Equilibrium Moisture Content of Wood
The moisture content wood drifts toward at a given air temperature and relative humidity.
Example
You enter
- Air temperature (°F) 70
- Relative humidity (%) 50
You get
- Equilibrium moisture content 9.2%
Details, formula, and sources
From the USDA Forest Products Lab (Hailwood-Horrobin) sorption equation - the reference a restoration drying setup moves a material toward. The exact value varies by species; the IICRC S500 dry standard and the dry, unaffected reference reading govern when a material is 'dry.'
EMC% = (1800/W)[Kh/(1-Kh) + (K1 K h + 2 K1 K2 K^2 h^2)/(1 + K1 K h + K1 K2 K^2 h^2)], h = RH/100, with W, K, K1, K2 temperature polynomials in degrees F (USDA FPL / Hailwood-Horrobin).
USDA Forest Products Laboratory Wood Handbook (Hailwood-Horrobin sorption equation) by name; the four temperature polynomials are bundled, no edition cycle.
The sorption equation is public; the exact EMC varies by species. The IICRC S500 dry standard and the unaffected reference reading govern.
Estimate. AHJ and licensed professional govern.
Field names used by the API: temperature_F, rh_pct, emc_pct
- Sorption model the USDA FPL Hailwood-Horrobin equation with bundled W / K / K1 / K2 polynomials (T in degrees F)USDA FPL Wood Handbook
- Species variation the value is a generic-wood reference; the actual EMC varies by species and historyUSDA FPL Wood Handbook
- Dry standard the IICRC S500 dry standard and the dry, unaffected reference reading - not a single computed number - define 'dry'IICRC S500