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Matt1945
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# Posted: 18 May 2014 00:19
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Hi! New to site... just bought a small 12 by 16 cabin in BC, needs a lot of work, have started with the roof as I am needing to fix the chimney, noticed that that rafters are only 2X4, I have re-insulated and vapor barrier with R14 roxul but no gap for any ventilation.
The cabin gets a lot of snow in winter and we will be using it occasionally year round... I was told the 2by4 rafter was not a big deal as the roof is a steep 12/12 pitch; however, I am concerned about ventilation, I am putting a metal roof on and was curious if I could just add a grid of strapping above the sheathing and tar paper and let the metal roof vent through the ridges in the metal panels?
The pic is the original cabin
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Don_P
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# Posted: 18 May 2014 09:27
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a friend called this week asking about the mold at the peak of the newly drywalled ceiling in what is to be an apartment above their gambrel garage. asking questions, they had stuffed the bays full to acheive the required R30 and had no vent space. I'm pretty sure that water vapor is getting through the R30, contacting the cool roof sheathing that is under a metal roof, condensing there and wetting the insulation. With no way to dry the moisture level increases until it will support mold. We're going to try monitoring interior humidity and nighttime dew point to see if the interior RH can be lowered below that dew point, maybe through the use of a dehumidifier during these shoulder seasons... I'm not sure if it'll work but worth a shot before tearing everything apart. I'm a little upset with the inspector for requiring an R value without warning them about creating an unvented assembly in too shallow rafter bays.
With less insulation and no effective venting to wick the moisture out of the insulation I suspect your problem could be worse. As you vent the sheathing it is probably going to be below dew point more often and condensation will form on the underside of it. If you are removing the metal I'd be tempted to cover the sheathing with several layers of foam, strap that and then roofing. This would keep the sheathing exposed to the insulation warmer than dew point so hopefully no condensation could form. The assembly would then need to dry to the inside. I could also see removing the sheathing, nailing 2x2's to the tops of the rafters to create a vent space, reinstall sheathing and creating a vented assembly. Another way would be to remove the roxul and sprayfoam the roof creating an unvented assembly. That's just .02 of armchair guessing, researching at buildingscience would be a good idea.
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Matt1945
Member
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# Posted: 18 May 2014 17:35
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Thanks for the reply Don, I was thinking of putting at least a layer of 1 inch foam panels on top of the roof sheathing then the 1 inch strapping over that then the new metal roof... was hoping that will be adequate, I realize not perfect but at least good enough for a weekend cabin, would you put the tar paper below the 1" foam board insulation? would you bother with radiant barrier or snow shield instead of tar paper?
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Don_P
Member
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# Posted: 18 May 2014 22:25
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I'm assuming the tarpaper is simply to redirect condensation and potential leaks from the metal out of the assembly, put it above the foam. I suspect the 1" foam isn't going to be thick enough to keep the sheathing above dew point. Keep the RH low in there.
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