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Community and Q&A

Plywood subfloor moisture damage & humid summers (Japan)- solutions?

rsmith02 | Posted in General Questions on

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?

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  1. user-2310254 | | #1

    I would start by buying a humidity sensor to track the indoor conditions along with running the minisplit. The minisplit should be able to lower the humidity level and may even have a dry mode for that purpose.

    It sounds like your home is built over a crawlspace. I would be curious to see what this space looks like since it may be contributing to your indoor problems. Do you have access?

    1. rsmith02 | | #5

      Hi Steve, thank you very much for this. I'll do that and keep a close eye on moisture levels. The previous occupant didn't seem to have used the AC downstairs and the house was left closed up for at least one summer. Yes, it will dehumidify and has a specialized dry mode.

      From what I can tell the house is built over a concrete slab. I'm thinking of trying to remove one of the plywood sheets to see what's under it exactly and what the condition is. This area gets a lot of rain in rainy season. There are small wells around the house (for what? unclear). There's no real land I could do any landscaping with, only a few feet to the next house.

  2. richmass62 | | #2

    Humidity must be coming from the ground. I had a full height basement and was able to replace the slab and seal the walls with thoroseal. In your case you might be able to dry out the crawl space with fans, treat the subfloor by cleaning off mold from the underside and then applying mold retardant, then sprayfoam (if it is a cold floor) or just fire resistant white poly (if the issue is just moisture). Also do you have access to dig down under the house? Some people have addressed this by creating a full height basement and a slab (could be just for a portion of your basement) where you could install outlets, dehumidifiers, whatever as long as you don't have water coming in.

    Fyi here is a link to my posts.

    1. user-2310254 | | #3


      Keep in mind that Roger is in Japan, which has a disposable-home culture.

    2. rsmith02 | | #7

      Hi Rich, I think there is less than 3 feet between this subfloor and what I believe is a concrete slab (built 2013). There's no accessible space I can get into but I want to lift a plywood board and see what it's like down there. Water table is likely high (we have a well) and nobody has a basement around here.

      Thanks for sharing your story, I'm reading through it and all you went through!

      Do you have a reference for white poly? Is that a sprayfoam too? What do you mean by cold floor? Cold from interior AC or from the cooler ground temps?

  3. Expert Member


    Steve is right. Japanese buildings are seen as short-term commodities and for some reason japanese culture had always had a blind-spot about insulation. The problem may well reoccur, but so far I don't see anything that necessitates more than cosmetic remediation. Wash the area with a beach solution, and perhaps encapsulate with something like Concrubium if it is available there. Skip the charcoal.

    1. rsmith02 | | #6

      Yes, building codes here are weak and the industry is only starting to do anything remotely green. Now I see home builders differentiate themselves as no gaps/leaks, event temperature rooms and net-zero all electric homes. That wasn't the case a few years back.

      This particular home wasn't built cheaply, but I think their understanding of moisture management was lacking. If excess moisture is the problem I'd prefer to limit the ability of moisture to build up below the first floor if that's possible to solve for a reasonable cost. The builder's representative who came to visit thought keeping the room used and ventilated was key. She suggested to just keep checking these boards and when they grow weak from moisture to swap them out from time to time.

      I did find online contractors that say they do antiseptic mold treatments, ventilation and even spray foam to deal with mold under the floor. (example: I wish I knew green builders here.

      If I can't find a way to solve, I think I'll swap out the plywood (apparently just $20/board) and do some kind of antifungal treatment. I'll control indoor moisture with the minisplit, though worry that if the floor gets cool, it might cause moisture to condense on the underside.

      1. Expert Member
        MALCOLM TAYLOR | | #8

        Sounds like a reasonable plan. Good luck!

        Over Christmas I re-watched a couple of Kurosawa movies, and all I could think was how cold everything looked in the traditional houses.

        1. rsmith02 | | #9

          Thanks Malcolm. Do you think the cool room surface and underside of the plywood is an issue?

          Yeah, the old homes are lovely but those mats over dirt floors invite insects in and it looked as hot as anything in summer. The sliding shoji doors are to let air pass through the entire house. More than winter I remember the bare light bulb and people wiping their brows in the hot and humid summer. I have lived in Japan's northeast, with a much cooler climate than Tokyo, and those old houses have big hearths in the middle to dry out the thatch roof and for the smoke to kill insects. I wish the craftsmen who managed to build wooden homes and temples that could survive centuries were still in the housing business today!

          1. Expert Member
            MALCOLM TAYLOR | | #10


            I build in the Pacific Northwest. I have a fair to middling understanding of this climate, and almost no useful kn0wledge about warm humid ones. Hopefully other posters will chime in.

  4. Deleted | | #11


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