Wood moisture and drying: what the Wood Handbook, EPA and GOV.UK say

This page reports what the official source says. It is not legal advice: ask the office named here. It describes what the sources say about moisture and drying and does not teach how to dry wood. The England rule is described in more detail on the England firewood sale page, and species heat values are on the firewood species and seasoning page.

How moisture content is defined

The Wood Handbook (General Technical Report FPL-GTR-282, from the US Forest Products Laboratory) gives moisture content as a percentage: the mass of water in the wood divided by the mass of the ovendry wood. In practice it is the wet mass minus the dry mass, divided by the dry mass, times 100. So a piece can read over 100 percent, because the divisor is the dry wood alone. The Forest Research page on woodfuel moisture says moisture content is a critical measure for woodfuel, and its spreadsheet tool also converts a moisture content quoted on a dry basis to a wet basis. Forest Research says that when fuel is bought by weight, moisture content decides how much is paid for the energy, and that this matters far less when it is bought by volume.

Green wood and fiber saturation

The Wood Handbook defines green wood as freshly sawn wood whose cell walls are completely saturated and in which more water may sit in the cell cavities. It says the moisture content of green wood runs from about 30 percent to over 200 percent, and that sapwood of green softwoods is usually wetter than heartwood. Its Table 4-1 lists the average green moisture content by species, which it calls typical, with considerable variation within and between trees.

Species, green	Heartwood percent	Sapwood percent
Ash, white	46	44
Oak, northern red	80	69
Oak, white	64	78
Maple, sugar	65	72
Hickory, mockernut	70	52
Pine, loblolly	33	110
Spruce, Sitka	41	142
Cedar, western red	58	249

The Handbook splits water in wood into free water, in the cell cavities, and bound water, held in the cell walls. The fiber saturation point is the moisture content at which the walls are full of bound water and the cavities hold none. It says the point averages about 30 percent moisture content but can differ by several percentage points between species and pieces, and that its exact definition is debated. Above it, the physical and mechanical properties of wood do not change with moisture. Below it, wood shrinks as it loses moisture and swells as it gains it, which can warp, check or split it. The Handbook says wood shrinks most along the growth rings, about half as much across them, and only slightly along the grain, with tangential shrinkage about twice radial.

Equilibrium moisture content

The Handbook defines equilibrium moisture content as the level at which the wood is neither gaining nor losing moisture, set by the relative humidity and temperature of the air. Its Table 4-2 gives values for each combination. At 21.1 degrees C (70 degrees F) the table gives these figures.

Relative humidity	Equilibrium moisture content, percent
30 percent	6.2
50 percent	9.2
65 percent	12.0
80 percent	16.0
90 percent	20.5

The same table gives 11.3 percent at 60 percent relative humidity and 4.4 degrees C. The Handbook says changes in moisture content can be slowed but not prevented by coatings such as varnish, lacquer or paint, and that the aim of drying is to bring moisture content close to the value the finished product will have in service.

Air drying and kiln drying

In its chapter on drying, the Handbook says lumber is usually dried by some mix of air drying, accelerated air drying or pre-drying, and kiln drying. It describes air drying down to 20 to 25 percent before kiln drying as a way to save energy, and says the rate is very slow in cold winter months. For 25 mm (1 inch) thick lumber it gives extremes of 15 to 30 days to reach 20 to 25 percent for low-density species such as pine, spruce, red alder and soft maple in favorable places and seasons, and 200 to 300 days for slow-drying species such as oak and birch in northern places at unfavorable times. It describes kiln drying as using higher temperatures and faster air circulation, with schedules that control temperature and humidity. It says moisture moves faster along the grain than across it, about 10 to 15 times, and that sapwood generally dries faster than heartwood.

It lists the targets used for lumber: dry lumber under the American Softwood Lumber Standard has a maximum moisture content of 19 percent, softwood framing lumber is usually dried to an average of 15 percent and not above 19, and hardwood lumber for furniture, flooring, cabinets and millwork is usually dried to 6 to 8 percent. Its recommended values at installation for interior woodwork, flooring and furniture in most areas of the United States are an average of 8 percent, with individual pieces at 6 to 10.

Firewood: the 20 percent figure

The EPA's Burn Wise page on best wood-burning practices says wood burns best when its moisture content is under 20 percent and advises testing with a moisture meter. It describes seasoned wood as darker, lighter in weight and hollow-sounding when two pieces are knocked together, and says to season wood for at least six months, stored outdoors and off the ground with only the top covered because covering the sides traps moisture. The page also notes that the Burn Wise program ran from 2009 to 2025 and is kept as a historic record.

In England, the GOV.UK guidance on selling wood for domestic use says wood fuel may be supplied or sold in volumes under 2 cubic metres only if certified as Ready to Burn, which it says confirms a moisture content of 20 percent or less. The scheme covers firewood in single retail bags and loose bulk deliveries under 2 cubic metres, and wood briquettes in the same forms. Sales of 2 cubic metres or more need no certification, but the supplier must give customers a notice on how to dry, store and check moisture before use. The page lists a fixed penalty of £300 from a local authority, or a larger fine from the courts, where wood is stored so that its moisture is over 20 percent or is not labelled correctly. It was last updated on July 7, 2022.

Forest Research adds that most woodfuel boilers are designed for a limited range of fuel moisture content, that fuel outside the range makes a boiler run inefficiently with more emissions or fail to run, and that the boiler or stove maker or installer is the source for the right range. GOV.UK and Forest Research text is adapted under the Open Government Licence v3.0.