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More questions about insulating a flat roof in Chicago

2430Cortland | Posted in Green Building Techniques on

[Editor’s note: This question appears to be a continuation of an earlier thread. Here is a link to that thread: Insulating a flat roof in Chicago.]

Hi Martin , first of all thank you so much for your help. I have learned more from you the past two days then the past 6 years I’ve been going through this. One more question then I will stop bugging you. We had to remove part of the ceiling in various room and remove the blown in Insulation and add rolled Owens insulation batts in its place. Thus the insulation in the ceiling is inconsistent in terms of R-value and tightness of fit. If we executed everything properly what is your opinion/best guess on which roof (vented/unvented) allows the most margin of error in term of air flow/ heat penetration from ceiling below into space above? Thanks, andy

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  1. Expert Member
    Dana Dorsett | | #1

    This is the flat-roof in Chicago with the rotting roof deck, right?

    Hands down the most reliable approach is unvented, putting ALL of the insulation above the roof deck, air-sealing & insulating the perimeter of the roof assembly over the exterior walls. That puts the roof deck and rafters/joists fully within conditioned space, where the humidity tracks that of the rest of the building. If none of the other structural wood inside isn't wet enough to rot, the roof deck will be in about the same condition.

  2. GBA Editor
    Martin Holladay | | #2

    Dana gave you good advice. If you follow his advice, the amount of blown-in insulation or batt insulation above your ceiling will be irrelevant, because you will be putting the insulation where it belongs -- above the roof sheathing.

    In all cases, attention to airtightness matters. Fortunately, most low-slope membrane roofs are airtight. When you put all of the insulation above your roof sheathing, air sealing becomes much easier -- although you still need to worry about air leaks at the perimeter of your attic, especially if your attic used to be vented. All vents must be carefully sealed, and the short 8-inch-high walls of your attic must be insulated.

  3. Expert Member
    Dana Dorsett | | #3

    BTW: If you put literally all of the insulation above the roof deck, you don't need R49. The R49 spec in the code presumes some amount of thermal bridging of joists/rafters.

    Code compliance can also be met with a calculated U-factor, the inverse of the "whole assembly R" after thermal bridging. In zone 5 that max U-factor is U0.026, which is R38.5 whole-assembly. The roof deck itself (not even counting the air film) is worth the R0.5, so you'd be code-legal with R38 of continuous foam. That's do-able with just 7" of roofing polyiso (but 8" would be better, being mindful of the temperature derating curves of polyiso.)

  4. 2430Cortland | | #4

    Follow up question:
    Hi Martin and Dana - As it relates to insulating exterior wall of ceiling joist, is it critical to spray foam (closed cell) the entire wall or can we leave existing cellulose insulation and just spray above it?
    Our building is a brick construction on the outside with cinderblock on the inside . The parapet wall on the roof is covered with a clay coping stone with a rubber membrane underneath as an addition barrier for water. The exterior brick was sealed a few years back and the interior parapet wall (cinderblock) wall was also sealed with a heavy white paint that is used to seal light houses. At the roof level (and on all 3 levels of the building) on the outside under a row of the brick is a rubber membrane with weep holes to catch any water that gets in wall. The roofer removed the coping stones and cut into the rubber membrane underneath and noticed that there was some moisture in between the brick and cinderblock. Is it normal for moisture to be in between the brick and cinderblock? A friend of mine developed a ventilation system that is positioned underneath the clay coping stone (on both sides of wall ) that allows airflow into and out of the parapet wall but doesn't allow water to get in from above. Assuming we go with a warm unvented roof, does it make any sense to vent the parapet wall itself? Do you get involved with local Chicago Roofers as we will need one?
    Thanks so much.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for posting your follow-up questions on this page.

    Your questions are the type that can't easily be answered on a web forum. I urge you to hire an experienced architect or engineer to review the situation at your building.

    Here are a few basic principles:

    1. Parapets are a thermal disaster, since they are giant thermal bridges. You can call them radiator fins if you like. They conduct heat away from your building during the winter, and release the heat to the open air. Ideally, if there are no historic preservation watchdogs looking over your shoulder, you would remove the parapets completely, so that the roof insulation could extend over the top of the masonry walls.

    2. Water shouldn't get trapped inside a masonry wall. If a masonry wall has an air gap between dissimilar materials like CMUs and coping stones, there should be a layer of continuous flashing at the bottom of the air gap, and this flashing should conduct any water to the exterior through weep holes.

    Q. "Is it critical to spray foam (closed cell) the entire wall or can we leave existing cellulose insulation and just spray above it?"

    A. You can leave the existing cellulose insulation in place and just spray above it.

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