Why wood on a Gulf Coast wall moves, and how much

Wood never stops trading water with the air around it. Leave a board in a room long enough and it settles at a moisture level set by that room’s temperature and relative humidity, a value foresters call the equilibrium moisture content, or EMC. On a coast where summer air is heavy, that settling point is the single most useful number to know before wood goes on a wall.

What sets a board’s moisture content indoors?

Mostly indoor humidity. The Wood Handbook explains that interior wood follows the room’s relative humidity, which in turn depends on moisture sources, ventilation, air conditioning and the weather outside¹. Temperature plays a smaller part. At 70°F, the Handbook’s EMC table climbs from 6.9% at 35% humidity to 17.9% at 85%¹.

Source: Wood Handbook Table 4–2. Values were derived mainly for Sitka spruce, and the Handbook says they can be applied to other species for most practical purposes.

Where does Florida fall on the federal map?

The Handbook’s Figure 13–1 splits the country into three zones for interior wood: about 6% in the dry Southwest, about 8% across most of the country and about 11% along the damp, warm coastal belt that includes Florida. Its Table 13–2 gives individual-piece ranges for interior woodwork of 6–10%, 4–9% and 8–13% respectively¹.

One caveat matters on this stretch of the Gulf Coast. The Handbook says these recommendations come from well-ventilated buildings without air conditioning, and that in air-conditioned buildings moisture depends largely on how well the system is sized¹. A tightly air-conditioned house in Wellen Park may hold drier than the coastal figure; a seasonal home with the thermostat set high while the owners are away may run wetter.

What indoor humidity should you aim for?

The U.S. Environmental Protection Agency advises keeping indoor relative humidity below 60%, and ideally between 30% and 50%, mainly to discourage mold². Read against the table above, that band corresponds to wood moisture of roughly 6% to 9% at 70°F. A home that holds steady in that range gives a wood wall the calmest life.

How much can a whole wall move?

Here is a worked example using Handbook Equation 13–2. Picture a wall 143 inches wide, a little under 12 feet, covered edge to edge in 26 boards of 5.5-inch face width, and suppose the boards go up at 8% and later settle at 11%¹.

Worked out for this guide: 143 in. × coefficient × 3 percentage points. The Handbook notes the method tends to overestimate width change for boards that are not purely flatsawn.

More than an inch and a half of growth across one wall is why installers leave small reveals at corners and ceilings, why boards are often spaced rather than butted, and why the moisture content at delivery matters as much as the species.

What does this mean for a new house?

New construction carries its own moisture, and it takes time to dry out. The Handbook’s finishing chapter advises that in new buildings wood should be given adequate time to reach equilibrium before it is finished¹. Running the air conditioning for a stretch before wood is delivered, and letting the boards sit in the room they will live in, are the practical versions of that advice. Our finishes explainer picks up the story from there.