8. BUILDING PHYSICS LVL beams, studs and panels can be used in structures similar to all other timber and engineered wood products. Building physics analyses of walls and roofs do not need any special methods and normal design tools are suitable for their thermal resistance calculation and dew point analyses. As LVL has fairly low thermal conductivity, the cold bridging effect of LVL beams or studs is minimal. In special cases LVL-C panels may be used alone as water vapour barriers with no separate plastic membranes required. 8.1 LVL AND MOISTURE LVL is a hygroscopic material similar to other wood-based products. Therefore, the moisture content of LVL products is dependent on the relative humidity (RH%) and, more specifically, on the direction of moisture content change (drying / wetting). LVL products swell when their moisture content increases and shrink when their moisture content decreases. A part of the swelling is permanent and the extent of these dimensional changes depends on the grain direction. Wetting can cause permanent deformations, and impair the visual appearance of surface veneers, such as colour changes due to water staining, surface cracks and falling of knots due to drying shrinkage after wetting, see also subsection 7.3.1. The hygroscopic surface of LVL has also advantages. If the surface is untreated or the surface treatment does not form a membrane on the surface, LVL can have a moisture buffering function absorbing humidity from the air when the RH% is high and releasing it when the RH% is low. This action levels out the peaks and can help to create pleasant indoor air conditions. In unheated storage spaces, the hygroscopicity prevents water condensation on cold surfaces and thus the risk of water dropping from, e.g., roof structures is smaller than in, e.g., steel structures. 8.1.1 Moisture content of LVL When leaving the factory, the moisture content ω of the LVL product is approximately 8 to 10%. Due to changes in ambient temperature and relative humidity, the moisture content of the product will continuously change. In service class 1 the moisture content usually varies between 6 and 10%, while in service class 2 it usually varies between 10 and 16%. Thus, the LVL products are delivered from the factory at a moisture content that is close to the end use conditions. Product moisture content ω is defined as follows: ω=(m_ω-m_0 )/m_0 (8.1) where mω is the product mass at moisture content ω; and m0 is the product dry mass. The average equilibrium moisture content of LVL products in different relative humidity conditions (RH%) can be estimated using a sorption isotherm Figure 8.1. When wood is drying (desorption) its equilibrium moisture content is higher in the same relative humidity than when the wood is wetting (absorption). This phenomenon is called hysteresis. 8.1.2 Measuring the moisture content Moisture meters based on electrical resistance give somewhat too high results for LVL due to the glue lines of LVL products. For exact determination of moisture content in a LVL sample, an oven drying test can be conducted according to EN 322. Surface moisture meters (non-invasive) are recommended for measuring the moisture content of LVL products. The measurements should be taken perpendicular to the grain direction from the face veneers at an undamaged location. E.g., through sanded areas of the face veneers cannot be measured reliably. To obtain the most reliable results, the moisture meter should be calibrated using samples of known moisture content measured, for example, by oven drying. Note: Examples of suitable moisture meters for measuring the moisture content of spruce LVL are: - Delta 2000H (setup: H3 Spruce) - Doser Messgerät HD5 (setup: material group3) - Doser Messgerät DM4A (setup: A2) - Esko 314 (setup: A2) - Exotec (setup: 500) - Merlin PM1-E (setup: 5) - Orion 930 (setup: 0,55) - Testo 616 (setup: F1) 8.1.1 Moisture content of LVL When leaving the factory, the moisture content ω of the LVL product 10%. Due to changes in ambient temperature and relative humidity, the product will continuously change. In service class 1 the moisture between 6 and 10%, while in service class 2 it usually varies betwee LVL products are delivered from the factory at a moisture content tha conditions. Product moisture content ω is defined as follows: = − 0 0 (8 mω is the product mass at moisture content ω; and m0 is the product dry mass. The average equilibrium moisture content of LVL products in differen conditions (RH%) can be estimated using a sorption isotherm Figure drying (desorption) its equilibrium moisture content is higher in the sa than when the wood is wetting (absorption). This phenomenon is cal Figure 8.1. Left: Average equilibrium moisture content of softwood L humidity at 20 °C. Right: Absorption and desorption isotherms of sof based on weather cycling tests, RH 65% → 92% → 40% 45. 8.1.2 Measuring the moisture content Moisture meters based on electrical resistance give somewhat too h the glue lines of LVL products. For exact determination of moisture c an oven drying test can be conducted according to EN 322. Surface moisture meters (non-invasive) are recommended for meas content of LVL products. The measurements should be taken perpen direction from the face veneers at an undamaged location. E.g., thro face veneers cannot be measured reliably. To obtain the most reliab meter should be calibrated using samples of known moisture conten by oven drying. Note: Examples of suitable moisture meters for measuring the moist LVL are the Delta 2000H (setup: H3 Spruce) and the Doser Messge group 3) 18. 0 5 10 15 20 25 30 0 10 20 30 40 Moisture content MC (%) Rela Moisture absorption and de Absorptio Desorptio LVL Handbook Europe 173
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