Relative humidity (RH) is the ratio of the current water vapour content of the air to the maximum the air can hold at that temperature. It is temperature-dependent — the same absolute moisture in the air reads as a higher RH in cooler conditions and a lower RH in warmer conditions.
For engineered timber and solid timber, the target service RH is 40 to 60 percent. Sustained excursion above or below that range drives dimensional movement — expansion at high RH, contraction at low RH. Cyclical movement between the two is the underlying mechanism behind long-term joint fatigue.
Site measurement
A calibrated hygrometer, held at floor level and clear of drafts, is the reference instrument. Readings are taken at delivery, at start of installation, and at handover, and are logged against the installation record. The log is the reference document against which future environmental claims are measured.