, older houses I have lived in (e.g., old capes, decidedly less tight) there have been full basements that communicate with the main floor through closed doors, and also have no grills for supply air and are not tied into ventilation systems.

      Building codes aside for a moment, it appears the 'tall crawlspace' in my current house does just fine without the strategies you provide above. And the older houses I refer to didn't have excess humidity issues. Is this just lucky or is there some reason to think a 4' tall crawlspace constructed as I described in my original question would behave differently than the full basements cited above? And, as a result, need to have additional steps taken to ensure acceptable humidities.

      It just seems to me that once the word crawlspace is uttered things get much more complex and risky even though less than code-perfect conditions are commonly observed in functioning full-height basements. What am I missing?

      1. Expert Member
        MALCOLM TAYLOR | | #10


        No I think you have things mainly right, although older houses and their basements benefitted from being leaky, which usually helped their indoor humidity levels - something you can't rely on with new tighter construction.

        Our code recognizes that a crawlspace sufficiently connected with the house above does not need other conditioning. Although y0ur present low-basement does not have floor grills, the open stairway serves a similar function, so I'd say it does use one of the strategies I listed, and without either that connection, or one of the other options, it might encounter problems.

        1. Expert Member
          NICK KEENAN | | #12

          "Older houses and their basements benefitted from being leaky, which usually helped their indoor humidity levels."

          During the heating season this is true. During the cooling season air infiltration leads to condensation problems in the basement. In the climate I'm used to the summer is a bigger problem.

          1. Expert Member
            BILL WICHERS | | #15

            In the DC steam sauna, an the near requirement to air condition, it's likely the exterior will typically have a far higher humidity level than the indoors. Any hot, humid outdoor air is likely to condense out when it comes inside.

            We have similar issues in SE MI, just without as much heat. The leaks tend to bring in enough humidity that the air conditioners don't run long enough to dehumidify before cooling stuff off, so you end up with a cool and muggy indoor enviornment.

            I always recommend doing a good job air sealing regardless of climate zone. It's always better to have a tight home with CONTROLLED ventilation than a leaky house that does what it wants.


      2. Expert Member
        NICK KEENAN | | #11

        "Partially finished" is one of those phrases that means different things to different people. Whether the walls, ceiling and floors have finished surfaces or not, the entire space needs to have damp-proofing, air sealing and insulation. If it does then natural air flow may provide all the ventilation it needs.

      3. Jon_R | | #13

        > my current house does just fine

        Also keep in mind that codes and advice need to be far more conservative and universal than "it often works". Which often leads to a generalization fallacy of "don't do X" and then "but I've seen X work OK, proving you wrong". Russian roulette players often survive.

  9. plumb_bob | | #14

    The house I live in now (zone 7a) is from the "old school"- the crawl is completely separate from the house, but un-insulated and also un-vented in the winter. But it works. Piping does not freeze even at -35c and no mould. This is only possible because the house is so incredible air leaky. The main problem I have with the system is that there is no radon mitigation which may or may not be a problem where I live. Energy efficiency is a moot point because i am still stupid enough to enjoy getting firewood...
    Try this crawlspace setup today with current insulation requirements and there would be trouble.

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