New outbuilding, mold, crawl space encapsulation/ventilation plan
I’m building a 190sf art studio/office in Zone 5B (Central Oregon, drier/sunnier eastern side of Cascade range, sandy/volcanic ash soil, 24″ frost line, ground freezes here). It has a one-year-old vented crawl space that developed mold beneath the plywood subfloor and I-joists. It sat with no roof and no ground moisture barrier through last year’s snowy winter. Last Spring, I scrubbed the mold off with diluted bleach and spread 6mil poly over the ground, but did not attach the poly to the stem wall or seal up the vents.
A year later, it’s closed in, unoccupied, un-insulated, no drywall, foundation still vented. Mold has re-appeared in the crawl space, and also seeing some on the above floor studs, interior OSB wall sheathing, and a little above on the TJI rafters. I assume my moisture problem is due to the ground, the stem walls themselves, and the four foundation vents. No rain leaks as I can still see all the framing. Crawl space currently averages 35° with 65% RH. Outside, it’s 45° and 30% RH. Moisture meter reads 15-18%.
Walls – R21 faced fg batts.
Cathedral ceiling – R30 unfaced fg batts with air-sealed site-built 2″ ventilation channel (have bird block vents and full ridge vent).
Floor – Was to be fg batts in the joists but, if I’m on the right track, I should change that to XPS/EPS on the interior foundation stem wall and also cut and cobbled into the rim joist.
Drywall – Airtight method.
HVAC – No plans for furnace/AC or HRV/ERV.
As this isn’t a house, I’m less concerned with R-value efficiency/human comfort as I am with affordably solving the moisture issues. Ideally I’ll be able to leave this building unattended, without constant conditioning, only heating with a space heater as needed when in-use. Rarely hot enough here to warrant AC, and it has plenty of operable windows. To exhaust oil painting art studio odors when I can’t open a window, I’ve planned a wall bath fan that exhausts directly out the 2×6 exterior wall (70 cfm, 0.1″ static pressure, DC motor rated for continuous use).
1) Seeing as this is not a dwelling, is it still recommended to encapsulate/condition the crawl space?
2) For crawl space ventilation without HVAC, would the sensible choice be a floor-mounted air register along with an exhaust fan positioned over one existing foundation vent? Would it have to run constantly even when the building is not in-use? Is this necessary?
3) Will the crawl space exhaust fan conflict with the painting odor bath exhaust fan?
4) How thick does the foundation interior foam need to be? R15? Same R-value in the rim joist, or bump up closer to walls/R20?
5) In cathedral ceiling, is it wise to add a vapor barrier (such as MemBrain) between drywall and unfaced batts?
After research over the last few months leading me here, I acknowledge my naivety and mistakes (went on a local builder friend’s old-fashioned advice of “houses need to breathe”). Building science is fascinating, thank you all for this great resource.
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