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Does basement insulation require waterproofing the concrete walls before rigid insulation?

Frank O | Posted in Energy Efficiency and Durability on

I plan to insulate my basement walls using 2″ XPS, air seal the seams, build a 2×4 wall 16″ o.c., insulate with batts, and complete with drywall.

I have also been advised to waterproof the concrete walls using either Drylock or Super seal. Is that needed?

The exterior basement wall has a mirror drain and water proofing and the burrito-wrapped footing drain is tightlined to an external sump. My basement is currently dry.

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  1. Riversong | | #1

    If the exterior of the foundation is really waterproofed, and not simply damp-proofed with black tar, then there should be no need for any additional interior sealing. You could do a test by taping a 12" square of clear plastic to the wall for 24 hours and see if there's any condensation. But, as long as you're isolating the stud wall and cavity insulation from the concrete with XPS and the basement is currently dry, there should be no problem.

    Don't forget to insulate and air seal the band joists and mud sill.

  2. Frank O | | #2


    Yes, I have insulated the band joist with rigid xps and air sealed...but you bring up a good point with regards to waterproofing...

    It rains often here in Seattle and I will need a few seasons in order to tell you my basement waterproofing details have indeed worked. So far, so good...even after this latest deluge.

    That said, it seems I should stick with R30 batts under the main floor and use a half wall insulation detail to eliminate 50% concrete wall heat loss (figure 11, page 12 of )

    This way, I can confine my envelope to my primary living spaces while keeping my concrete radiant heat on the main floor, tackle a good deal of heat loss from the uppper half of the basement concrete walls, and lastly, save the expense and time of firring out the entire basement while I ensure my basement isn't a swimming pool in disguise.

    Have you seen the half wall-insulation detail work? Does the foil faced polyisocyanurate also need air sealing on the bottom? the detail only shows an air sealing bead on the top of the rigid board against the concrete wall.

    thank you

  3. Riversong | | #3

    If you've had heavy rains and no sign of dampness in the basement - AND you've done the 24 hour patch test I described before - then there should be no problem with finishing the basement walls.

    The decision should rest on the planned use for the basement. If you need finished space, then by all means stud it out and drywall it. If it's just utility and storage space, then keep the thermal envelope at the first floor and insulate the upper half of the basement walls. Use adhesive caulk to bond the polyiso to the walls and tape the seams with aluminum foil tape.

    When you say "concrete radiant heat on the main floor", do you mean a thin light-weight concrete over a wood-framed floor with radiant tubing? If the first floor is radiantly heated, then I would install a radiant barrier 2" under the subfloor (there are foil-bubble-foil rolls that are designed to fit between joists with a staple flange) and fill the remaining space with batt insulation.

  4. Frank | | #4

    - the 24 hr patch test yielded NO moisture behind the plastic
    - yes, the concrete radiant is 2" concrete over a wood framed floor with pex tubing

    the .pdf ref''d above discusses the considerable heat loss of the above grade portion of the concrete wall...but doesn't talk about the heat loss of the rim joist. Do you know the heat loss of the rim joist (as compared to the concrete wall)?

  5. Riversong | | #5


    The rim joist and the upper half of the concrete wall would lose about the same amount of heat per square foot of surface.

  6. Steve El | | #6

    My current basement (pennsylvania hills) floods in spring when the ground is frozen but the snow melts hard. I think it's got something to do with how the frost line intersects some subsoil drainage planes on the sloping lot where the house stands. That may not be your case, but my point is if you are in a wintry part of Washington and haven't lived there through a melt it may be worth waiting to see how your basement performs under those conditions also.

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