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Green Building Blog

Best Practices for Dry, Healthy Basements

To avoid water problems and comfort issues down the road, build your basement right the first time

Growing up in New England, I thought that all houses had basements. Whether it had a dirt floor and stone walls or was a nicely finished space, in my mind basements were synonymous with foundations. And, I assumed that even the nicest of basements were ultimately doomed to flood.

In my memory, there was always a small pool of water in our unfinished basement, and it seems like one at a time, each of my friends’ finished-basement rec rooms, where we played ping-pong and watched movies, eventually suffered water damage and became wastelands that were never rebuilt. I also remember basements being cool, temperature-wise—a reprieve from summer heat, but a bit uncomfortable in the winter.

Basements are useful, but I’m not sold on them as the best foundation type, even in areas like the Northeast, where they are still common. And I tend to recommend that people proceed with caution when considering finishing an existing basement. That a basement be a dry space before it is finished is nonnegotiable. If you are not sure—as sure as you can be—that your basement will stay dry, don’t finish it. Humidity and moisture issues can be prevented with the right materials and mechanicals, but water will ruin your finished basement—slowly or quickly, but surely. To avoid problems with basements, we must build them right from the beginning.

It is common for basement walls to be built on top of a footing that is formed and poured. The footing sits below the frost line around the basement’s perimeter. We can’t talk about building a dry basement without talking about the footings because the cold joint between the footing and the basement walls is vulnerable to leaking water if water management details are not well executed. The primary defense…

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11 Comments

  1. User avater
    Ryan Lewis - Zone 4A | | #1

    We completed a basement renovation last year. We replaced our basement slab, in our 1930 Tudor home, excavated an extra 12”, added filter fabric, drain pipes every 6’, and on perimeter, R-15 of XPS (due to space requirements), vapor barrier. We added metal chairs and hydronic tubes for floor heat across the full 1000 sqft.

    On the outside we excavated 3/4 of the foundation wall, and did all the steps above with R-15 insulation, vapor barrier, and drainage.

    The basement was regularly wet&flooding, it now is dry year round, even during the worst rains.

    Our house is very much a WIP, I’m not sure we have seen a noticeable impact on energy use, but it’s so leaky still. We will get there.

    1. User avater GBA Editor
      Brian Pontolilo | | #2

      That's a hell of a project Ryan. Glad to hear you have a better basement now.

      1. User avater
        Ryan Lewis - Zone 4A | | #6

        Thanks! All the detailing came from GBA lots of Q&A here.

  2. Kohta Ueno | | #3

    In the introduction to his thesis, Kohta Ueno wrote: “One observation consistently noted in studies and field investigations is that the failure of interior basement insulation is strongly linked to bulk-water leakage...

    To put this in less-stilted language when I'm *not* writing an academic thesis: I've looked at boatloads of basements with bulk water issues, and then you walk around the outside, and the wet spot lines up with the downspout that's dumping water from half of the roof right next to the wall. Falls into the category of, "Yep, there's yer problem, lady."

    1. User avater
      Peter Engle | | #4

      Yes, but the same lady already got quotes from three different waterproofing contractors who told her that the only solution is to install interior perimeter French drains and three different sump pumps, even though they're already in place from the original construction. And she looks at you like you're crazy, because everybody knows that French drains are the answer to basement water issues.

    2. User avater GBA Editor
      Brian Pontolilo | | #5

      When I was a bit younger, I went with my boss at the landscape construction company I was working for to visit some homeowers who wanted a curtain drain installed to stop their basement from flooding. As we walked around the house, I didn't understand why my boss seemed to be looking at the roof. When the homeowners came out to greet us, the first thing he said to them was, you don't need a "curtain drain, just fix your gutters." We never did install a curtain drain there, so I guess he was right.

      Thanks for the follow up Kohta.

  3. John Prospect | | #7

    Brian, thanks for an excellent summary article. Lots of great information.

    I had a question for the wider GBA community: Does anyone have data on footing capillary breaks and seismic shear resistance?

    In our part of the world (Southern Vancouver Island), we would be using rebar to pin the walls to the footing, but I wondered if anyone had a study or a general engineering practice since this break might make the "cold joint" have even less friction.

    I would think that this kind of question might come up in larger commercial structures?

    1. Expert Member
      Malcolm Taylor | | #10

      John,

      Given the high sheer strength of rebar embedded in concrete, I doubt the friction between the footings and slab of a cold joint contributes any significant amount to the resistance to lateral movement. I suppose you could compensate by increasing the reinforcing at the joint been the two, or forming a keyway in the footing as they used to.

      1. John Prospect | | #11

        Thanks, Malcolm. That makes intuitive sense.

        In your experience, do your local code officials (or engineers) require any additional rebar to compensate? Does anyone check or care?

  4. Benj Wadsworth | | #8

    Similar to John's question above, what is the best practice for insulating under an interior basement shear wall? I will be starting a house in Seattle soon and the engineer has specified two interior walls in the basement that sit on 18" wide footers. it seems easiest to pour these footers in conjunction with the slab pour. Is it possible to insulate under these walls somehow?

    1. Expert Member
      Malcolm Taylor | | #9

      Benji,

      It is possible to insulate under any load-bearing concrete element if you use foam with the correct compressive strength. In the case of a shear-wall where continuity is very important, I'd imagine the insulation would have to go under the footing, not between the footing and slab. One compromise solution would be to pour the footing with a small stem-wall on it up to the level of the top of the slab, and insulate between the footing and slab. That would limit the uninsulated area. Of course all this is speculative - it's up to your engineer to decide how comfortable they are with any insulation, and the energy penalty for not including any is negligible in the PNW.

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