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Community and Q&A

Non-typical installation of XPS and 8 ft. wall with 9 ft. ceilings

Austin K | Posted in GBA Pro Help on

I am in the early stages of framing up a basement wall. Exterior of foundation is almost all exposed. I plan on 2″ xps, backed by a 2×4 stud wall, then possibly filled with insulation.
I originally wanted to have the rim joist accessible for future leak detection and planned to incorporate a perimeter soffit for wiring/ and access. This would give a finished tray cieling for the basement. And still have 9ft in the center of the room.

I now have concern that possible condensation on this 12″ of exposed concrete is going to cause problems. Will it even condescending if no insulation is covering it? Will the room temp transfer into the soffit and keep the dew point where it needs to be to not condesate?

It sounds silly, but I am not overly concerned about the 12″ of uninsulated space. It is inefficient, but so it the other 25% of the storage area in basement that will. Not be insulated.

If thus doesn’t make sense I can explain further.

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  1. Steve Knapp CZ 3A Georgia | | #1

    Austin. Where do you live?

  2. Austin K | | #2

    I live in cincinnati ohio. My main reason for accessibility is the obvious plumbing components, and a deck which is thru bolted to the rim joist. Top of wall is connected to a treated cleat on the wall and fir blocked

  3. Expert Member
    Malcolm Taylor | | #3


    Why are you anticipating leaks at the rim joist?

  4. Austin K | | #4

    Exterior doors and penetrations through veneer from ledger bolts. Also, future ledger board replacement for deck expand sion

  5. GBA Editor
    Martin Holladay | | #5

    It's certainly possible that your concrete foundation wall will get cold enough during the winter for condensation to occur. In colder climates, such an uninsulated foundation wall can even develop interior frost.

    The coldest part of your wall -- the part most in need of insulation -- is the part that you have chosen not to insulate.

    Unless you need to leave an inspection strip due to a concern over a heavy termite infestation, the usual practice (for the type of work you are undertaking) would be to cover all of the exposed concrete with rigid foam on the interior side. It would also be a good idea to remove the fiberglass batts that you are using to insulate the rim joist, and to replace those batts with rectangles of rigid foam (sealed in place with canned spray foam).

    For more information on these issues, see:

    How to Insulate a Basement Wall

    Insulating rim joists


  6. Austin K | | #6

    Ok, a couple of questions relating to your recommendations.
    1. I do plan to remove the fiberglass. Batts in the rim joist (this was the builders doing) but how do I "fireblock" the joist bay after rigid insulation? And this will 100% conceal my ledger bolts, which need to be removed when the deck addition gets done.

    2. Can I just somehow insulate the concrete above the wall without having to extend the framing? Obviously rigid is the choice, but again....back to fireblock in the soffit. This adds anot her dimension to the equation. If fireblock isn't required in joist bay, would it be requiredin this area?
    (The top of wall has 3/4 ply to fireblock the current design)

    3. The 3.5 treated cleat used to anchor the wall top will be exposed, without cover.

    I'm not at the point of no return, I just don't want to add uneccesay work for myself.

  7. GBA Editor
    Martin Holladay | | #7

    Fire blocking is required to separate certain cavities with an air barrier. See the detail below.

    You may be thinking about ignition barriers and thermal barriers. Most building inspectors require rigid foam to be protected by a layer of 1/2-inch drywall (a thermal barrier). For more information on your local code requirements, talk to your local building department.

    You may also want to read this article: Thermal Barriers and Ignition Barriers for Spray Foam. (Although the article discusses spray foam, similar requirements usually apply to rigid foam.)


  8. Austin K | | #8

    Martin, you are correct. The term I'm looking for is thermal barrier. The 2x4 wall will encase the already installed xps. 5/8 drywall on the wall face, and six inch wide 3/4 ply to cover the xps and top of stud wall.

    The proposed soffit will have decorative wood on horizontal surface, and drywall on the vertical surfaces.
    If I were to insulate the remaining concrete wall (inside the soffit), what material options do I have? I'd rather not extend framing, as there are multiple plumbing obstructions.

    I'm aware fiberglass, mineral, etc are out due to air being allowed through, which will definitely condensate on the cold surface. Is there a foam product that can be used without requiring a therMal or ignition barrier?

  9. GBA Editor
    Martin Holladay | | #9

    According the most building codes, rigid foam in a finished basement needs to be protected by 1/2-inch drywall. This code provision is sometimes unenforced.

    If the area was only used for storage and for mechanical equipment, you might get away with a product like Thermax (a brand of polyiso that has passed certain fire safety tests). When in doubt, talk to your local building official.

  10. Austin K | | #10

    I was thinking polyiso this whole time for the soffit area if it were to be insulated. Gives me easy access when time comes to replace deck ledger. And it limits the need to extend the framing to the ceiling/ cover with drywall. I don't expect a guaran tee , but does this solution sound resonable?

    In summary: construction of wall stays same. Cut 12" polyiso to fill in remaining concrete and pipe areas. Along with rim joist. Seal everything off with a fire rated expanding foam/caulk?

  11. GBA Editor
    Martin Holladay | | #11

    The way I read the code -- and I hate to sound like a broken record -- is that you need to protect the polyiso with a layer of 1/2-inch drywall (or a material that has been shown through testing to perform as a thermal barrier that is equivalent to 1/2-inch drywall).

  12. Austin K | | #12

    not a broken record at all, i understand the importance of covering the rigid board. i was misunderstanding the previous post about the thermax. i figured thermax/ polyiso wouldn't need to be covered(even though it is concealed in a soffit), which seemed like an easy solution to glue it to the exposed concrete and call it a day.
    at this point, i am gathering the idea that the only solution is to tear out the wall, add xps to top of concrete, rebuild the wall to same height, and close it off accordingly with thermal barriers. unless there is a valid material that can be used against the concrete inside of a soffit.

    i guess the confusion arises from the fact that rim joist insulation is an exception to the rule.

    the original thougt process was to leave access to any and all possible future problem areas. was willing to sacrifice a little efficiency for ease of access. i know that if things cant be seen, problems dont get noticed until significant damage is present. a rotten rim joist from a failed door flashing is a much larger problem for me than a few bucks a couple months a year on my utility bill from inefficient insulation.
    my concern developed over possible condensation on the exposed area of the wall. i have never seen this basement drop below 58 degrees (single digit outdoor temps), and i had hoped that the temperature inside was enough to keep that part of the concrete warm enough to prevent the sweating. i have never seen condensation anywhere in the basement which initially was no concern of mine)

  13. GBA Editor
    Martin Holladay | | #13

    Thermax is one brand of polyiso. Most polyiso brands have not undergone the testing and approval process that Thermax has. (Thermax has a special facing.)

    If your soffit is finished with drywall, you should be all set.

  14. Austin K | | #14

    Even if the inside top of the soffit is exposed floor joist?

  15. GBA Editor
    Martin Holladay | | #15

    That would be my ruling. But I'm not a code official. My interpretation of the code is irrelevant. The only interpretation that matters is that of your local code official -- so pick up the phone and call the office.

  16. Austin K | | #16

    Thank you very much for the expertise in the matter. I appreciate it. And I now feel confident I have enough to bring a valid plan to the building department. Thank you

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