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Community and Q&A

Gambrel Ventless Roof

PapaK | Posted in Energy Efficiency and Durability on

Hello everyone. I have searched the forum and have not found an answer so I apologize if this has been covered. 

I have a large shed (10×16)that I am turning into a bunky. This isn’t to any code so I am looking for answer for best practices that fit into the existing structure. I am near Edmonton Alberta.

The structure is 2×4 walls with 2×4 horizontal purlins. The gambrel roof is built the same way. I am in the process of adding steel siding and roofing to it. My plan was to fill the wall cavities with Roxul insulation and add 1.5” of roxul behind the studs where the purlins have created a gap to reduce thermal bridging. The exterior would be covered with typar over the purlins and the 26gauge steel siding attached directly to that. The interior would have poly vapour barrier and this space won’t be air conditioned. 

While reading about ventless roofs, it seems like a big issue is the possibility of wet OSB when warm air hits the cold steel, condenses, and falls onto the the OSB. 

Since this bunky isn’t to code, there will be no OSB. Would it be foolish to build a ventless roof in the same way I described building the wall with typar and the steel attached directly to it? Due to the steel’s 3/4” ribs there is minimal venting (I realize 3/4” isn’t ideal, I could add furring strips). I was planning for my interior vapour barrier on the ceiling potentially being 1” durofoam gps for a bit more R value 

This is an existing structure so I’m not necessarily looking for the perfect solution just something that gives me a low probability of future head aches. 

Thanks for your time. I’ve definitely learned a lot from this site. 

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  1. Expert Member


    Unfortunately unvented cathedral roofs with permeable insulation accumulate moisture. That usually occurs on the sheathing because it is the first surface cold enough for water to condense on, but if there is no sheathing it will happily accumulate on the roof underlay, strapping, or top side of the rafters. The activities in Bunkies don't produce much moisture, and I presume it won't occupied all the time, but it's still a risky roof assembly.

    Just a note on the code because you mention it twice. You may not have permits or enforcement where you are, but the Alberta Building Code covers the entire province. Buildings where people sleep have specific requirements related to safety, so you may want to make sure you meets those whether it is inspected or not.

  2. walta100 | | #2

    Can you make one of these 5 options work for you?


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