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What is the best way to insulate interior finished basement walls that have 2″ rigid foam on the exterior?

Jason Hackworth | Posted in GBA Pro Help on

This is in a cold climate and the basement slab is also insulated with 2″ rigid foam. Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jason,
    The best way to insulate your basement walls is with more rigid foam: XPS, EPS, polyisocyanurate, or closed-cell spray polyurethane foam.

    What you will end up with is a kind of home-made ICF wall.

  2. Jason Hackworth | | #2

    Ok, thanks. There should only be only one vapor barrier on the inside, right? I know that the wall needs to dry to the inside.

  3. Travis T | | #3

    frame with 2x3 or 2x4 studs held an inch off the block. then spray open cell foam.

  4. Travis T | | #4

    ...and no vapor barrier if your code allows this. if it does require vb i would use certainteen membrain.

  5. GBA Editor
    Martin Holladay | | #5

    Jason,
    Don't worry about drying. After all, ICF foundations work fine. Each layer of foam is an effective vapor retarder, whether you want it or not.

    Whatever you do, don't include a layer of poly.

    I disagree with Travis, who likes open-cell foam. In damp areas like basements, closed-cell foam is far preferable. Open-cell foam acts like a sponge when it comes in contact with moisture.

  6. Travis T | | #6

    good point on the moisture absorption Martin. I don't do much with closed cell here unless it is the small quantity 2 part stuff. above grade i much prefer open cell but below grade i could see the closed cell being a better choice. My initial thought with the open cell was you would detect a bulk moisture issue (leak) a lot sooner than you would with the closed cell, and the open cell would allow for some drying.

  7. Chad Ludeman | | #7

    We're big fans of Dow's Thermax poly rigid insulation for interiors of basements. It includes a white coating on one side that does not require you to cover the foam in drywall. This saves time and money by eliminating the need for additional framing and drywalling. The seams can be taped if desired.

  8. Jason Hackworth | | #8

    Thanks for your input.

  9. Riversong | | #9

    Jason,

    Martin is wrong about both ICFs and the need for basement walls to dry to the interior.

    Most ICFs are made from EPS foam, which is relatively vapor-open (perm 3.5 per inch). Unless you've completely isolated every part of the concrete foundation from ground water, including the footings, and have given the concrete several months to dry, all cementitious basements must be able to dry to the interior. Building Science Corp stresses this in all their reports.

    Do not place any wood in direct contact with the concrete walls and use some kind of vapor open insulation, like open-cell foam, EPS or mineral wool. Use NO interior vapor barrier.

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