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Metal roof, cathedral ceiling, condensation, mistake??

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  • Metal roof, cathedral ceiling, condensation, mistake??

    Well here goes.. Last Spring I put an imperial rib metal roof on a cabin. The interior is I guess what you could call timber framed with about 2 1/2" thick tounge and groove finished cedar for a ceiling. This also doubles for the roof deck. The roof was shingled and I removed them down to the deck. damn. I then fastened 2x4's on edge perpendicular to the ridge on 16" centers. We were trying to add some room to insulate as well as create a cold roof to control ice. We then insulated the cavities with fiberglass faced insulation. We attached 3/4" perlings for the metal on 24" centers parallel with the ridge, and screwed the metal down. I now have what I believe to be a condensation nightmare in the middle of a northern michigan winter. Its leaking very bad. I know now that I should have at least tar papered over the existing deck but I didn't. I'm thinking my only solution is to remove all the metal and perlings, then sheath over the 2x's w 7/16" osb, add a vapor barrier and hope for the best.. But I'm wondering if the osb will condensate as well??? I dunno any help or advice would be greatly appreciated.

  • #2
    Re: Metal roof, cathedral ceiling, condensation, mistake??

    First, you need to create an air barrier at the ceiling plane. You will need to open up the roof from the top and throw away the wet fiberglass batts. The air barrier should be installed on the top side of the deck boards. Although you could use polyethylene as the air barrier, it's tricky to install poly on a roof. If it were me, I would install two layers of rigid foam insulation with offset seams; each layer should be 3 inches thick. (This is going to cost more than fiberglass, but since you have a big mess on your hands, you want to do it right this time.) It wouldn't hurt to tape the seams of the top layer of rigid foam, using contractors' tape. Then install your purlins, fastened through the foam to the deck boards or rafters with long screws. The purlins are installed parallel to the ridge. Finally, install the steel roof.
    In case the mechanism is unclear to you, the condensation occurred when warm, humid interior air migrated between cracks in the ceiling boards until they hit a cold surface.


    • #3
      Re: Metal roof, cathedral ceiling, condensation, mistake??

      Thanx for the advice Martin. As of right now the bottoms of the corugated metal are closed off. Would it help at all to open them up and get some air flow clear to the ridge? Or would that just complicate things more? Just a thought for a possible temporary fix or slow down until it stops snowing long enough to stand on the metal roof.
      Thanks again


      • #4
        Re: Metal roof, cathedral ceiling, condensation, mistake??

        You need a vapor barrier under the metal. I use 30lb asphalt felt under my steel roofs and have never had a problem with condensation. You definetly need to remove the steel and toss the fiberglass. the foam is a good idea but I would still put the felt on top. The felt will let water vapor through and then when it condenses on the cold steel it will form droplets that will not penetrate the felt and will run out the bottom. You absolutely need to remove the "plugs" from the bottom of the metal. Let it breath as much as possible and make sure that the ridge of the roof can breath too. Since you don't have a way of venting above the insulation with any vents you have to get air flow under the steel, you may want to raise the pulins you fasten the steel too a little more just for more air flow.
        WFM Total Construction, LLC.

        Specialty Timber Products, LLC.


        • #5
          Re: Metal roof, cathedral ceiling, condensation, mistake??

          TimberRidge and Wes,
          There are two kinds of condensation that can occur in a roof assembly like this. The most common is the one I described -- warm, humid interior air leaks into the roof assembly and condenses on cold surfaces. Another type can occur in the spring: when the steel roof is cold or snow-covered, and the weather warms up during the day, warm exterior air passing under the ribs of the metal roofing can condense on the underside of the steel roofing. For those conditions, a layer of asphalt felt may be useful, to be sure the condensate is conveyed to the eaves. However, this is not a vapor barrier; it is roofing underlayment. Taped rigid foam insulation would serve the same purpose.
          Vapor diffusion is unlikely to be a factor in either type of condensation situation. The method of transport is air movement. That's why an air barrier near the ceiling plane is so important.
          To answer the question about whether increasing airflow under the steel roofing will help or hurt the current problem: It's hard to say. It certainly won't cure it. Increasing the airflow might even make things worse, since it might contribute to sucking more interior air into the roof assembly, encouraging condensation.


          • #6
            Re: Metal roof, cathedral ceiling, condensation, mistake??

            I agree mostly with Martin, but I would recommend an extruded styrene like the blue Styrofoam, not an aluminum faced insulation like Thermax. Then I'd install the 2 layers, and foam the edges with spray foam to stop air movement. That air movement is carrying the moisture from the interior of the house. Then add another 2" of foam over the 2x4s - that will give you an R-30 and will insulate the 2x4 purlins. THen add another row of 2x4s flat perpendicular to the ridge screwed thru the foam to the 1st row of 2x4s (or you could use 1x3). That will be your air channel. Then the strapping parallel to the ridge to fasten the asphalt felt & metal to.

            Be sure to wrap the felt over the ridge; you will get snow and possibly wind driven rain in there & you want to make sure it can run out & not work its way into the house.


            • #7
              Re: Metal roof, cathedral ceiling, condensation, mistake??

              Martin is exactly right. I install roof systems for a log home builder and prefer to use SIP's but you can do as martin has described. An other option would to strip the roof cover the T&G wood decking with 6 Mill poly. sealing all holes or penetrations. Screw down an insulated panel ( any where from 3" to 12" thick. This would have a 5/8" thick osb or plywood top to which you can screw the steel roof to. This will of course require you to re do the fascia and mabey other work to account for the thickness.

              The second issue is properly install the steel roof. This includes a #30 felt underlayment ( or other product, I use Try-flex). Install the steel according to the manufactures recommendations. This will include closers, fasteners and trim pieces. Do not try short cuts and just screw down a few pieces of steel and expect it to keep the roof dry.