These material data constitute an indispensable minimum without which a calculation is not possible:

• Bulk density [kg/m³],
  serves to convert the specific heat by mass to the specific heat by volume.

  The bulk density rhobulk is the ratio of the mass m of the sample and the total volume Vtot of the sample:
  rhobulk = m / Vtot.
  The true density rhotrue, by contrast, is the ratio of the mass m of the sample and the volume taken up by the material matrix only:
  rhotrue = m / (Vtot - Vpores) = m / Vtrue.

  The bulk density should be available for virtually every building material. If not, it can be measured very easily. Since it only affects the specific heat value entering into the calculation, and hygrothermal simulations usually don't depend very sensitively on this value, it need not be known with great precision.

• Porosity [m³/m³],
  determines the maximum water content wmax (by multiplication by rhowater = 1000 kg/m³).

  Since most calculations are not sensitive to the exact value of the maximum water content (you'll rarely encounter water contents above free saturation), it is usually sufficient to estimate it if no value is available for the material in question.

  The porosity can be estimated from the true density rhotrue and the bulk density rhobulk:
  rhobulk = m / Vtot = m / (Vtrue + Vpores) = rhotrue / (1 + Vpores/Vtrue) = rhotrue * Vtrue/Vtot = rhotrue * (1 - Vpores/Vtot) = rhotrue * (1 - porosity),
  therefore
  porosity = 1 - rhobulk / rhotrue.

  rhotrue can in turn be estimated from other materials which have the same composition but different bulk density, if their bulk density and porosity are known.
  Example: a cellular concrete brick with rhobulk = 600 kg/m³ and porosity = 0.72 has rhotrue = 600 / (1 - 0.72) kg/m³ = 2140 kg/m³. The porosity of a cellular concrete brick with rhobulk = 400 kg/m³ can then be estimated as porosity = 1 - 400/2140 = 0.81.

  If the true density must be measured, this may for example be done with a helium pyknometer.

• Heat capacity [J/kgK],
  the specific heat capacity by mass of the dry material.

  Using the specific heat capacity by mass has the advantage that this value only depends on the chemical composition of the material, but not on its porosity. For example, cellular concrete bricks with bulk densities of 400 kg/m³ and 600 kg/m³ have the same specific heat capacity by mass.

  To convert into heat capacity by volume (which enters into the transport equations), WUFI multiplies the mass-specific heat capacity by the bulk density.

  Rough values are 850 J/kgK for mineral materials and 1500 J/kgK for organic materials. In most cases, these estimates will be sufficient since hygrothermal simulations usually don't depend very sensitively on this value.

  WUFI automatically allows for the additional heat capacity of the water content, if any.

• Heat conductivity dry [W/mK],
  the heat conductivity of the material in dry condition. A moisture-dependent heat conductivity is optional.

  Please note that experimentally measured heat conductivities of vapor-permeable materials may include the effect of vapor transport with phase change (i.e. water evaporating at one side of the specimen and condensing at the other side, thus in effect transporting latent heat without a corresponding heat flow being conducted across the specimen). Since WUFI explicitly computes this thermal effect of vapor flow, it should not be included in the heat conductivity, if possible. However, it is usually difficult or impossible to separate this effect out of the measured data.

  Furthermore, 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.

• Diffusion resistance factor dry [-],
  the diffusion resistance factor (µ-value) of the material in dry condition.

  The µ-value states by how much the diffusion resistance of the material in question is higher than that of stagnant air. A moisture-dependent µ-value is optional.

  Please note that even if you do not explicitly use a moisture-dependent µ-value, WUFI will treat it as moisture-dependent for moisture contents above free saturation wf. WUFI will reduce the permeability of the material, δ/µ (*), in proportion to the moisture excess over wf, until it reaches zero at maximum saturation wmax (that is, µ reaches infinity). This reflects - in a first approximation - the fact that at very high moisture contents even the larger capillaries become clogged with water and can't contribute to vapor transport any more.
   
  (*) δ is the permeability of stagnant air; δ/µ is the permeability of the porous material, smaller by a factor µ than the permeability of air.

Page created: 30 Apr 2001; last update: 08 Jun 2010