Solving a mold/moisture transfer challenge under an office shed
I have a 8′ x 12′ office that is a kit shed in the back yard. I purchased the house and the shed has a mold issue that is from below. I have removed the carpet and will remove the plywood subfloor to expose the treated 4x4s that form the base of the structure. Which right now sits on the ground which is covered with gravel. I plan to raise the structure and place it on 4″ thick 8″x16″cement blocks to prevent moisture transfer. And treat the 4x4s for mold. My question is four-fold:
1) I am not yet 100% sure the mold is only in the floor. Is there a way to locate mold in walls to rule that out without demo in the case the mold is from an external leak that isn’t showing/testing as moisture on the inside?
2) What would you treat the 4x4s with? RMR-86 was recommended.
3) I also considered cinder blocks with a cutout that the 4×4 would sit in, this would result in a lift height of 6″ vs. the 4″ for the solid blocks. What recommendations do you have for what I should put under the shed to make it more secure in an earthquake so it doesn’t slide off the blocks, and what is best to prevent moisture transfer?
4) What should I put on top of the 4x4s if the space inside is heated and cooled with minisplit with outside temp range from upper 40s to occasional high 80s (I am near ocean in zone 15-16/fog so less temp range). The 4x4s will continue to be open to the air to allow circulation from the sides of the structure. Should I put a vapor barrier on top of the 4x4s before subfloor and flooring? Ideally I would also like to put some rigid insulation in over the subfloor and top it with luxury vinyl planking, but have concerns about the moisture transfer the temp difference will create.
Thank you for your guidance.
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Replies
Duncan,
There are two mechanisms bringing moisture into the structure. Capillary action is moving moisture directly from the ground into the 4"x4"s, and water vapour is rising and being taken into the the floor by adsorbtion. The solution to both is the same: a non-permeable surface between the ground and structure above. I would suggest:
- Raise the structure and lay a sheet of 10mm poly underneath, making sure it slopes so it does not hold water, and does not extend out past the walls. Cover this with a thin layer of gravel.
- Take ground contact PT lumber (either 2"x4" or 4"x4"s), cover the tops with a strip of peel & stick membrane, and attach them to the underside of existing 4"x4" base.