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Bathroom ceiling and walls

user-466364 | Posted in General Questions on

Working on a project in Quebec. It’s an older home and we are remodeling in steps. We rebuilt one wall of the house, leveled and rebuilt a sub floor in this area. This process required us to tear out the bathroom and two closets on each side of the bathroom. This entire section of the house appears to be an add-on at some point in time and the roofline over this section runs at a slightly lower pitch than the main roof. The ceiling in this area is treated as a cathedral type since the rafters hit the top plate at around 6 feet. We plan to redo the roof but cannot afford to do it right now, and we need a way to finish the ceiling that will get us through perhaps 3-years. The rafters are 2×4. Just to complete the scenario, we have had closed cell spray foam installed in the walls.

I have two questions based on this scenario:

1. Given that we do not want to sink a lot of money into a ceiling that we will rip out in the near future, what would be the best way to insulate and finish this this space, especially given that it is a moist bathroom environment?

2. Once we address the issue in my first question we want to start getting drywall up. However, there seems to be a lot of controversy on how to finish the walls. What we have concluded is that walls with closed cell spray foam should not include an additional vapor barrier (that would cause condensation inside and eventual mold and rot). We are planning to drywall with green board right over the spray foam. Are we on the right track? Is there any other considerations given that this in a bathroom?

Thank you!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Rob,
    Q. "Given that we do not want to sink a lot of money into a ceiling that we will rip out in the near future, what would be the best way to insulate and finish this this space?"

    A. In your climate zone, a properly insulated roof assembly will be between 9 and 14 inches thick. You can't install adequate insulation in a 3.5 inch cavity. If you don't have any plans to lower the ceiling, that means that most of the insulation needs to go above the roof sheathing -- in other words, you will eventually be installing multiple layers of rigid foam, nailbase, or SIPs above the existing roof sheathing.

    In the meantime, you can do almost anything you want, since the ceiling will barely have any insulation in it. I would avoid any vented assemblies -- in other words, don't try to install a vent channel under to roof sheathing. Probably closed-cell spray foam is your best option here.

    For more information, see How to Build an Insulated Cathedral Ceiling.

    Q. "Walls with closed cell spray foam should not include an additional vapor barrier (that would cause condensation inside and eventual mold and rot). We are planning to drywall with green board right over the spray foam. Are we on the right track?"

    A. You are correct that closed-cell spray foam is an effective vapor retarder. However, I don't see why an additional layer of interior polyethylene would cause any problems. The interior polyethylene is unnecessary but in this case harmless.

    Installing green board right over the studs (without poly) is just fine.

  2. charlie_sullivan | | #2

    For temporary insulation, consider also cut and cobble:

    https://www.greenbuildingadvisor.com/blogs/dept/musings/cut-and-cobble-insulation

    It could save you money vs. spray foam, especially if you use EPS or reclaimed foam. It's a lot of work but it sounds like this is a small area so it might not be bad.

  3. user-466364 | | #3

    Hi,

    Getting ready to begin this ceiling insulation project using the "cut-and-cobble" method. I have a few more questions.
    1. would it be better to use XPS?
    2. I will be using can spray foam for the gaps, but how do we attach to underside of the roof sheathing?
    3. The article you forwarded mentioned the pancake method, but there were no details on what that is. Can I assume that means stacking two or more layers together to get the desired thickness (in my case 3.5 inches)? If so, how do you keep them sandwiched together? Should the foam be flush with the bottom of the rafters?
    4. should we use a plastic vapor barrier once we've installed the foam and before installing drywall.

    Thank you.

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