It smells a little damp down there is a fair observation, but it’s hard to act on. A number is easier. If you own a raised-floor house in La Jolla, two measurements tell you more about the crawl space than a sniff test: the relative humidity of the air under the floor, and the moisture content of the wood framing. This post covers how to take both, where to take them, how to keep a log, and how to read the numbers without over-reading them.
This is a measurement guide. If you’re already seeing standing water, staining or soft wood, start with our wet crawl space warning signs post. If you want to know what vents, liners and closed crawl spaces are supposed to do, those are covered in vapor barriers in crawl spaces and crawl space encapsulation vs. repair. And before anyone goes under the house, read our access and working conditions post. Tight crawl spaces aren’t a place to improvise.
Two different numbers
Relative humidity (RH) describes the air. It’s the amount of water vapor in the air compared with the maximum it could hold at that temperature, expressed as a percent. Cool air holds less, so the same air reads a higher RH as it cools. That’s why a crawl space can read damper than the yard on the same afternoon.
Wood moisture content (MC) describes the framing. The USDA Forest Products Laboratory’s Wood Handbook defines it as the weight of water in the wood compared with the weight of the wood when fully dry, as a percent. Wood and air trade moisture back and forth. When wood is kept out of liquid water, “its moisture content is a function of both relative humidity (RH) and temperature of the surrounding air.” So RH tells you what the wood is being pushed toward, and MC tells you where the wood actually is.
You want both. Air readings change by the hour. Wood readings change slowly and are closer to the question you care about, which is whether the framing is staying dry enough.
Tools for the job
A digital hygrometer or temperature/RH data logger. A simple hygrometer gives you a spot reading. A data logger records on its own at set intervals so you can see the pattern over days and seasons. The EPA’s mold course notes that “humidity gauges may be useful for checking or monitoring humidity throughout the building.”
Read the spec sheet before you trust the decimal. As one example of how manufacturers state accuracy, Onset lists its HOBO MX1101 logger at “±2% from 20% to 80% (maximum of ±4.5% including hysteresis at 25°C or 77°F),” and says “Below 20% and above 80%, the accuracy is ±6% typical.” Its humidity range is listed as “1% to 90% (non-condensing),” with “a typical drift of less than 1% per year.” Other sensors will have their own numbers. The point is that a reading of 78% and a reading of 81% may not be a real difference, and a sensor sitting in condensation isn’t giving you a clean number at all. We aren’t recommending a brand. Use whatever you have and learn its limits.
A wood moisture meter. There are two kinds. Pin meters push two metal pins into the wood and measure electrical resistance between them. Pinless meters sit on the surface and read through a sensing pad. The Wood Handbook is blunt that “moisture determinations using electrical methods are always subject to some uncertainty,” and it explains why the two types read differently:
- With pins, “Uninsulated electrodes will sense the lowest resistance (highest conductance) along their length, thus highest moisture content level.” Insulated pins read only at the tips, so you can check the inside of a board.
- For an average reading, the Handbook says to “drive the pins about one-fifth to one-fourth the thickness of the wood to reflect the average moisture content of the entire piece.”
- With pinless meters, “the surface layers of the specimen influence the readings of dielectric (pinless) meters predominantly.” A damp surface skin can make a dry board look wet.
A thermometer, a flashlight and a notebook or phone. Temperature matters for both readings, and dated notes matter more than any single number.
Corrections that change the reading
Moisture meters are calibrated for a particular wood and temperature. The Handbook says to “Apply a species correction supplied with the instrument when appropriate.” For resistance (pin) meters it gives a rule of thumb of “adding or subtracting about 0.5% for each 5.6 °C (10 °F) the wood temperature differs from the calibration temperature.” Your meter’s instructions win over any rule of thumb.
Range matters too. Resistance meters read reliably from “about 6% to 30%,” and the Handbook warns that “Readings greater than 30% must be considered only qualitative.” Above that, a meter can tell you the wood is wet. It can’t tell you how wet.
Where to take readings
Pick spots and stick with them so your log compares like with like. A starting list:
- Outside, in shade. An outdoor reading at the same time gives you a baseline.
- Near the crawl-space access, and at the far end of the crawl space, away from vents and openings.
- Low spots and the uphill side, where water would gather or seep in first.
- Wood that touches or sits near concrete or soil: sill plates, the rim joist, post bases and the bottom of girders. The residential code’s decay section already flags wood joists “closer than 18 inches (457 mm) to exposed ground” as needing protection, so those are worth a reading.
- Under bathrooms, kitchens and laundry, where plumbing leaks show up first.
- A few spots in the middle of joists and subfloor for comparison.
Mark each spot with tape or a numbered tag and photograph it, so the next reading lands in the same place.
How to log readings
For air, set a logger in a fixed spot, off the soil, out of direct airflow from a vent, and leave it. Download it monthly. For wood, take spot readings at your marked locations on a schedule, at roughly the same time of day.
Write down the date, time, outdoor RH and temperature, crawl-space RH and temperature, the MC at each spot, the meter setting you used and any species or temperature correction you applied. Note the weather too: a marine-layer morning, a rainy week, a Santa Ana day.
How long? The Building America Solution Center, writing about crawl-space retrofits, suggests monitoring “preferably for one year, to track the crawlspace temperature and relative humidity conditions, and to test the moisture content of the rim and floor joists.” A full year makes sense here even without a retrofit, because coastal seasons swing.
