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
Fugtindholdet i træ (MC — moisture content) er defineret som forholdet mellem vandets masse og den tørre masse: u = (m_vand / m_tør) · 100 %. Fibermætningspunktet (FSP) er ca. 28 % MC — det punkt, hvor cellehulederne er tomme men cellevæggene er fuldt mættet. Under FSP svinder og udvider træet sig; over FSP sker kun sorptionsændringer i hulederne uden dimensionsændring.
| 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.
Praktisk eksempel: et 145 mm bredt fyr-stolpetræ i tangentiel retning, der tørrer fra 18 % (AK2 driftsvilkår) til 12 % (leveringsfugt) — en ΔMC på 6 %: svind = 145 · 2,75 · 6 / 1000 ≈ 2,4 mm. I et bjælkelag kan det bemærkes som fuger i et gulv.
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