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Control layers for unconditioned space

darrin_k | Posted in General Questions on

I’m planning to build an off-grid unconditioned shed that may be used for rough camping a few nights each summer and into the spring and fall shoulder seasons as temperatures permit. This is a temporary (5-year?) plan in anticipation of a permanent home on the site. I do want to insulate sumewhat, though not necessarily to code levels for a conditioned building (CZ6A). My main concern is mold and milkdew accumulation common in sheds and cabins that are shut tight for long periods of time resulting from the increase in relative humidity as temperatures drop.

The plan is a 12’x16′ shed with cathedral ceiling allowing for a loft above on an extremely tight budget.  The walls will be (from outside in): T1-11 as siding and structural sheathing (I know…), Tyvek directly on 2×4 framing with R15 mineral wool batts.  Then it seems I’d want a vapor retarder.  A smart vapor retarder (SVR such as Intello) might open up too much when temps drop due to increase in RH, correct? I won’t have any drywall and will probably do plywood or shiplap so assume open seams not airtight for the interior finish.

The roof design is less settled, but assuming mineral wool-filled rafter bays in an unvented cathedral ceiling with Zip sheathing and a vapor diffusion port at the ridge.  The SVR would be on the ceiling also, of course.

I know I’ll have to control humidity and will need to use large desiccant pouches (any experience with these?) when the shed is shut tight.  Maybe that works, maybe it doesn’t.

1. Am I creating more opportunities for mold or fewer?
2. Can mineral wool be in direct contact with the Tyvek on the walls when there is no sheathing?
3. Can T1-11 be used as a rain screen with 1/2″ strapping directly over the Tyvek-covered studs?  (Assuming nail length is increased to provide the proper attachment to the studs).  Am I missing something structurally with that? Or other problems?  Would it even be necessary?

Thanks in advance for any insights.  I can follow the science for conditioned spaces well enough, but unconditioned spaces leave me baffled for some reason.

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  1. Jon_R | | #1

    > mold and mildew accumulation common in sheds and cabins that are shut tight

    More data on unconditioned spaces is needed. My guess is a) vent it really well or b) insulate and air seal to some extent, block ground moisture and use some south facing windows to frequently warm it up (which will dry it out).

    Not what you asked for, but a solar panel/inverter/dehumidifier could also work.

    > large desiccant pouches

    I doubt they have sufficient capacity.

    1. darrin_k | | #2

      Thanks Jon. Venting in the summer months should be easy enough. And yeah, I’m not sold on the claims of the “XL” desiccant pouches,but others have said they work. Each $8 pouch is supposed to be good for 1000 cubic feet for a few months. Putting out 2 or 3 when I shut up the shed won’t hurt though.

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