Timber floor acclimatisation and site conditions: reconciling the advice

At a glance

Letters are the sources under "Source by source". NWFA figures are US units converted (1 in = 25.4 mm). RH is relative humidity; WME is a moisture-meter reading of a timber subfloor. Figures are what each source states, not a standard; the maker's written instructions take precedence.

Why the advice conflicts

Published advice looks contradictory. The period in the room runs from none to four weeks. Moisture content runs from 6 to 9% to 8 to 12%. Room humidity runs from 30 to 50% to 45 to 65%. Most of the conflict dissolves once four differences are separated.

• Days stand in for a moisture reading. The US National Wood Flooring Association (NWFA) defines acclimation, more accurately conditioning, as aligning the flooring with the environment it is to be installed in. It gives no number of days, and says flooring that already arrives at the in-use moisture content, with the subfloor to match and the conditions held, may be installed straight away (A). A UK magazine quotes a parquet specialist saying the purpose is not to leave the boards for a set number of days but to reach an appropriate moisture content, and quotes a timber supplier in the same piece recommending 10 to 14 days (J). The days are a proxy and the target is a reading.
• The humidity is not the same. Wood settles at a moisture content set by the humidity and temperature of the air around it. The US Forest Products Laboratory table (Wood Handbook 2021, Table 4-2) gives about 6.2% at 30% RH and 9.2% at 50% RH at about 21 °C, and about 8.5% at 45% and 12.0% at 65% (E). So the NWFA band of 30 to 50% RH goes with roughly 6 to 9% moisture content, and the UK band of 45 to 65% RH goes with roughly 8.5 to 12%. Each set of figures is consistent with itself. They describe different rooms.
• The pairs are sometimes mixed. One UK maker's moisture sheet pairs 6 to 9% moisture content with 35 to 65% RH (G). On the Forest Products Laboratory table 65% RH is about 12% moisture content, so those two figures do not line up. Whichever pair you adopt, check the two against each other.
• The products differ. A pre-finished engineered board delivered already conditioned starts in a different place from a solid unfinished board or reclaimed timber. The NWFA says imported and tropical species often need two to three times longer (A). The UK retailer ranges run from 24 to 48 hours for engineered to 14 days or more for solid (H).

Source by source

The figures on this page come from eleven documents we read and one standard we could only see listed. The table lists each by a key, so any number can be traced. We name neutral bodies and describe the rest by type; the names and dates are held in our notes for review. Dates are the dates we could see, and several UK pages are undated.

The NWFA is a US trade body and its figures are in US units, converted here (1 in = 25.4 mm; 60 to 80 °F is 16 to 27 °C). The UK distributor describes its guidelines as complementing BS 8201, and is a flooring distributor, not an independent trade body.

The specifier's rule: conditions first, then readings

Specify the conditions and the readings, not the number of days. The order below works with any of the sources above, because it does not depend on which humidity band you choose.

1. Decide the in-service regime. That is the humidity and temperature the finished building will hold in use, which depends on its heating and ventilation design, not on a catalogue. UK guidance uses 45 to 65% RH and 15 to 25 °C (F). The NWFA works to 30 to 50% RH (B). Choose the band the building will actually deliver, and write it into the specification.
2. Hold the building at that regime before the floor arrives. The sources agree that the building should be weathertight and the wet trades finished and dry (C, F, G). The NWFA's sample specification asks for the occupied humidity and 65 to 75 °F for at least seven days before installation, during it and for seven days after (C).
3. Take readings on delivery. The NWFA asks for the moisture content of at least 40 boards for the first 1,000 sq ft (about 93 m²), from a variety of bundles, then 4 more for every 100 sq ft, averaged, with unusually high or low boards set aside. It asks for 20 subfloor readings for the first 1,000 sq ft and 4 for every 100 sq ft after (A). Set the meter for the species. Screeds and slabs are tested separately, by the hygrometer test that UK sources attribute to BS 8201 Annex A, with limits of 65% RH (75% with a surface damp-proof membrane) in the distributor's guidance (F).
4. Compare with the target. The flooring should read close to the equilibrium moisture content for the specified humidity, using the table in the Wood Handbook (E). For a timber subfloor, the NWFA's rule of thumb is that the subfloor is no more than 4 points above strip under 76 mm and parquet, and no more than 2 points above plank 76 mm and wider and end grain (A).
5. Hold on the readings. There is no fixed period. If the flooring arrives inside the target, there is nothing to wait for. If it does not, hold it in the room until it does. The Forest Products Laboratory notes that wood in service responds gradually and that short-term changes tend to reach only the surface (E), which is a reason to measure rather than assume that a few days is enough.
6. After fixing, keep the conditions. UK guidance calls the time during and for 48 hours after fixing the most critical (F). The NWFA asks for 5 to 7 days before sanding an unfinished solid floor (A), and UK guidance asks for 10 days (F).

