Details:HeatConductivityMoistureDependent
Heat Conductivity, Moisture-dependent
The heat conductivity of the dry material is a basic material parameter and thus indispensable. If, in addition, the dependence of the heat conductivity on the moisture content must be taken into account, WUFI can optionally employ a table with the relevant data. WUFI interpolates linearly between table entries.
If a simple linear dependence of the heat conductivity on the moisture content is sufficient, a two-line table may be generated by entering the moisture-induced heat conductivity supplement. The linear interpolation in this table is then equivalent to evaluating the formula
| l(w) = lo·(1 + b·w/r s) | ||
| l(w) | [W/mK]: | heat conductivity of moist material |
| lo | [W/mK]: | heat conductivity of dry material |
| r s | [kg/m³]: | bulk density of dry material |
| b | [%/m-%]: | moisture-induced heat conductivity supplement |
The supplement b gives the fractional increase [in %] of the heat conductivity per mass-% moisture. Its value depends on the material; in hygroscopic materials, however, it is largely independent of their bulk density.
Examples:
| Material | Bulk Density | l | b |
| [kg/m³] | [W/mK] | [%/M.-%] | |
| cellular concrete | 400-800 | 0.09-0.19 | 4 |
| lime silica brick | 1800 | 0.7 | 8 |
| expanded clay concrete, pumice concrete | 1400-1800 | 0.5-1.0 | 4 |
| light-weight concrete with EPS aggregate | 300-900 | 0.07-0.28 | 3 |
| normal concrete | 2300 | 1.3-1.5 | 8 |
| wood | 400-700 | 0.08-0.15 | 1.5 |
In organic insulation materials, there is in general no linear relationship between the heat conductivity and the moisture content.
In this context, 'heat conductivity of moist materials' means exclusively the influence of stationary water on heat transport. Water vapor diffusion with phase change (evaporation and condensation of water) also contributes to heat transport, but this is allowed for by separate terms in the transport equations. However, since in the standard measurement techniques this effect of water vapor diffusion is usually included, results of measurements in the plate apparatus for permeable materials (e.g. mineral wool) have to be regarded with caution.
During the calculation WUFI performs iterations where it samples small regions of the tabulated curve. Very sharp bends in the curve may throw the iteration off and thus impede its convergence. In such a case, the curve should be smoothed by inserting additional points. A large number of entries may slow the search in the table, however.
Please note that design values, such as the data given in
German Standard DIN 4108, may already contain the contribution
of a typical water content and, if so, are not strictly dry values.
If you want to perform the calculation with a constant (i.e. not
moisture-dependent) heat conductivity (for example because you have
no detailed data on the moisture-dependence), you may use these
design values to allow for moisture content at least in a crude
approximation. However, if you explicitly use a table of
moisture-dependent heat conductivities, you should make sure that
the value for moisture content = 0 is really the dry value.
On the other hand, hygrothermal simulations (in particular the resulting moisture contents and distributions) usually don't depend very sensitively on the precise values of the heat conductivities, so the difference may be generally negligible unless you are specifically interested in heat flows.
