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Community and Q&A

Open-cell insulation beneath roof sheathing in Chicago

user-7422445 | Posted in General Questions on

After the polar vortex a day or two into the thaw we observed water dripping through a drywall seam on a dual height ceiling. After which we found an ice dam in front of both our scupper boxes and lots of water sitting on top of the ice/snow build up.  We manual cleared the snow/ice to get the water moving and the drip immediately stopped. A few days later we had a roofer come out who said he didn’t see any problems with our roof.  Very little interior damage if any was observed so we decided to “ignore the problem” and possibly add some heating coils by the the scupper boxes to prevent future problems. A few days later, the breaker for the bedroom with the leak kept flipping.  An electrician then found a closet fixture completely full of water about 12 feet away from the leak along with lots of wet insulation near the light fixture. Our contractor came out an opened up some of the drywall.  They found wet insulation, surface mold on underside of our roofing deckboards, and rusty nails.  No mold was observed on the trusses of the attic between the drywall and the underside of the deck boards of the flat roof. Since then we have received all sorts of input about what kind of problem we have and what should be done.  Majority of it we have dismissed as a “boiler plate solution”.  It seems that most people the have any experience are VERY busy. A contractor we have used in the past and do trust has suggested that we add open cell insulation between the deck boards and fill the whole cavity down to the ceiling.  Rather than doing the work from the interior I suggested that we consider replacing the roof and adding the insulation from the exterior while replacing all the deck boards.  We are also considering getting a PVC roof.  However, I am really confused when trying to figure out if we need a vapor barrier, venting, or if I should be concerned with wall insulation and stack effect. From what I can gather we currently have an unvented attic structure.  We live in Chicago so I believe we are considered Zone 5 .  Our house is a 2004 brick/block build, flat roof, third floor shingled doghouse walkout with interior staircase with lots of skylights over it.  Our second floor ceiling has recessed can lights in the master bedroom and the whole second floor has hvac returns in the ceiling. Since moved in in 2015 we have replaced the flashing on the top of the house between the limestone and brick and added stainless steel drip trays, tuck pointed, replaced 1/3 of the roof and ceiling joist that was rotted from what we assumed was water that came in through the masonry issues.  Our family also has a lot of respiratory issues (we had them before we lived here) and we need to run humidifiers through the the winter. My apologies for my ignorance…trying to educate myself the best we can before we drop a lot of money on a solution that might not even fix the problem.  In the end efficiency is important but more than anything preventing future moisture problems is the priority.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    I have never heard of the term "scudder box." I Googled it -- and I found an online discussion in which it was suggested that this was a spelling error for "scupper box." Is that what you are talking about?

  2. GBA Editor
    Martin Holladay | | #2

    You have ice dams. Before we discuss solutions, you need to understand the causes of ice dams. Here are links to two relevant articles:

    "Prevent Ice Dams With Air Sealing and Insulation"

    "Ice Dam Basics"

    You have an unvented low-slope (flat) roof. Before you come up with a plan to address your ice dams, you need to understand the right way to insulate your type of roof. Read this article: "Insulating Low-Slope Residential Roofs."

    If you have an unvented flat roof with recessed can lights in the ceiling, it's almost certain that your problem is cause by air leakage and insufficient roof insulation. To solve the problem, you'll need to remove the recessed can lights (replacing them with some type of surface-mounted fixtures), create an airtight ceiling, and install an adequate thickness of insulation.

    Q. "I am really confused when trying to figure out if we need a vapor barrier, venting, or if I should be concerned with wall insulation and stack effect."

    A. You need an air barrier, not a vapor barrier. My guess is that you don't need venting -- it's almost impossible to convert an unvented flat roof assembly to a vented flat roof assembly. This problem has nothing to do with wall insulation, but it is almost certainly affected by the stack effect (which is why you need an air barrier).

    Open-cell spray foam in roof assemblies is associated with damp sheathing, so it's not a good choice. For more information on this issue, see "Open-Cell Spray Foam and Damp Roof Sheathing."

    You mentioned the possibility of opening up the roof on the exterior to make repairs. If you are willing to install new roofing, the best solution involves adding an adequately thick layer of continuous rigid foam on the exterior side of the roof sheathing, followed by new roofing. More information on this solution can be found in this article: "Insulating Low-Slope Residential Roofs."

    1. user-7422445 | | #8

      After some reading & a moisture intrusion inspection we have decided to go with a new roof with insulation on the exterior side of the roof sheathing. We did also find some areas on the south side of our house that need spot tuck-pointing but we are operating under the assumption that we have multiple problems.

      For the insulation I do have a question. If you could direct me to any appropriate articles it would be very beneficial. One of our estimates for the roofing work stated such, “ –In addition to the work above we will be tearing off the existing roofing to the wood decking. We will then install 4” Insulation board, ½” plywood decking, and then 2” of tapered insulation board which will run the water towards the two new scupper boxes installed.” They said the second layer of plywood was necessary to allow the insulation to be screwed down. Is this the best way to do this? Thanks - Kelly

      1. Expert Member
        Dana Dorsett | | #9

        In Chicago's climate >40% of the total R-value has to be above the roof deck for this to work reliably. Whether 4" of foam board is going to be enough depends on how much is currently under the roof deck.

        Installing a 5/8" nailer deck above the foam board through-screwed to the structural roof deck with pancake head timber screws is the most common way of dealing with exterior foam board. That allows for a standard roofing layup to be used on the exterior. It's also common to use a fully adhered self-healing weather resistant barrier eg Grace Ice & Water Shield) between the foam board and structural roof deck, which self-seals around the screws that are holding the sandwich together.

  3. Peter Yost | | #3

    While I agree with Martin that your moisture problem is most likely related to condensation and air leakage, I don't think we can rule out bulk water leakage as part of the problem.

    You need to get an experienced building investigator who understand hygrothermal physics as applied to building assemblies to work through your roof assembly problems.


    1. GBA Editor
      Martin Holladay | | #4

      I wasn't implying that this was condensation. If in fact this is a type of ice dam, the usual mechanism is heat leakage, snow melt, and water entry through roofing.

  4. Jon_R | | #5

    See below (Table 2) for vapor barrier recommendations (you want class II or smart equivalent):

    > we need to run humidifiers through the the winter.

    Then you should be more conservative (ie good interior side air sealing, additional insulation and follow BSC recommendations). And monitor the humidity so it isn't excessive.

  5. user-7422445 | | #6

    Thank you for all your responses. I believe I have read all these articles but I clearly need to spend more time re-reading them. I’m have a hard time understanding the cause/effect and/or correlating effects of everything. Yes, I meant scupper...I have edited. As far as the ice dam is concerned Chicago did have the perfect weather patterns for it but I’d still like to do what we can to avoid it again. From what my contractor mentioned to me he thought the ice dam and condensation issue were two different issues, both of which need addressed. Our roof is not in perfect condition but very few roofers feel it needs replaced however we’d like to see less water pooling than we currently have and if a new roof is the best way to fix that especially if we are already going to be working towards updating our insulation and I’d prefer to do it from the outside to avoid the mess inside if the alternative is to tear out all my ceilings. Any suggestions for an inspector in Chicago?

    Thank you,

    1. GBA Editor
      Martin Holladay | | #7

      Although I assumed it was an ice dam and roof leakage, it may well be condensation -- especially if you operate a humidifier, which is (generally speaking) a big building science no-no.

      Determining whether you have an ice dam problem, or a condensation problem, or both, will take an experienced sleuth, as Peter noted.

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