Inconsistent mu values for wood?

[Alexander]

Hi, me again.
I have looked through the wood materials in the catalog "Wooden materials" in the material database. But now i'm confused. I always thought that wood belongs to those materials where the mu value strongly decreases if the wood becomes wetter. And for some of the wood materials in the database the mu values show this effect, but for other materials the mu value is simply constant. How can that be? Could this be an error in the material data or do different kinds of wood really behave so differently?

Thanks. Alex

[Thomas]

for other materials the mu value is simply constant. How can that be?

Hi Alex,

the data in the material database should be okay.

It is true that diffusion measurements on wood and wood-based materials
usually show that the mu-value depends strongly on moisture content. It
does not always decrease, BTW; our measurements on materials made from
processed wood occasionally also show increased mu-values at higher
moisture contents.

However, in most materials, this apparent variability of the mu-value is due
to the fact that at higher moisture contents some liquid transport occurs in
the materials in addition to the diffusion transport to be measured. The
total amount of transported water is thus higher and the mu-value appears
to be reduced. In reality (at least for most mineral materials) the pore
structure of the material and thus the mu-value are independent of the moisture
content.

WUFI allows to simply use the 'moisture-dependent' mu-values provided
by the measurements. It is better, however, to separate out the liquid
transport and to describe it with appropriate liquid transport coefficients
while the mu-value is held constant (at the value measured for low moisture
content).

This may be important when the building component is exposed to different
temperatures on both sides. Liquid transport is driven by differences in
relative humidity, diffusion transport is driven by differences in vapor
pressure. If the temperatures are the same on both sides (as is the
case with the laboratory measurements for the mu-value), liquid and
diffusion transport go in the same direction. If the temperatures are
diifferent, however, the side with higher partial pressure may have
the lower relative humidity, and then liquid and diffusion transport
will partly cancel each other; this situation cannot be described by a
lowered mu-value.

Which of these two possible approaches has been used for the individual
materials in the database depends on the data that were available. For
those materials that have a constant mu-value in the database, the liquid
transport has been separated out from the diffusion measurements and
has been accounted for by appropriately adjusted liquid transport coeffi-
cients. The mu-value has been set to the value measured at low moisture
content.
Please note that materials whose mu-value has been treated as variable
usually have no liquid transport coefficients at all (the entire moisture
transport is assumed to happen by diffusion), while the materials whose
mu-value is treated as constant have detailed liquid transport coefficients,
derived from water absorption and drying experiments.

Now, wood materials do change their pore structure in dependence
of the moisture content (swelling, shrinking), so that for these materials
setting the mu-value constant is certainly not perfectly correct either, but
even here it should be appropriate to separate out the liquid tansport. Test
calculations show that (at least under the investigated conditions) both
approaches (variable mu-value versus constant mu-value plus liquid
transport) give very similar results.

In other words: under the conditions for which you will usually do WUFI
calculations, wood materials will remain relatively dry and shrinking/
swelling effects or liquid transport will only have a small effect on the
results anyway. Which of the two approaches to use is then more or less a
philosophical question.

Bye,
Thomas