Moisture control in unconditioned storage space under detached garage
Hello, we are building an unconditioned detached garage on a sloped lot in MD with a lower level that will be used as an additional garage space as well as storage for tools, power equipment, and even kids’ toys and bikes. We would like to do our best to prevent rust and and mold in that space, but we have struggled with figuring out the best approach. This lower level space will be a 25’x35’x7′ concrete cave with a slab on grade, a structural slab for a ceiling, and concrete walls that are on one end fully below grade, on the other fully above grade.
We will set a drain line for a dehumidifier just in case, but we would prefer to build the structure in a way that does not require running one. If there is such a way.
Question #1: Our thinking is to provide mechanical ventilation and eliminate condensation surfaces. Will that be enough in the mixed, humid MD climate to make this space useful for our intended purpose or there is no other way than to build it as air tight as we can and dehumidify?
Question #2: How to build it? What we have so far:
Floor slab: 4″ clean gravel + 2″ XPS foam board + poly vapor barrier under the 4″ slab to prevent moisture from coming up from ground as well as from condensating on the slab surface in warm months.
Footing: capillary break between the footing and the foundation walls (no gravel or insulation underneath the footing)
Upper level slab: no insulation since the garage above will be unconditioned
Concrete walls: the walls will be unfinished and just have exterior waterproofing. But what about insulation? Would it make any difference to try to keep the walls from becoming condensation surfaces? If so, would it make any sense to insulate just the below-grade part of the walls from the exterior with XPS? (full height exterior insulation is not appealing because it would cause trouble for the stone veneer; while interior insulation isn’t appealing because we assume it would need to get covered with drywall per fire code)
We have been struggling to find information on best practices for building unconditioned spaces, so any help would be greatly appreciated!
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