LVL Handbook EUROPE

8. BUILDING PHYSICS

LVL beams, studs and panels can be used in structures simi-

lar to all other timber and engineered wood products. Build-

ing physics analyses of walls and roofs do not need any special

methods and normal design tools are suitable for their ther-

mal 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 ANDMOISTURE

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 specif-

ically, on the direction of moisture content change (drying /

wetting). LVL products swell when their moisture content in-

creases and shrink when their moisture content decreases. A

part of the swelling is permanent and the extent of these di-

mensional changes depends on the grain direction. Wetting

can cause permanent deformations, and impair the visual ap-

pearance of surface veneers, such as colour changes due to wa-

ter 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 condi-

tions. In unheated storage spaces, the hygroscopicity prevents

water condensation on cold surfaces and thus the risk of wa-

ter 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 mois-

ture 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

is the product mass at moisture content ω; and

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 (absorp-

tion). 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 recommend-

ed for measuring the moisture content of LVL products. The

measurements should be taken perpendicular to the grain di-

rection 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 the Delta 2000H (setup: H3

Spruce) and the Doser Messgerät HD5 (setup: material group

8.1.1

Moisture content of LVL

When leaving the factory, the moisture content

of the LVL prod

10%. Due to changes in ambient temperature and relative humidit

the product will continuously change. In service class 1 the moistu

between 6 and 10%, while in service class 2 it usually varies betw

LVL products are delivered from the factory at a moisture content

conditions.

Product moisture conte t ω is defined as follows:

−

is the product mass at moisture content ω; and

is the product d y mass.

The average equilibrium moisture content of LVL products in diffe

conditions (RH%) can be estimated using a sorption isotherm Fig

drying (desorption) its equilibrium moisture content is higher in the

than when the wood is wetting (absorption). This phenomenon is

Figure 8.1. Left: Av rage equilibrium moisture content of softwood

humidity at 20 °C. Right: Absorptio and des rption isotherms of

based on weather cycling tests, RH 65% → 92% → 40%

8.1.2

Measuring the moisture content

Moisture meters based on electrical resistance give somewhat to

the glue lines of LVL products. For exact determination of moistur

an oven drying test can be conducted according to EN 322.

Surface moisture meters (non-invasiv ) are recommended for me

content of LVL products. The measurements should be taken per

direction from the face veneers at an undamaged location. E.g., t

face veneers cannot be measured reliably. To obtain the most reli

meter should be calibrated using samples of known moisture cont

by oven drying.

Note: Examples of suitable moisture meters for measuring the mo

LVL are the

Delta 2000H

(setup: H3 Spruce) and the

Doser Mess

group 3)

Moisture content MC (%)

Moisture absorption an

Absorptio

Desorptio

LVL Handbook Europe

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