Moisture meter checker
Crawl space wood moisture checker
Took a moisture meter under the house? Enter the reading and the wood temperature to see what it means for your joists, rim and sill plate, and how far to trust it. No meter yet? Start with the no-meter moisture test.
A reading tells you how wet the wood is, not whether it is still sound. It is not a rot diagnosis.
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Short answer
What should wood moisture be in a crawl space?
Crawl space framing should read about 15% moisture or less, and stay under 19%. From 20%, the wood has lost its safe margin against decay. At about 28% or more, liquid water is usually reaching it. Pin meters read low in cold crawl spaces and vary by species, so correct the reading before you judge it.
Checker
Check your moisture meter reading
One reading at a time. Take several around the crawl space and check the highest ones.
Your reading
Corrected moisture content
18.5%
Acceptable — watch it (16–19%)
Still inside the dry-service range lumber is designed for. Recheck in the most humid month.
- • Temperature: on wood at 60°F a pin meter reads about 0.5 points low — cold wood reads drier than it is. Your 18% has been corrected for that.
- • Species not confirmed: the true value is likely within about ±2 points (16.5–20.5%). That range crosses a band edge, so set the meter's species and read again.
- Wood settles at this moisture in air held near 86% relative humidity at 60°F — compare with wood moisture from your crawl space humidity.
What to do next
- • Check the air that is driving it: wood moisture from your crawl space humidity.
- • Read the same spots again in the humid season; a reading creeping toward 20% is the early warning.
Thresholds
What your reading means
Four bands, each tied to the source it comes from.
| Band | Moisture content | What it means | Where the line comes from |
|---|---|---|---|
| Dry | 15% or less | Where framing in a well-controlled crawl space usually sits, and the target after drying. | Drying target used in our treatment guide; framing lumber is dried to 19% or less before sale (KD-19). |
| Acceptable — watch it | 16–19% | Still inside the dry-service range lumber is designed for. Recheck in the most humid month. | American Wood Council NDS: 19% is the dry-service limit for sawn lumber. |
| Past the safe margin | 20–27% | Wetter than the 20% treated as a safe margin against decay. Mold can grow and existing decay can keep going. Find the moisture source. | USDA Wood Handbook ch. 14: wood dried to 20% or less and kept dry gives a reasonable margin of safety against fungal damage. |
| Wet | 28% or more | Around fibre saturation, where serious decay happens. Humid air alone rarely gets wood this wet, so look for liquid water. The exact number is no longer reliable. | USDA Wood Handbook ch. 14 (serious decay needs moisture above fibre saturation, about 30% on average); James 1988 (meters lose accuracy above it). |
Search this question and you will find 16%, 19%, 20% and 28% all given as “the” number. They answer different questions. 19% is the line lumber design uses between dry and wet service. 20% is the safety margin the USDA Wood Handbook gives against decay. Around 28–30% is fibre saturation, where serious decay happens. 16% is a comfortable target for a dried-out crawl space.
Humid air alone tops out near 27.7% even at 99% humidity, and wood reaches 20% only when the air around it stays near 89% humidity. A reading at 28% or more therefore points to liquid water: a leak, seepage, standing water, or wet insulation pressed against the wood. To see what your crawl space air does to wood, use the dew point calculator's wood moisture mode.
Meter corrections
Why the raw number can be wrong
Temperature first, then species. That is the order the USDA's moisture meter guide gives.
| Meter reads | Wood at 40°F | Wood at 50°F | Wood at 60°F | Wood at 70°F | Wood at 95°F |
|---|---|---|---|---|---|
| 12% | 13.5% | 13% | 12.5% | 12% | 11% |
| 16% | 17.5% | 17% | 16.5% | 16% | 15% |
| 20% | 21.5% | 21% | 20.5% | 20% | 19% |
| 25% | 26.5% | 26% | 25.5% | 25% | 24% |
Temperature. Pin meters are calibrated at room temperature, and wood conducts electricity less when it is cold, so a cold joist reads drier than it is. The USDA rule for pin meters: no correction between 70 and 90°F. Otherwise add 1 point for every 20°F the wood is colder than 70°F, or take 1 off for every 20°F warmer. In a 50°F crawl space, 19% is really about 20%, which moves it across the 20% line. Some meters correct for temperature themselves; check the manual before correcting twice.
Species. Pin meters are commonly calibrated for Douglas fir. For North American woods the USDA puts species corrections at usually under 2 points. If your meter has a species setting, use it (Southern Pine for most Southeastern framing). If not, the checker shows the reading as a ±2-point range and tells you when that range crosses a band edge.
Pressure-treated wood. Sill plates are usually pressure-treated, and salt-type preservatives make wood read high. Above about 10% the error grows erratically and can't be corrected, so judge a sill plate by the untreated rim or joist resting on it, and by probing it. Plywood and OSB glue lines can also skew pin readings.
Method
How to take a reading under the house
- Read the likely-wet spots: the sill plate, the rim joist, joists near vents, under plumbing, and under sagging or wet insulation. Also read a few spots that should be dry.
- Pins along the grain, driven to their full depth. For a joist that is about a quarter to a fifth of its thickness. Above 20%, readings across the grain can come out up to 2 points low.
- Away from board ends. End grain dries faster. The USDA guide says to read at least 50 cm (about 20 inches) from the end where you can.
- Dry the surface first. Condensation or dripping water on the surface makes any meter read too high.
