Details:Membranes

Membranes

Membranes may serve different purposes in building elements, such as airflow control, waterproofing, surface protection etc. Within the scope of one-dimensional hygrothermal simulations only vapor retarders and vapor barriers are of interest.

Generally, the only effect of these membranes is their water vapor diffusion resistance. At most, a membrane at the exterior surface may make itself additionally felt by its radiation absorptivity (for example, 0.9 for bituminous felt) and its rain reduction factor (0 for watertight membranes), which are treated separately in WUFI (namely, in terms of surface transfer coefficients). The thermal resistance of a membrane is usually negligible.

If the membrane is on the exterior or interior surface, its diffusion resistance may simply be allowed for by choice of an adequate vapor diffusion thickness for the surface.

A membrane in the midst of a building component, however, has to be explicitly included in the component assembly.
Generally, a membrane will have a thickness of a few hundredths to a few tenths of a millimeter. Since every layer of the assembly should be composed of at least ca. five grid elements, the individual grid elements of the membrane will be very small. For numerical reasons, neighbouring grid elements should not differ too much in their thickness; in the material layers adjacent to the membrane, this would necessitate a succession of smaller and smaller elements towards the membrane in order to avoid a large mismatch. The resulting large number of grid elements would impose a heavy burden on the calculation without any other benefit than to smooth the grid.

It is therefore preferable to use an 'effective' membrane which is thicker than the real membrane and whose material data have accordingly been adapted to result in the correct heat and vapor diffusion fluxes. The membranes in the database that comes with WUFI have their data adjusted so that these membranes must be used as 1 mm thick membranes in the component assembly.

The 'effective' membranes have the additional advantage that they are easier to identify in graphical representations of the component assembly.

The conversion is simple: Often you'll neither know the thickness of the membrane nor the diffusion resistance factor of its material, but the vapor diffusion thickness of the membrane (i.e. the thickness of a stagnant air layer with the same diffusion resistance). That's all you need: divide the vapor diffusion thickness by 1 mm or another convenient thickness, use the result as the effective diffusion resistance factor and insert the membrane with the chosen thickness in the component assembly.

The other basic material parameters are treated as follows:

• if you happen to know the bulk density of the membrane material and the thickness of the membrane, you may determine the effective density as r · (real thickness) / (effective thickness) in order to adjust the heat capacity. Otherwise, choose a value that is small enough to result in a plausible heat capacity if multiplied by the specific heat capacity. (The heat capacity of a thin membrane is negligible, but you should take care that the heat capacity of the thicker effective membrane is negligible, too.)

• set the porosity to an arbitrary, preferably very small value (e.g. 0.001, since the usual membrane materials are not porous)

• if you know the specific heat capacity of the membrane material, enter this value unchanged. Otherwise enter e.g. 1500 J/kgK for organic materials

• if you know the heat conductivity of the membrane material and the thickness of the membrane, determine the effective heat conductivity as l · (effective thickness) / (real thickness). Otherwise, choose a value that is large enough to result in a negligible heat resistance for the membrane.

The definitions of the vapor diffusion resistance factor (µ-value) and the vapor diffusion thickness (sd-value) are discussed in reference: Water Vapor Diffusion.

Please note that replacing a thin layer with a thicker layer with effective material parameters is only permissible in one-dimensional calculations! In two-dimensional cases you also have heat and vapor flows along the membrane instead of only across it, and in general you can't use an arbitrary effective length of the membrane (e.g. a membrane wrapped around a rafter or one following a winding path through the assembly).