Fiber saturation point ~28 % MC, tangential shrinkage 2.5–3.0 mm/m per % MC, vapour barrier placement and moisture technical requirements in BR18 and SBi-instruction 224.
Introduction
Moisture is the most important cause of wear, shape changes and fungal attacks in wood structures. Understanding what happens to wood when its moisture content changes is the basis for choosing the right materials, building right and avoiding errors that can cost renovation.
Moisture content and fiber saturation point
The moisture content of wood (MC) is defined as the ratio of the mass of water to the dry mass: u = (m_water / m_dry) · 100 %. The fibre saturation point (FSP) is approx. 28 % MC — the point where the cell cavities are empty but the cell walls are fully saturated. Below FSP the wood shrinks and swells; above FSP only sorption changes occur in the cavities without dimensional change.
| Parameter | Value | Note |
|---|---|---|
| Fiber saturation point (FSP) | ~28 % MC | Limit for shrinkage/expansion |
| Fungal Risk | > 18–20 % MC | Thrives over this threshold |
| Delivery moisture, construction timber | 12 % ± 3 % | Typical DK delivery target |
| EMC outdoors DK, summer | ~16–17 % MC | Lowest equilibrium moisture |
| EMC outdoors DK, winter | ~23 % MC | Highest equilibrium moisture |
| EMC indoors heated | ~8 % ± 2 % MC | Furniture dry climate |
Source: trae.dk – Humidity and wood.
Waste
Shrinkage only occurs during FSP and is different in the three directions. It is by far greatest tangentially (perpendicular to growth rings) and slightly less radially (along growth rings). Axially (along the grain) it is negligible.
Practical example: a 145 mm wide pine post in the tangential direction that dries from 18 % (SC2 service conditions) to 12 % (delivery moisture) — a ΔMC of 6 %: shrinkage = 145 · 2.75 · 6 / 1000 ≈ 2.4 mm. In a joist floor this can be noticed as gaps in a floor.
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