Moisture, Humidity and Room Climate
Wood moves with the air around it. What that means in a British home, what is normal and what is not, the readings the standards ask for before a floor goes down, and how to keep a room stable through the year.
On this page
Why wood moves at all
Timber is hygroscopic. It takes moisture out of the air when the air is damp and gives it back when the air is dry, and it changes size as it does so. This never stops. A wood floor is not a static object and was never sold as one.
In a British home the swing is predictable. Winter heating dries the air, boards give up moisture and shrink slightly, so fine gaps can appear at the joints. Summer brings the moisture back, the boards take it up and the gaps close. A floor that behaves this way is behaving correctly, and it is not a sign that anything was fitted badly.
Construction decides how much it moves
Two floors can look identical and behave completely differently, because what governs movement is how the board is built.
What to measure, and when
Two instruments answer almost every question here, and neither is expensive. The readings below are the ones UK practice works to.
Keeping a room stable
You are not trying to hold the room at one number. You are trying to avoid violent changes.
The difficult rooms
Some rooms are harder than others, and it is worth knowing before you order rather than after.
What the published standards say
Each entry is quoted from its source and linked to it. The full set is in the standards reference.
BS 8203 — relative humidity limit for a subfloor
A hygrometer maker's data sheet that cites BS 8203, BS 5325 and BS 8201 treats a subfloor as dry enough for most floor coverings when hygrometer readings do not exceed 75% relative humidity (65% RH for solid timber coverings), measured with a hygrometer sealed to the surface so that it reaches equilibrium with the base.
The exact wording
“The base can be considered dry enough to install most floorcoverings if a series of tests produce readings not exceeding 75% R.H. (65% R.H. for solid timber floorcoverings).”
Room conditions at the time of fitting
The MMFA, the European association for multilayer modular floor coverings, requires the flooring to acclimatise for at least 48 hours in the same conditions as the installation area, stored flat and cross-stacked in unopened boxes at roughly 15-25 degrees Celsius and 35 to 65 per cent relative humidity.
The exact wording
“Leave the floor covering to acclimatise in the same environmental conditions as the installation area for a period of at least 48 hours prior to installation. Always store the boards flat and cross-stacked, in their original unopened boxes and at approx. 15-25 °C and between 35 % and maximum 65 % relative humidity.”
Acclimatisation before fitting
The Contract Flooring Association's guide asks for a steady room temperature of 18 to 27 °C, reached 48 hours before installation, so that the flooring can be kept in the room and reach room temperature before it is laid.
The exact wording
“They also advise that this temperature should be attained 48 hours prior to the installation, so that floorings can be kept in the room and brought to room temperature (acclimatised) before installation.”
Moisture content at delivery — what EN 13489 requires
EN 13489, the product standard for multi-layer parquet, requires the moisture content of the elements to be between 5 % and 9 % at the time of first delivery, measured by the oven-dry method of EN 13183-1. The corresponding figure for solid parquet under EN 13226 was not available from a source we could read, so it is not stated here.
The exact wording
“The moisture content shall be between 5 % and 9 % at the time of the first delivery of the product.”
How long a screed takes to dry
BS 8204-1 gives a drying allowance of one day per millimetre of thickness for the first 50 mm of a levelling screed, with longer per-millimetre times above that thickness, so a 50 mm screed drying in good conditions takes roughly two months.
The exact wording
“with levelling screeds, one day should be allowed for each millimetre of thickness for the first 50 mm, followed by an increasing time for each millimetre above this thickness. It is therefore reasonable to expect a levelling screed 50 mm thick, drying under good conditions, to be sufficiently dry in about 2 months.”
Damp-proof membrane — Approved Document C
Approved Document C, paragraph 4.2, states that floors next to the ground should resist the passage of ground moisture to the upper surface, not be damaged by moisture from the ground or by groundwater, and resist the passage of ground gases.
The exact wording
“Floors next to the ground should: a. resist the passage of ground moisture to the upper surface of the floor; b. not be damaged by moisture from the ground; c. not be damaged by groundwater; d. resist the passage of ground gases.”
Questions about moisture and room climate
What humidity should a room with a wood floor have?
How dry does a concrete floor need to be before flooring?
How long should flooring acclimatise before fitting?
Why are there gaps between my boards in winter?
Is laminate affected by humidity?
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