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Interior foundation drainage plane

snfh | Posted in Energy Efficiency and Durability on

Question for the experts. I’ve been thinking about the ramifications of installing rigid or spray foam insulation directly to the interior foundation wall of a basement. Over the past 15 years I have built approx. 60 custom homes and I can think of 5 foundations that developed some type of leak after construction was complete. All leaks were easily fixed from the interior side. Two were tie leaks and three were hairline foundation cracks. One of the tie leaks showed up when a customer installed a lawn sprinkler system and a sprinkler head soaked a planting bed along the foundation wall.

Any type of foam insulation applied directly to an interior foundation wall will mask the effect of a leak. It seems like some sort of drainage plane would be a good idea to allow water from a leak to drain and the possibility of a leak to be discovered. Thoughts? Would a drainage plane cause other problems (ie. air sealing)?


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  1. Billy | | #1


    You really need to deal with the leaks from the outside. That being said there are some who advocate a drainage system inside the wall, with a drain that connects to the sump. I personally don't like it.

    But consider the possibility of using this type of wall insulation panel in basements.

    The vertical channels would allow water to drain to the bottom of a wall where you could have a drainage system such as a trench that goes to a sump. It sound like too much work to account for something that should be addressed from the exterior.

    What type of waterproofing system do you use on your foundations?


  2. snfh | | #2


    Not looking to get nutty with the drainage plane. I should add that when I say drainage plane I'm really thinking of an air space or gap between the foundation wall and insulation. If a leak ever developed the water would just drip to the floor and become visible to the occupant who could have the leak repaired. Most foundations will never leak, but the possiblity will always exist.

    Typical spray-on damp-proofing has been sufficient for most foundations in the areas I build. I generally waterproof when we run into wetter soils. However, since my methods have changed along with the energy codes I no longer have the luxury of leaving exposed interior foundation walls to see post construction should a leak develop. All foundation walls must be insulated before we complete construction. Exterior rigid foam is not an option since most of the houses I build have at least some brick or stone.


  3. wjrobinson | | #3

    Bruce, can you explain.... " Exterior rigid foam is not an option since most of the houses I build have at least some brick or stone."

  4. Billy | | #4


    In that case the product I linked to may work for you. The vertical channels in the foam insulation would allow water to drain down, depending on how you attach this product to the wall. A trickier detail is that you're not supposed to allow interior air come in contact with the foundation in order to eliminate condensation of humid air on a cool foundation, so you'll have to work that out.

    In basements I keep the drywall an inch off the floor and I use PVC baseboard so a little leak won't cause secondary problems.

    On the exterior foundation a quality peel and stick membrane such as offered by Grace and a plastic waffle mat and fibrous drainage plane mat will help you avoid foundation water issues. A simple paint-on waterproofing approach may not be sufficient, but I don't know what you're using.

    AJ's question is a good one.


  5. snfh | | #5

    AJ and Billy,

    Beyond the rather poor aesthetics of rigid foam protruding above grade and beyond the face of brick or stone, there would still be a thermal bridge to the interior foundation via the brick/stone ledge. Years ago I used rigid foam (like many builders) on the exterior foundation wall until I read some material from Building Science Corp. and Build America about the futility of using rigid foam with brick or stone.

    I looked again at the insofast panels. You are right. The vertical channels would allow water to run down. I would rather use something that I could buy/assemble locally but I will check it out further. Thanks for the info.


  6. user-1091831 | | #6


    If it's aesthetics that concerns you, one future construction option could be to insulate with xps just to grade, and, in addition, insulate interior walls fully with xps as well to prevent your thermal bridge between the foundation. Just be sure to insulate your perimeter cavities as well with either carefully cut rigid foam or spray foam for an air tight cavity and no thermal bridge. Good moisture control as well.

    Take a look at this as it may help explain what I'm talking about, just keep your exterior insulation below grade (unlike the image).

    I'm also assuming you're using a brick veneer wall construction? There are some issues I have heard of as far as protecting exterior rigid foam, but I guess I'm not too sure of issues you mentioned of the brick and rigid foam combo. If its a brick veneer, and standard concrete foundation, I wouldn't think you would have any issues, but that is something I'd consult the experts about. Hope that is helpful in some way.

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