The local baseline
Crawl-space air starts as outdoor air, so it helps to know what normal looks like here. The National Weather Service’s climate study for San Diego says “Relative humidity averages 69 percent at San Diego on an annual basis.” Over a day, “The average daily maximum relative humidity is 82 percent, generally occurring in the early morning hours around sunrise,” while “The average daily minimum is 54 percent, usually occurring around noon.” By season, “Humidities are higher than the annual average, by 10 to 20 percent, May through October,” and “Near 100 percent relative humidity is typical for the late nights and early mornings of the marine layer seasons of Spring and Summer.”
Those figures come from the airport-area station, not from La Jolla and not from under anybody’s floor. Still, they explain why a single summer-morning RH reading near the coast can look alarming. Compare crawl-space readings with outdoor readings taken at the same time, and look at the trend, not one spike. Our coastal moisture and drainage patterns post covers the water side of living near the ocean.
What the readings mean
This is where the numbers can look contradictory, because the sources give different numbers for different purposes.
Wood moisture content. The Wood Handbook says “Decay generally requires a wood moisture content equal to or greater than fiber saturation (usually about 30%) and temperatures between 10 and 43 °C (50 and 100 °F).” It adds that “To maintain a safety margin, a 20% moisture content is sometimes used during field inspections as the maximum allowable level.” Building America’s crawl-space guide is stricter: “The moisture content of the wood in the crawlspace must remain below 17%. Moisture content greater than 17% is of concern and should be investigated.” Read together, sustained readings above 17% are worth a closer look, 20% is a field-inspection margin, and readings near or past 30% mean wet wood. The Handbook also notes that “Wood installed in basements or over a crawl space may experience moisture contents greater than the range given” for indoor wood, so crawl-space wood reading a bit higher than your interior trim isn’t a surprise by itself.
Relative humidity. There isn’t one code number for the air in a vented crawl space. The EPA’s guidance to “keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity” is written for living space. A closed-crawl-space quick reference published at crawlspaces.org describes contractors guaranteeing RH “below an agreed-upon target (typically 70% or lower)” in closed crawl spaces. Those are targets for specific situations, not a pass/fail line for every house. For a vented crawl space near the coast, RH is best read alongside outdoor RH and the wood readings.
When numbers disagree. High air RH with dry wood can point to humid air passing through. Wet wood with moderate air RH can point to a local source, like a leak, a wet sill at grade or water at a low spot. The Wood Handbook notes that in crawl-space houses “The primary source of condensation is soil moisture.” Wet readings concentrated on the uphill side or at low spots are a reason to look at foundation drainage before anything else.
A reading is a clue, not a diagnosis. It can’t tell you whether wood is decayed, whether a sill is sound, or what to do next.
Questions worth asking
- Are my readings taken at the same spots and times, with the meter set the same way?
- How do crawl-space RH readings compare with outdoor RH taken at the same time?
- Is any wood reading above 17% for weeks, or above 20% at all?
- Are the wet readings concentrated on one side, at low spots or under plumbing?
- Does the pattern change with the marine layer, rain or irrigation?
- Should someone probe the wood and look at the framing in person?
Cost factors, not prices
We don’t publish prices. What shapes the cost of evaluating a crawl space with moisture readings includes the size and height of the crawl space, how hard it is to reach every area safely, how much framing needs checking, whether readings point to plumbing, drainage or both, whether wood needs probing or opening up to confirm condition, and whether the findings call for engineering input.
Where we fit
We’re a foundation and structural repair company serving La Jolla citywide, and raised-floor crawl spaces are part of that work. We can take readings, check the framing they point to and help you understand what your log is telling you before decisions get made. We aren’t engineers. We work with verified contractors and coordinate with licensed engineers as needed. This post is general education, not an assessment of any property. Our raised foundation repair and crawl space foundation repair pages explain what happens when readings turn into repairs.
Related evaluations & services
- Crawl space foundation repair: repair methods and process
- Foundation drainage: finding and moving the water
- Raised foundation repair: posts, piers, sills and stem walls
- Foundation inspection: getting an evaluation
- Earlier post: Wet crawl space warning signs
- Earlier post: Vapor barriers in crawl spaces
Next step
If your crawl-space readings are climbing or the wood is reading wet, request a Free Quote for an on-site visit anywhere in La Jolla, or call (650) 739-6537.
Sources (accessed October 31, 2026)
- USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material, FPL-GTR-282 (2021), Chapters 4, 13 and 17
- U.S. Department of Energy Building America, Guide to Closing and Conditioning Ventilated Crawlspaces (IBACOS, January 2013)
- U.S. Department of Energy Building America Solution Center, Water Management of Existing Crawlspace Floor
- crawlspaces.org, A Quick Reference on Closed Crawl Spaces
- U.S. Environmental Protection Agency, A Brief Guide to Mold, Moisture and Your Home
- U.S. Environmental Protection Agency, Mold Course, Chapter 3
- National Weather Service, NOAA Technical Memorandum NWS WR-270, Climate of San Diego, California (revised September 2004)
- Onset Computer Corporation, HOBO MX1101 Temperature/Relative Humidity Data Logger product page
- San Diego Residential Code (2025 California Residential Code as amended), Chapter 3, §R304.1, via UpCodes