Vented roof assembly for tiny house
I am building a tiny house on a trailer in southeast BC (climate zone 6). I am seeking some advice on how to protect the vented roof assembly from condensation.
The 2×8 rafters span the 8 foot trailer width. They slope only 1/2 in 12, the low slope chosen to maximize headroom in the lofts while keeping within height restrictions. The roof membrane is liquid applied EPDM over plywood sheathing. I’ll have about an inch gap for air flow below the sheathing between continuously vented eaves on either side. I understand that with such a low slope, the stack effect won’t help me much to push air through so I’m relying on wind and pressure differences between the two sides, and I thought the short distance between eaves probably helps too. Because I know this isn’t optimal, I would like to do what I can to guard against condensation on the underside of the sheathing.
My plan is to use Intello smart vapour/air barrier between the ceiling and rafters/insulation and not have any pot lights or unnecessary protrusions through the ceiling, and to carefully air seal those that are necessary. I’m using Havelock Wool batt insulation in the stud/rafter bays. To ensure the ventilated roof gap is maintained, and to avoid air currents through the batt insulation, I thought to use rigid mineral wool boards cut to rafter bay widths and with 1″ thick blocks fastened to the top surface here and there to maintain the space and contain the wool batts below.
(Does that make sense? So from the inside: ceiling (probably t&g ship lap), Intello, 5.5″HavelockWool, 1.25″ Comfortboard, air gap, sheathing, EPDM.)
My questions are about air and bulk water barriers between the insulation and air gap.
– I know it’s not an air barrier, but will the rigid mineral wool work well enough to prevent air currents penetrating my insulation? Or should I use a vapour permeable air barrier membrane between the insulation and air gap, air sealing the insulation from both sides?
– I’ve heard of rigid mineral wool serving as a drainage plain behind a rain screen on a vertical wall, but with the low slope, perhaps an air barrier membrane would also improve bulk water drainage.
– Any recommendations for such a membrane? Just seam-taped house wrap or something more high performance/technical?
– And lastly, is there a benefit to protecting the underside of the wood sheathing from the condensation? Like painting it with a wood sealer? Or will sealing the wood on both sides be worse, since if it does get wet, it won’t be able to dry?
Any input will be much appreciated.
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