Plywood subfloor moisture damage & humid summers (Japan)- solutions?
I recently moved in to a 2013 standalone home near the coast in the Tokyo area. This area is apparently quite humid in summer. In the main bedroom on the first floor there are tatami (straw) mats on the floor. I was told that before I moved in this fall, what were new mats had to be replaced as the room got too humid over the summer. This room as a minisplit “aircon” in it but doesn’t appear to have been used in summer as the foam under the grill and condensate drain were in pristine condition when I opened it up to clean it.
Out of curiosity I pulled up the mats and saw that the plywood sheathing looked like it had gotten soaked. I ended up in contact with the builder who said that this area has a lot of issues with humidity. Their suggested solution was to put bags of charcoal down in the subfloor. To me that seems nuts as a solution. You’d need a lot of charcoal to soak up the moisture from humid summers and would have to size it right so that they expel the same amount of moisture in winter as they take in in summer. Otherwise they’d lose effectiveness quickly I think.
I haven’t yet seen what’s under the plywood but it’s hollow when I knock on it. The builder said there are pillars there and I assume another subfloor. I am not sure what’s that over but am guessing a concrete foundation. No idea where there is insulation.
So I have a few options. #1 is to do nothing as it’s not wet at the moment or weak from water (yet). #2 replace the plywood (fairly cheap) #3 replace the plywood and go for the charcoal bags (expensive) #4 other- replace plywood and install rigid or spray foam on the underside?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part