What would cause drywall moisture to raise during the day and drop at night?

Question

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?

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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.

Answer 1

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.

Answer 2

I have the same moisture meter and when held against the drywall it reads OL with all the red bars lit up in all walls that face outdoors.. on interior walls that are not exposed to the outdoors on the other side, the meter reads 10 to 15 %.. i had a licensed specialist come and their fancy meter did verify the OL readings I had were accurate.. They also used a thermal imaging meter which helped locate a few water entry points in the external siding joints.. After about 50 tubes of caulk and 15 gallons of kilz primer and 2 semi gloss top coats, i feel confident water can no longer penetrate the exterior siding, however my meter still shows 100 % moisture on exterior facing drywall.. one interesting thing to note, is that on the 2nd floor of the home, i get normal readings of appx 15%.. only the 1st floor drywall seems to be affected.. I haven't figured out why yet.