Fuktutslag

Fuktutslag is a term used in building and materials science to describe the dimensional changes a material undergoes when its moisture content varies. It is a hygroscopic phenomenon, most pronounced in wood and wood-based products, but it is also relevant for textiles, paper and some concrete systems. The change occurs as water molecules diffuse into or out of the material, causing it to swell or shrink.

In wood, fuktutslag is highly anisotropic: swelling is typically greater in the tangential direction than in the radial direction, and much less along the grain. The magnitude of the effect depends on the change in equilibrium moisture content, ambient temperature, wood species and density, and whether the material is coated or sealed. In non-wood materials, moisture-driven deformation can occur as well, but is generally less dramatic.

Practical implications include warping, bowing, cupping of boards, gaps at joints, and buckling of flooring or paneling. In buildings, uncontrolled fuktutslag can lead to structural and finishing problems, moisture migration, and reduced durability. Sensitive assemblies, such as timber floors, furniture, or veneer laminates, may require design accommodations to allow for moisture movement.

Measurement and mitigation: moisture content is commonly measured with handheld or in-situ meters, and environmental relative humidity is monitored to limit rapid changes. Prevention strategies include acclimating materials to expected service conditions, using breathable coatings, detailing that accommodates movement, and maintaining stable temperature and humidity. In design and standards, fuktutslag is often considered under hygroscopic expansion and wood physics, with emphasis on material-specific swelling coefficients and service moisture ranges.