What to log, and who signs it off

The record is what turns a site condition into something that can be checked. Agree it in the specification and name who signs each line.

When the conditions cannot be met

If the building is not ready, delay the delivery rather than store the floor in the wrong room. Our installation guide notes that temporary heating and dehumidification can mimic normal conditions until permanent services run. The NWFA allows off-site conditioning where the off-site conditions reflect the expected in-use conditions (A).

If the occupied building will not hold the band you chose, say so to the client. The NWFA's magazine says all solid flooring should be expected to show seasonal gaps in dry months where no humidification is added (L), and UK guidance describes small winter gaps as natural movement, not a fault (F). That is movement the room conditions cause, and no length of acclimatisation will remove it. See solid vs engineered wood flooring for how much a board moves.

Wood flooring is not to be delivered until the building is weathertight, the wet trades are complete and dry, and heating and ventilation have held the in-service conditions of [45 to 65]% relative humidity and [15 to 25] °C for not less than [7] days, as recorded by a logger at floor level in each room receiving flooring. The log is to be issued to [designer] and [floor contractor] before delivery. Conditions are to be held during fixing and for not less than [7] days after. Unfinished floors are not to be sanded until [10] days after fixing, or the maker's period if longer.

No fixed acclimatisation period applies. Flooring is to be held in the room, or in an equivalent conditioned space, until the moisture content of not fewer than [40] boards per [93] m², taken from [several] packs with a meter set for [species], averages [x]% [+/- y]% and no board exceeds [z]%. Over a timber subfloor, the flooring is to be within [2] percentage points of the subfloor for planks [76] mm and wider, and within [4] for narrower strips and parquet. Readings, meter and date are to be issued to [designer] before fixing. The maker's written instructions take precedence.

Standards and sources

• NWFA (US trade body): Acclimation/Conditioning (c 2019): Source A, with B, C and D (linked in the source table). All read.
• US Forest Products Laboratory, Wood Handbook (2021), chapter 4: Table 4-2. Read. Source E.
• BS 8201:2011: BSI lists it as current (31 October 2011). Not read: confirm every figure attributed to it. Source K.

Questions we are asked

How long should wood flooring acclimatise before installation?

There is no single answer. UK sources give from none (for some pre-finished engineered boards) through 72 hours to 10 to 14 days, and up to four weeks for reclaimed timber. The NWFA gives no number of days at all: flooring that arrives at the in-use moisture content, with a matching subfloor and held conditions, may be installed at once. Specify the room conditions and the moisture readings, and follow the maker's written instructions.

What moisture content should wood flooring have at installation?

The moisture content it will reach in the finished building, which depends on the room humidity. On the US Forest Products Laboratory table, about 6 to 9% goes with 30 to 50% RH and about 8.5 to 12% with 45 to 65% RH at around 21 °C. Over a timber subfloor the NWFA also asks for the two to be within 2 points (planks 76 mm and wider) or 4 points (narrower strips and parquet). Ask the maker for the figure and the tolerance.

What humidity should a room be for a wood floor?

The NWFA works to 30 to 50% RH at 60 to 80 °F (16 to 27 °C), with regional exceptions. A UK flooring distributor's guidelines work to 45 to 65% RH and 15 to 25 °C for the life of the floor. The bands overlap at 45 to 50%. Use the maker's range, and the humidity the building's heating and ventilation will actually hold.

Does engineered wood flooring need to acclimatise?

Most UK guidance says yes, for less time than solid: 24 to 72 hours in the retailer and distributor guidance we read. One maker's specification says its pre-finished engineered boards need none but asks for 7 to 10 days over underfloor heating. The NWFA says that flooring already at the in-use moisture content may go straight down. Follow the maker's instruction for the product.

Should I open the boxes to acclimatise the floor?

It depends on the product. The NWFA's sample specification asks for sealed packages of unfinished flooring to be opened. A UK magazine expert says not to open every box unless the manufacturer recommends it, and one maker asks for the ends of the cartons to be opened over underfloor heating. Follow the maker's instruction.

What does BS 8201 say about acclimatisation?

We have not read it. BSI lists BS 8201:2011 as the current code of practice for installing wood and wood-based panel flooring, covering the material, the environment and the installation. Others quote it as asking for storage as close as possible to the conditions of the finished building and as describing a hygrometer test for concrete bases in an annex. Check the standard before quoting it.