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Community and Q&A

Would the condensation be explained by the lack of insulation?

2430cortland1 | Posted in Energy Efficiency and Durability on

We own a 3 unit building in Chicago built in mid 2000s. Last fall we put on a new warm roof with vapor barrier on top of main roof decking but below the new insulation (insulation on outside) but did not insulate rim joist from ceiling below. We noticed strong odor in unit.

We opened the interior walls of 3rd floor in August and noticed condensation on the walls and rim joist. We read your article on insulating rim joist and NOW plan on doing this using closed cell foam.

The interior walls has only R-12 Owens insulation with a plastic vapor barrier. We are wanting to also spray foam these walls with closed cell foam.

Would the condensation be explained by the lack of insulation? And would insulating the interior side walls with closed cell foam address the condensation within walls and ceiling rim joist?

I am also assuming the plastic vapor barrier on walls is a bad idea?

Building is constructed of exterior brick with cinder block on the inside. Is it OK to still have insulation within the closed ceiling cavity below new roof? Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Insulating an old building with brick walls, or walls made of brick and concrete blocks, is tricky. Here is a link to an article that you may want to read: Insulating Old Brick Buildings.

    If you see wintertime condensation on a rim joist, you can be fairly confident that the components where the condensation is occurring are too cold. Proper insulation can prevent this problem. For more information on rim joists, see Insulating rim joists.

    The area you are discussing -- the top of your walls (where your exterior walls meet your roof) -- can be a tricky area. We would need to know a lot more to give good advice.

    Is this a low-slope (flat) roof?

    Is there an attic (or a space between the ceiling and the roof sheathing)?

    Are there studs on the interior side of the CMU (concrete masonry unit) wall?

    If so, is there an air space between the studs and the CMUs?

    Are there hidden air flow channels in these stud walls?

  2. 2430Cortland | | #2

    Roof is a flat roof. Engineered ceiling joist is the only space separating the ceiling and roof above. There are 1 1/2 wood studs on the interior side of the cinder block. There is no air space. We are planning on using closed cell foam on rim joist (perimeter of joist above and on the perimeter of the interior walls) and then will put back drywall.

  3. GBA Editor
    Martin Holladay | | #3

    Closed-cell spray foam is a good choice for this type of building. Good luck.

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