Moisture and acclimatisation

A wood floor takes on and gives up moisture until it reaches equilibrium with the air around it, so its dimensions follow the room's humidity. The practical job is to install it when the room, the boards and the subfloor already agree. The USDA Forest Products Laboratory defines equilibrium moisture content (EMC) as the moisture content at which wood is neither gaining nor losing moisture, set by the relative humidity (RH) and temperature of the surrounding air¹.

What is equilibrium moisture content?

The Wood Handbook's table of EMC at 21.1°C (70°F) shows how strongly humidity drives moisture content¹:

The same chapter notes that changes in service are usually gradual, that short-term fluctuations mostly affect the surface, and that coatings such as varnish or lacquer retard moisture changes but cannot prevent them¹. It also stresses that table values are suitable for practical applications and that actual EMC can vary considerably between species and pieces¹. Wood shrinks and swells about twice as much tangentially as radially¹, and NWFA notes that plainsawn flooring is less stable in width³.

For interior wood, NWFA cites USDA figures of 8% average moisture content (6 to 10% range) in most of the United States, 6% (4 to 9%) in dry southwestern regions and 11% (8 to 13%) in damp, warm coastal areas, and warns that actual conditions in any location may differ significantly³.

What humidity should a wood floor live in?

Makers set different working ranges, so match your product's own sheet.

NWFA also says wood flooring made for use above 9% moisture content or above 50% RH should not be installed over radiant heating unless the maker approves it³. The underfloor heating guide covers that case.

How do installers test the moisture content of the boards and the room?

NWFA frames testing as essential quality control: installers must know the ambient conditions, the moisture content of the flooring and that of the substrate, using thermo-hygrometers and moisture meters³. Its procedure for solid wood is to record jobsite temperature and RH on delivery and take at least 40 board readings for the first 1,000 square feet, plus 4 more per extra 100 square feet, averaging the results and isolating unusually high or low boards³. Engineered and parquet floors follow a similar schedule, and parquet and end-grain use 40 panels or blocks³.

Meter choice matters. NWFA describes pinless (dielectric) meters whose signal penetrates from 1/4 to 1 1/2 inches and pin-type (resistance) meters, notes that most meters are calibrated to pine, Douglas fir and hemlock so species correction is needed, and warns that engineered floors' cores and adhesives can change a pinless reading³. It also says moisture content values above the fibre saturation point are only qualitative, and that the laboratory oven-dry method (ASTM D4442, 102 to 105°C) is the reference method used to calibrate other methods³.

How should you test a wood subfloor?

NWFA asks for at least 20 test locations for the first 1,000 square feet plus 4 per additional 100 square feet, with at least three tests per room and extra attention to exterior walls and plumbing³. Its rule of thumb where the subfloor is not aligned with the room's conditions is a subfloor moisture content no more than 4% above strip flooring (under 3 inches wide) and no more than 2% above plank flooring (3 inches or wider)³. In hot, humid climates it says the subfloor should not exceed 13% moisture content, and notes mould growth is possible above 15% and fastener corrosion above 18%³.

Junckers expects wooden subfloors to sit at 6 to 12% moisture content in balance with room humidity⁶. NWFA also says to compare against other properly conditioned wood in the building when the subfloor is hard to read, and to find and fix any elevated reading before flooring is delivered³.

How do you test a concrete subfloor?

NWFA says to test every job, even when the adhesive maker states no moisture testing is required, and notes that a test shows the slab's condition at the time of the test, not a permanent state, especially without an effective vapour retarder under the slab³. Its main tests, with the thresholds it states:

Makers add their own limits. Junckers requires residual moisture not to exceed 65% RH for glue-down, with its liquid moisture barrier for 65 to 95% RH or underfloor heating; for its clip floors it allows 90% RH (UK: 75% RH according to BS 8201)⁶⁵. Junckers also drills in-situ holes to about half the slab thickness and measures at a subfloor temperature of 17 to 25°C⁶. For heated screeds, Weitzer gives maximum CM residual moisture of 1.8% for cement screed and 0.3% for anhydrite screed⁷.

The sources therefore disagree on depth and limits: NWFA's probe depth is 40% or 20% of the slab with an 80% RH trigger, while Junckers uses about half the slab and stricter RH ceilings for glue-down. Use the flooring maker's number.

How do you acclimatise a floor?

NWFA prefers the word conditioning: aligning the flooring and the building³. If boards arrive at the expected in-use moisture content, the room conditions are right and the subfloor matches, flooring may be installed immediately³. If not, NWFA says moisture content may need adjusting, and cautions that altering it can make boards shrink or swell outside their manufactured size and that solid wood does not change uniformly³. Its conditioning steps:

1. Separate solid flooring into small lots or open the packaging, then cross-stack with 3/4 to 1 inch stickers between layers until EMC is reached³.
2. Allow two to three times longer for imported or tropical species³.
3. Condition off-site only where those conditions mirror the building's expected conditions³.
4. For unfinished solid wood, let the installed floor stabilise 5 to 7 days before sanding and finishing, longer if adhesive off-gassing needs time³.

For end-grain flooring NWFA says it tends to equilibrate relatively fast and conditioning is faster when blocks are restacked on edge and spread out³.

Other sources differ. The Wood Handbook's finished-flooring guidance, which cites a 1961 Forest Products Laboratory publication, says to break open bundles, close the house at night and raise the temperature about 8°C (15°F) above outdoor temperature for about three days before laying². NBS (2007) advises acclimatising in the room for at least seven days before installation⁹. Junckers instructs differently for its boards: before delivery the building must already be at 35 to 65% RH and about 20 °C, and the boards are then laid immediately, with wrapping removed only just before laying and no acclimatising on site⁶. The aim is the same, matching floor and room, but the method differs. The practical rule is to follow the instructions for your product, and read our installation methods guide alongside.

What conditions does the building need first?

Junckers says the building must be weathertight, the heating installed and tested, and wet trades such as screeding, tiling and plastering finished and dried⁴. The Wood Handbook adds practical timing: deliver flooring after masonry and plaster are fully dry and install the heating plant before flooring arrives². For reclaimed wood, NWFA says milled boards must be dried to a moisture content that matches the home and then conditioned like solid, engineered, parquet or end-grain flooring³.

What happens when moisture is wrong?

Seasonal change is normal. Junckers says all hardwood floors expand as RH rises and contract as it falls, producing occasional minor gaps between boards in winter⁴. NWFA's firm rule is never to install a wood floor over a known moisture condition, and always to test so unknown conditions become known³. Kährs says floors can lift from over-wet cleaning or from moisture rising from below when a vapour barrier is missing⁸. Humidity management afterwards is covered in cleaning and maintenance.

This page compiles published data from the sources listed above. Read our editorial policy. Last updated 3 Nov 2026.