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Insulating a flat roof in Chicago

2430Cortland | Posted in Green Building Techniques on

I have had continued problems with my flat roof on our 10-year-old 3-unit building. This will be the 3rd time having to do major work.

The ceiling below has about 8 inches of blown in insulation with an 8 inch or so open space above. All of which is within the engineered floor joist that should allow for air flow.

On top of the joists is the main roof deck and on top of the deck is an additional deck that is sloped with the roof membrane on top. The wood is completely rotted out but the main deck appears OK.

Your April 12 2013 article that was updated September 2014 addresses the “right way to vent low sloped roof.” It says to vent perimeter and center along with sealing the ceiling below to stop air from escaping unit into above. My question is should you also insulate the roof on the outside, beneath the roof membrane?

Thanks very much,

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  1. GBA Editor
    Martin Holladay | | #1

    As I explained in my article, Insulating Low-Slope Residential Roofs, a flat roof like yours can be either vented or unvented.

    It sounds as if you have a vented roof. Clearly, something is wrong with your details if "the wood is completely rotted out" on your 10-year-old building's roof, and if "this will be the 3rd time having to do major work."

    A vented approach will only work if (a) your ceiling is close to airtight, and (b) you have a large cupola or dog house in the middle of the flat roof to encourage air flow. Without these essential elements, your roof will fail. Your building is Exhibit A.

    Q. "My question is should you also insulate the roof on the outside, beneath the roof membrane?"

    A. There is no reason to add insulation above the roof sheathing unless you have decided to throw in the towel and change your roof from a vented roof to an unvented roof. Considering the fact that you have a major failure, I don't blame you for throwing in the towel. If you want to make this change, here are the steps you need to take:

    (a) You have to gain access to the perimeter of your attic, either by opening up the roof sheathing from above or the ceiling below, so that you can insulate the exterior walls of your attic, right up to the roof sheathing. I know that your attic is only 8 inches high -- but this work is still essential. Spray foam insulation might be a good choice for this work.

    (b) While you are doing the work described above, seal all vent openings at the perimeter of your attic.

    (c) Replace the rotten roof sheathing.

    (d) Make sure that your roofer installs enough rigid foam insulation above the roof sheathing to meet minimum code requirements in your climate zone (R-49).

  2. Dana1 | | #2

    (e) Make sure that the R-value of the insulation above the roof deck is at least 40% of the total R (=>0.67x the R value of the cavity insulation). The IRC prescribes R20 above the roof deck in your climate (zone 5A) but that is predicated upon an R49 total R. In an unvented assembly, the average temperature at the roof deck in winter determines it's moisture content, and it's moisture content is what determines it's rot-risk. With 16" of blown cellulose you would have about R60 in the cavity, and if you only added R20 above the roof deck the roof deck would be too cold (=wet). You would need at least R40 above the roof deck to hit the minimum ratio, which is a super-insulated roof.

    It may make more financial sense to remove the fiber insultion, put R49 above the roof deck, seal off the venting, insulate only the perimeter below the roof deck, and remove any interior side vapor barriers at the ceiling level (assuming ther is any.)

  3. GBA Editor
    Martin Holladay | | #3

    As Dana pointed out, there are two ways to create an unvented roof assembly. One way is to install all of the insulation (usually rigid foam) above the roof sheathing. The other way is to install a certain amount of rigid foam above the roof sheathing, and the rest of the insulation (fluffy insulation like fiberglass) under the roof sheathing and in direct contact with the roof sheathing.

    Both approaches are explained in my article, Insulating Low-Slope Residential Roofs.

    If you decide to implement the hybrid approach (rigid above + fluffy below), you'll need to insulate the attic walls, and you'll need to perform air sealing work at the ceiling plane. My guess is that your ceiling is very, very leaky, which is why your roof sheathing is now rotten.

  4. charlie_sullivan | | #4

    If you decide to try to get your vented assembly to work right, rather than changing to unvented, I would install a temperature/humidity monitor in the vent space and perhaps a moisture probe in the deck before closing it up, so you can find out whether it's working right before the next failure.

  5. GBA Editor
    Martin Holladay | | #5

    Editor's note: Andrew Rothstein apparently decided to continue this discussion on a new thread. Here is a link to the continuation of the discussion: More questions about insulating a flat roof in Chicago.

  6. Trishamegan | | #6

    One thing to keep in mind...Chicago doesn't follow IBC or IRC. It has its own code.

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