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8. BUILDING PHYSICS
8.1.4 Water vapour resistance
The water vapour resistance factor μ and water vapour
diffusion coefficient in air
δ
p
defined in EN ISO 10456 may
be used for LVL products in their thickness direction.
Thickness direction values are normally the essential val-
ues 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 thick-
ness 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 speci-
fied for LVL products can be used without any modification
for temperatures below or equal to 50 ºC for a prolonged pe-
riod of time. LVL can be continually used in temperatures less
than 100 ºC and has a maximum short-term exposure tem-
perature 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/m
3
, which is recommended to be
used in the calculation of thermal insulation. The tabulated
value is defined for 20 °C, RH 65% conditions. Lower den-
sity 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 structur-
al 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 · 106/°C · 10 000 mm)
= 10 001 mm
In normal ambient temperatures the properties of LVL prod-
ucts 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
c
p
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 δ
p
of softwood LVL.
Water vapour resistance factor μ [-]
Water vapour diffusion coefficient in air δp [kg/(Pa·s·m)]
Density
ρ
mean
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