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There is a portion of drywall to the side of a window in my bedroom (4' W x 5' H) that was saturated with moisture (my pinless moisture meter calibrated for drywall would read 100%). Not sure when the moisture started as the wall does not look discolored, does not feel soft or moist to touch (I've only discovered because I bought a moisture meter). I've had a water mitigation company out, who recommended I get some dehumidifiers and fans and dry out the wall. Rented 19G/day dehumidifier, ran for 4 days, was able to drop all moisture to 0% except for a small 2" x 4" spot by the window to 15%.

Since then moisture in a 0.5'W x 1.5'H area by the window had been fluctuating: raising during the day, and falling during the night. I measured at 9 pm and 8 am. Went from 20% evening 15 % morning to 30% evening 18% morning, and now 70% evening 30% morning. Air humidity in the house is 65% (San Diego), all windows are always open and house is well ventilated.

I'll give water mitigation company a call for advice on next steps, but would like to understand this better myself and get a second pair of eyes.

What would cause the moisture to increase during the day and drop at night? Feels counterintuitive. It hasn't rained in a few weeks.

Is this something I can solve by just dehumidifying longer? Should I let it be? Should I cut out a piece of drywall and see if anything inside needs drying?


Exterior is stucco, there are some visible hairline cracks a few inches above where the moisture is. I have stucco contractors scheduled to patch that out in a few months. We haven't had heavy rains in ~5 months (San Diego).

There are no leaks in the house, had plumber out.

House is on the raised foundation. There was some clean standing water in crawlspace and mud, possibly from the rain or ground water rising up. I've had the water pumped and mud mixed with absorbent sand to be taken out. Neighbors say water table is pretty high and they all had similar issue, I'll grade the dirt in the crawlspace and put up a sump pump.

House is built in 1927, but well maintained overall, framing is redwood.

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    I don't mean to minimize your concern, but this sounds like a solution in search of a problem. If you would edit your question to state what the actual demonstrable problem is, you may get some answers. If the only problem is that your admittedly amateur-level moisture meter gives a higher than expected reading, and that any number of water mitigation companies are willing to take your money and do nothing, then the solution to the problem is to stop measuring moisture.
    – MTA
    Commented Aug 14, 2023 at 16:12
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    @MTA Hey thanks for the comment! The water mitigation company came out with their equipment and have confirmed my readings. The problem is that there's a high level of moisture in drywall: from what I understand drywall shouldn't be getting moist past 17%, but there's no damage I can see. I don't have DIY skills or sense yet, and I don't know if I need to be concerned, or wet drywall isn't a problem. I've updated my question more explicitly to ask "should I be concerned or should I leave it be". Commented Aug 14, 2023 at 16:32
  • Wet drywall is a concern, usually leads to replacement. Moisture/humidity on the painted surface of drywall is common. Are your readings inside the drywall?
    – crip659
    Commented Aug 14, 2023 at 16:42
  • @crip659 This is the moisture meter that I use, so the readings are on the outside of the drywall: amzn.com/dp/B07SZX8QXH Commented Aug 14, 2023 at 17:26
  • @crip659 Actually since I'm learning more now, it's a pinless meter, so the readings penetrate the drywall from what I understand. Commented Aug 14, 2023 at 18:54

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Wet drywall would be a concern, but your wall is not wet. You are getting an instrument reading that does not comport with your own observation that the wall is actually dry.

Gypsum, the main component of drywall, is a compound of calcium sulfate, with each molecule bound loosely to 2 water molecules. The normal state of gypsum when exposed to an atmosphere with any humidity at all is to be fully saturated with water. "Saturated" in this sense means that the calcium sulfate can't accept any more water; it doesn't mean that it's moist or dripping.

The hydration is reversible. Heating gypsum drives off some of the water, and the bulk gypsum can then serve as a dessicant to absorb water from the air. Eventually, all gypsum will return to 100% saturated when exposed to the atmosphere.

You mentioned different readings between night and day. Your moisture meter is probably measuring electrical conductivity. Conductivity of solids is strongly affected by temperature, and your amateur-level moisture meter may not be compensating for temperature.

Want to find out if this is why you get different readings night and day? Obtain a small piece of drywall, say a 4-inch square, and leave it somewhere in your living space to stabilize for 24 hours. Take a moisture reading with your meter. Now wrap the piece tightly in aluminum foil so that it can neither gain nor lose moisture. Place the piece in the refrigerator for one hour. Now rip off the aluminum foil and immediately measure the moisture.

Does it read lower? If so, your meter is not compensating for temperature. The moisture level has not changed -- you prevented that with aluminum foil -- but the reading has changed.

If your bedroom wall is not actually damp, the readings that you are getting are of no concern.

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  • Thank you! I believe the pinless meter I use detects via an electromagnetic field - amzn.com/dp/B07SZX8QXH. I'll keep an eye on the drywall state visually to see if things change. Commented Aug 14, 2023 at 18:15
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    @RuslanOsipov Oy, I wish you had said that before I wrote an answer about conductivity (pin-type) meters. You are measuring capacitance, but there are 3 problems: (1) The instructions say the material must be at least 3/4" thick and you don't have 3/4" drywall in a bedroom. So you are also measuring something behind the drywall. If it's metal foil or a duct, readings are NG (2) The instructions say avoid areas with energized wires. Any wires where you're measuring? (3) It says drywall measurements are relative, not absolute. So you're not measuring moisture content.
    – MTA
    Commented Aug 14, 2023 at 18:32
  • @RuslanOsipov So the bottom line? My advice is: Fuhgeddaboudit!!
    – MTA
    Commented Aug 14, 2023 at 18:35
  • Ah, thank you! I'm new to the world of moisture meters and didn't realize the types matter! (1) Hope there's no wet insulation or anything... (2) There could very well be wires there. (3) Re: relative vs absolute - it's definitely reading much higher than the surrounding drywall, and the shape of the higher moisture area changes over time (it's not always in the same spot). What I'm hearing is - if the area doesn't grow much, I should forget about it. If the readings in the surrounding area increase in size and the spot grows in size by a few times - I'll investigate further. That right? Commented Aug 14, 2023 at 18:49
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    @RuslanOsipov Sounds good to me. But if this is causing anxiety or you're losing sleep over it, the best approach is the direct approach: remove the drywall and directly examine inside the wall. Then you can either fix it or forget about it and stop worrying about it. (And then repair the wall, of course.)
    – MTA
    Commented Aug 14, 2023 at 19:03

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