8. BUILDING PHYSICS 8.1.4 Water vapour resistance The water vapour resistance factor μ and water vapour diffusion coefficient in air δp defined in EN ISO 10456 may be used for LVL products in their thickness direction. Thickness direction values are normally the essential values for building physics analyses. For special cases, however, the following estimates for the other directions may be used: • Due to the wood cell structure, water vapour resistance in the length direction is only 5% of the water vapour resistance in the thickness direction • For LVL-P the values are similar in both the thickness and height directions • For LVL-C, due to the cross band veneers, resistance in the height direction is about 15% of the resistance in the thickness direction. Note: the glue lines between the veneers do not have a significant influence on the water vapour resistance of LVL, which is of a similar level to solid sawn spruce or pine. 8.2 THERMAL PROPERTIES OF LVL 8.2.1 Influence of temperature on the mechanical properties of LVL The characteristic values of the mechanical properties specified for LVL products can be used without any modification for temperatures below or equal to 50 ºC for a prolonged period of time. LVL can be continually used in temperatures less than 100 ºC and has a maximum short-term exposure temperature of 120 ºC. Wood products resist cold better than heat and the minimum suitable temperature for LVL is -200 ºC 46. In structural fire design, however, the decrease in strength and stiffness properties of LVL products due to high temperature must be taken into account, similar to other types of softwood members. The reduction factors may be evaluated from the Figures 6.3. 8.2.2 Thermal conductivity of LVL products The design thermal conductivity λ of LVL products is 0,13 W/ (m K), according to the tabulated values in EN ISO 10456 for a product density of 500 kg/m3, which is recommended to be used in the calculation of thermal insulation. The tabulated value is defined for 20 °C, RH 65% conditions. Lower density decreases the thermal conductivity and higher moisture content increases it. Within the practical range of density and moisture content, their influence on thermal conductivity λ may be ±0,02 W/(m K). 8.2.3 Temperature deformations Since the dimensions of LVL products remain stable in normal temperature change conditions, it is usually not necessary to consider any effects of temperature variations on the structural design, unlike swelling and shrinkage due to moisture. The coefficient of thermal expansion in the direction of wood fibres is in the range from 3,5 to 5,0 · 10-6/K. Example: If the temperature changes from 5 °C to 30 °C, the length of a 10 000mm long LVL beam changes as follows: 10 000 mm → 10 000 mm + (25 °C · 4.0 · 10-6/°C · 10 000 mm) = 10 001 mm In normal ambient temperatures the properties of LVL products are unaffected by temperature variations. 8.2.4 Heat combustion and specific heat capacity The heat of combustion of LVL products is 17 MJ/kg 28. The specific heat capacity cp is 1600 J/(kg K) according to EN ISO 10456. Table 8.3. Water vapour resistance factor μ and water vapour diffusion coefficient in air δp of softwood LVL. Water vapour resistance factor μ [-] Water vapour diffusion coefficient in air δp [kg/(Pa·s·m)] Density ρmean Dry cup Wet cup Dry cup Wet cup 440 kg/m3 180 65 0,73 · 10-12 2,3 · 10-12 510 kg/m3 200 70 0,96 · 10-12 2,7 · 10-12 The dry cup values are tested in 23°C - 0/50 RH % and apply when the mean relative humidity across the material is less than 70 %. The wet cup values are tested in 23°C - 50/93 RH % and apply when the mean relative humidity across the material is greater than or equal to 70 %. 176 LVL Handbook Europe
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