- Repeat in the humid season and write down where each reading was taken, so next year's readings compare like with like.
Meter types
Pin vs pinless meters
A pin meter measures the electrical resistance between two pins. It works the same regardless of density and has a simple temperature correction, which makes it the better tool for framing. A pinless meter reads a signal through a flat pad. It is quick and leaves no holes, but it reads mostly the outer layer and depends on wood density, so its species setting matters more and wet surfaces fool it. Use pinless to scan for wet areas, then confirm with pins. The checker skips the temperature correction for pinless meters.
Limits
What this tool does not tell you
Not strength
Moisture says nothing about strength already lost. Wood that is soft, crumbly or lets an awl sink in is damaged at any reading.
Not the core
Pins read the outer part of a timber. A beam can be wetter inside than at the surface.
Not a single spot
One reading can be a local wet spot or an error. The pattern across several spots matters more.
Not treated wood
Above about 10%, readings on pressure-treated wood can't be corrected.
Not panels
Plywood and OSB readings are only useful for comparing wet and dry areas.
Not mold
Mold can grow on surfaces in humid air even when the wood reads dry. Check the air humidity too.
What happens next
What to do with your reading
| Corrected reading | Usual next step |
|---|---|
| 15% or less | Nothing urgent. Keep crawl space humidity where it is and recheck next humid season. |
| 16–19% | Check the air humidity driving it; recheck the same spots in late summer. |
| 20–27% | Find the moisture source, then dry the framing. Probe the wood for softness. |
| 28% or more | Look for liquid water first. Probe the wood; soft wood needs repair, not just drying. |
For the moisture side, see the humidity range that keeps framing dry and drying out the crawl space. If the wood is soft, read the signs of wet rot vs dry rot, the treatment steps, in order, and repairing a soft or damaged joist.
Related
Related pages
How to test a crawl space for moisture
The no-meter first check: hygrometer and plastic-sheet test.
OpenDew point calculator
Condensation on ducts and joists, and the wood moisture your air drives toward.
OpenCrawl space wood rot
Wet rot vs dry rot, the causes and the signs.
OpenHow to treat and prevent crawl space rot
Stop the water, dry the wood, then treat or replace.
OpenFloor joist span chart
What damp wood does to a joist's span, plus notch limits.
OpenHigh crawl space humidity
What sustained humidity above 60% does to framing.
OpenFAQs
Wood moisture readings: common questions
What should a moisture meter read on crawl space floor joists?
About 15% or less is dry, and up to 19% is still within the dry-service range framing lumber is designed for. From 20% the wood has lost the safety margin the USDA Wood Handbook gives against decay, and at about 28% or more liquid water is usually reaching it. Correct a pin meter reading for temperature first: in a cool crawl space it reads low.
What is a high moisture reading for wood framing?
Anything at 20% or above, held for weeks. Readings of 28% or more mean the wood is near fibre saturation, which humid air alone rarely causes, so look for a leak, standing water, seepage or wet insulation against the wood. One high spot is a reason to take more readings, not a diagnosis.
Are wood moisture meters accurate?
Within a point or two in the range that matters, if you use them correctly. The USDA Forest Products Laboratory's guide to moisture meters lists what throws them off: wood temperature (pin meters read low on cold wood), species (corrections are usually under 2 points for North American woods), salt-type preservative treatment (reads high), surface water, and readings above fibre saturation, where accuracy falls away.
Pin or pinless: which is better for joists?
A pin meter. It measures between the pins at a known depth, needs no density setting, and the USDA guide gives a simple temperature correction for it. A pinless meter is faster and leaves no holes, but it reads mostly the outer layer and depends on wood density, so use it to scan for wet areas and confirm high readings with pins.
Does wood moisture change with the seasons?
Yes. Wood slowly trades moisture with the air around it, so framing in a vented crawl space usually reads highest late in a humid summer and lowest in winter. At 70°F, air held near 89% relative humidity brings wood to about 20%. Take readings in the humid season and compare the same spots year to year.
Why are my floor joists wet?
Usually one of three routes: liquid water (a plumbing leak, seepage or standing water), ground moisture rising from bare soil, or humid outdoor air reaching cooler wood in summer. Wet insulation held against joists keeps them wet too. Where the wet readings cluster usually points to which route it is.
Sources
Sources and method
- James, W.L. (1988). Electric Moisture Meters for Wood. USDA Forest Products Laboratory, FPL-GTR-6. Temperature correction rule (p. 13), species corrections (p. 12), treated wood and plywood (p. 14), grain direction, board ends and pin depth.
- Arango, R.A., Lebow, P.K. and Glaeser, J.A. (2021). Chapter 14: Biodeterioration of wood. Wood Handbook, FPL-GTR-282. Serious decay needs moisture above fibre saturation (average 30%); dried to 20% or less and kept dry gives a reasonable margin of safety.
- Glass, S.V. and Zelinka, S.L. (2021). Chapter 4: Moisture relations and physical properties of wood. Wood Handbook, FPL-GTR-282. Equation 4–5, used for the equivalent humidity line.
- American Wood Council, 2018 NDS Supplement: 19% moisture as the dry-service limit for sawn lumber.
- Method: the checker applies the temperature rule to pin meters, then shows a ±2-point range when the species setting isn't confirmed. It never invents a species offset. All figures on this page are computed from the same module as the checker.
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