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Community and Q&A

Cedar tongue and groove on walls – Remove or not?

chrisakus | Posted in Energy Efficiency and Durability on

Some contractors have recommended that we take off the drywall and remove the tongue and groove cedar under the drywall (some of the drywall is rotten/black mold) to lay in insulation and put in a vapor barrier. But we are wondering if it would be better to leave the cedar in place and blow-in insulation and then just patch-fix the drywall. We are also replacing windows during this process, and the house is empty.


What should we do?

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  1. davidmeiland | | #1

    I don't think the cedar is an issue, and I don't think you should install a warm-side vapor barrier in your climate, but why is there drywall that is "rotten/black mold?"

    The main reason you might remove the interior wall covering is to do plumbing, electrical, or air-sealing work, or to inspect for/repair structural issues. Since you are replacing windows, I assume you are handling the flashing and moisture management issues from the outside. Insulating walls without doing that is dangerous.

    It would help if you could provide more info on the house, and some photos.

  2. chrisakus | | #2

    There was some mold damage to the drywall from a leaky window that wasn't fixed. When we started pulling out the drywall, it was pretty minimal. So, do you think we should just blow in loose insulation without adding a vapor barrier instead of adding a foam insulation and barrier? Why not a vapor barrier on the inside walls?

  3. davidmeiland | | #3

    I think that dense-pack cellulose is a great insulation method for closed walls, but you need to be sure that there is not going to be water leaking into the walls from the exterior. How are you going about replacing the windows? What age is the house and what is the siding? Is there a proper water-resistant barrier under the siding, and are the flashing details correct? Any signs of other problems aside from the window leak issue you mention?

    Sorry for the questions, but I don't think an answer to your question can be given without knowing a fair amount about the house. Your uninsulated walls may cause high heating bills, but they also let things dry out if water gets in. Insulation changes the equation dramatically.

  4. chrisakus | | #4

    We haven't seen any other indications of leaking from the outside. Also, looking through the open knots in the cedar, I can tell that there is a black fabric between the outer cedar and the vinyl siding. We don't know much more than that, because we haven't taken off any of the siding. We are replacing wood frame windows with vinyl. The house was built in 1958. I think the flashings are okay...? What about a closed cell spray foam?

  5. GBA Editor
    Martin Holladay | | #5

    I agree with David. The first step is to be sure that you have diagnosed the source of the moisture and mold.

    If you are sure that the source of the moisture is bad window flashing, then it it critically important to address the flashing at ALL windows, not just the window with the most obvious failure. It's possible that all of your windows are leaking -- but that your wall has so much drying potential (without insulation) that the leaks aren't showing up. Once you insulate, small amounts of moisture (which may dry quickly now) might begin to rot your wall.

    Window flashing is a complicated topic. Ideally, you should pull out the windows and flash each rough opening. Flashing should include a sill pan on the rough sill. In some cases, however, this isn't done for retrofit work. Your level of flashing depends on judgment.

    Sometimes improvements to the window head details can go a long ways to helping protect the window and to encourage rain to be kicked away from the building. It's even possible to create window trim details that include a "hat" or mini-shed-roof over each window; a good designer can help with these details to be sure they look good.

    I don't believe you need to remove the cedar boards, by the way.

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