Wood Moisture Content: Why 16% Matters

The wood moisture content mold threshold most restorers work to is 16 percent. Below that, wood is too dry for mold to grow on it. Above it, and especially above 19 or 20 percent, mold can take hold if the wood stays that way for more than a few days. That is why we keep drying equipment running until framing reads under 16 percent, even when the room has looked dry for days.

What wood moisture content means

Moisture content is the weight of the water in a piece of wood compared to the weight of the wood itself when completely dry. A stud at 12 percent is carrying water equal to 12 percent of its dry weight. Wood always holds some water. It is never at zero inside a house.

Wood also trades moisture with the air around it all the time. Left alone in steady conditions, it settles at a moisture content that matches the humidity of the air. In round numbers, at normal room temperature:

• Air at 30 percent relative humidity, a typical heated house in January: wood settles near 6 percent.
• Air at 50 percent: wood settles near 9 percent.
• Air at 65 percent: wood settles near 12 percent.
• Air at 80 percent: wood settles near 16 percent.
• Air at 90 percent: wood settles at 20 percent or a little higher.

Those pairs are the key to the whole subject.

Where the 16% wood moisture content mold threshold comes from

Mold does not need liquid water. It needs a damp surface. Building science has long put the line at roughly 80 percent relative humidity in the thin layer of air right at the surface of a material. Hold a surface there for long enough, at ordinary temperatures, and the common household molds can grow on it.

Look back at the list. Wood in balance with 80 percent humidity sits at about 16 percent moisture content. So a stud reading 16 percent is telling you the air at its surface is at the edge of what mold needs. A reading under 16 means the surface is too dry. That is the whole reasoning behind the number.

You will see other figures quoted, and they do not conflict. Lumber is graded as dry at 19 percent or less, and many guides say mold growth becomes likely at around 20 percent. Those describe where trouble is expected. Sixteen is where trouble becomes possible. Drying to below 16 leaves a margin for the spots the meter did not land on, and for the fact that readings vary a little with species and temperature.

Time and temperature matter too

Moisture content is one of several conditions. Mold also needs time, a temperature it likes and something to eat. Wood, the paper on drywall and household dust are all food, so that condition is always met.

A stud that spikes to 25 percent after a spill and is dried back down within a few days will normally be fine. A joist that sits at 18 percent all summer in a damp crawlspace is a different story, even though it never looked wet. Warmth speeds things up, which is why wet materials in a heated house can show growth within a couple of days, while a cold attic can stay damp for weeks in winter before spots appear.

How framing gets above 16% without a leak

Plenty of the mold we see has no burst pipe behind it. The wood simply spent too long in humid air or against a cold surface. Common examples around London:

• Attic roof sheathing in winter. Warm, moist house air leaks into a cold attic and condenses on the underside of the roof. The sheathing can sit well above 16 percent for months.
• Crawlspaces and unfinished basements in summer. Humid outdoor air meets cool framing and concrete, and the joists and rim boards slowly climb.
• Finished basement walls. Wood framing tight against cool foundation walls, in clay soil that holds water, picks up moisture from the concrete.
• Bathrooms without a working exhaust fan, and around windows that sweat in cold weather.
• Slow leaks. A drip at a shower valve or a weeping drain joint can hold one stud bay above 16 percent for years.

What to do if your wood reads above 16%

First, confirm it. Take several readings, a few centimetres apart and on neighbouring pieces, and compare them to wood in a part of the house you know is dry. One odd reading can be a knot, a nail or a stain.

Second, find the source. Wood does not get damp by itself. It is a leak, condensation or humid air, and drying the wood without fixing the cause only resets the clock.

Third, bring it down and check that it stays down. After a leak that means dehumidifiers and air movement until the readings pass. For a humidity problem it means ventilation, air sealing or a dehumidifier that runs through the damp season, with indoor humidity held at around 50 percent or lower in summer.

If you see staining or fuzzy growth on wood that reads high, leave it undisturbed and get it looked at. A single reading with a date written beside it is a good start. A second one a week later tells you which way things are heading.

Common questions