Hi, everyone. I’ve been visiting this forum for a while while doing some renovations on a small off-grid cabin in southern Vermont that was built very badly and recently. Now I am building an accessory outbuilding for the first time, and inferring the right thing to do with the roof from existing posts seemed like a recipe for disaster. So I’d appreciate some guidance. 

 

The structure will be an outhouse with a self-contained Nature’s Head composting toilet and on-demand propane shower (unvented) and eventually propane wall heat (vented). When the shower is on for three or four minutes at a time, the windows will be cracked for air circulation. 

 

The structure will have an exterior footprint of 8×8′ on a platform that’s 8×10′, with those extra two feet reserved for a step-up deck. Ground contact 6×6 posts resting on tamped-down glacial soil about 24″ deep, band joists sitting on notches in the posts and lag-bolted in. (Lots of interesting salvage and reuse in the construction, but that’s a story for another time.) 

 

My big question here concerns moisture and ventilation as it pertains to the choice of roof. The building will be heated and used for no more than 4–5 days at a time by a couple of people in any season. At the same time, this is a very wet part of the Northeast lately (5b). Exterior moisture is a potential issue, as is the interior moisture generated by the shower in colder weather. Warmer weather there will be an outdoor shower that will take some of the demand off the interior. The building is in the woods and will get a modest amount of direct and indirect sun during the day. The cabin (nearby) gets less direct and indirect daytime sun and gets mildewy if you leave it for a long time. 

 

1. Should there be any interior vapor barrier in the walls over the R19 insulation? 

 

2. What the heck to do with the roof. I am planning a shed roof with 4.6:12 (21º) pitch and steel or aluminum over tar paper and OSB, and overhangs of about 12″ except going to 24″ over the exposed deck section. I am planning to do rigid 250 foam board above the subfloor between the stud walls and leave the joists open from beneath. When it comes to the roof, I cannot figure out what the best plan is for this scenario, and I have read around this board a fair amount. Options with proper soffits, options with no soffits, options with rigid board, options with fiberglass bats. What’s the sweet spot for decent moisture management with decent insulation and not an extreme level of complexity? 

 

Thanks very much.