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Thermal Bridging Problem?

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  • Thermal Bridging Problem?

    Need some opinions regarding a church roof problem. See the attached roof detail provided. This is a cathedral ceiling, no attic space, no roof ventilation. During the summer we have had situations on a clear warm or hot day that there is moisture dripping at the interior, off of the laminated arches. Space is air conditioned, occupancy is approx. 450 people. There are also stains up the ceiling along the arch crown molding about 2" wide . Have been reading about Thermal Bridging. Do you think it is possible that the laminated arches are heating up from the roof surface and exterior, transferring that heat down to the air conditioned air and condensation results because of the temperature difference and humidity and dew point? AC registers are in the floor at the outside walls, cold air return is in the floor at exterior wall end of the structure. There are no can lights in the ceiling.

    We have determined there is no rain passing through the shingles.

    Church is considering reroofing with a 50 year roofing product in the near future and wants to address this issue in case something needs to be done to the exterior side of roof before installing the new roofing.

    Thank You
    Original Construction Photos 1993 Section of roof

  • #2
    Added pictures from interior.


    • #3
      "Do you think it is possible that the laminated arches are heating up from the roof surface and exterior, transferring that heat down to the air conditioned air and condensation results because of the temperature difference and humidity and dew point? "
      "Heat transferring" doesn't cause condensation. Condensation happens when humid air hits a surface below its dew point. Heating things reduces liklihood of condensation.
      Best guess from afar, the A/C cools down some of the surfaces in the ceiling area. (The beams should be a bit colder than the roof decking, as they're less warmed up by the sun/outside heat.) Then 450 people come in and breathe humid warm air out, which tends to rise to the ceiling. The low return doesn't collect, cool, and dehumidify the (now) humid, warm air in the higher parts of space. Result, humid warm air hits surfaces (beams) below the dew point of the air.
      Incidentally, did this happen when it was hot outside, or now in December? big difference...
      Solutions: If my guess is right, based on my experience a high return would make a huge difference. You can hack something together with flex duct and try it to see.
      If you want to try sorting out the issue(s) yourself, a few little dataloggers with temp & humidity will help, as will an infrared camera (IMHO the phone-accessory versions are great). Spend some time on if you're determined to become an expert.
      A real solution will be to find someone who pretty much knows what they're doing. Could be a mechanical engineer, building consultant, local engineering professor...your church community may have someone who can help. Good luck,
      Last edited by ThingOfBeauty; 12-13-2019, 08:43 PM.

      Favorite tool this week: Makita double-battery "worm drive" framing saw


      • #4
        Where are you located? Does this occur all summer or just as seasonal transition from spring to summer or otherwise? Fiber sheathing could be acting as a moisture storage medium and accumulates water vapor all winter. How is structure heated and humidified/dehumidifier?


        • #5
          Reply to: calvert1,
          Building is located in Northwest Iowa, about 50 miles north of Sioux City, We get the extremes here. This only occurs in the summer. Only occasionally when it is hot and humid. Structure is heated with NG forced air units, and cooled with central AC units. No supplemental humidifiers or dehumidifiers used.
          I think you are on to something with the fiber sheathing, that may explain the black streaking on the laminated wood arches. Since I believe most fiber sheathing contains asphalt as a wood fiber binding agent. Perhaps a heavy dew is being solar driven into the plywood sheathing and fiber sheathing. I believe I do recall seeing in the summer that the shingles have a slight expansion buckle to them in the area of the laminated arches. ( I would have to monitor that more closely this summer ) Any theory why the shingles would only buckle every 12' over the laminated arch?


          • #6
            Do you have poly installed anywhere in the roof?

            If it was an Air Conditioning or excess humidity issue, then you should be seeing condensation on the windows at the same time. If this is not happening, then your A/C is fine and problem is that exterior hot/humid air is somehow entering into the 2x8 space. You might want to check typical sources of air-leakage:

            1) review the penetrations on the roof are air-sealed
            2) review the locations where rafters (2x8) meet walls
            3) if the fresh air-intake duct is located in the roof, then the duct should sealed as well and not leaking humid air into 2x8 cavity. This duct should also be insulated thoroughly.
            4) check if hot air from eaves is not entering into the roof and reaching the ridge where the beam is located

            If that doesn't fix it, then either install spray foam or taped-styrofoam on inside where you see most amount of condensation happening. If you decide to change the roof, consider exterior insulation with peel-and-stick vapor barrier installed on top of roof plywood (split-insulated roof). Your contractor will have to do dew point calculations to figure out how much exterior insulation would suffice.


            • #7
              Can you also pin-point the location in your pictures where the shingles are buckling every 12 feet? Framing picture would be ideal. Do you also have the exterior picture of the church - how it looks today and satellite view?


              • #8
                Dennis, some interesting problems.
                I am in Iowa, you can contact me directly. there is a lot to sort out here and more info is needed. Contact info is on my website.
                Mark Parlee
                BESI(building envelope science institute) Envelope Inspector
                EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                EDI Seminar Instructor
                Level one thermographer (Snell)
                You build to code, code is the minimum to pass this test. Congratulations your grade is a D-