Modeling the Heat, Air, and Moisture Response of Building Envelopes: What Material Properties are Needed, How Trustful Are the Predictions?

The physical models which form the basis for the different software tools used to predict heat, air, and moisture response of building envelopes seem quite diverse. For example, various “potentials” are used, and each individual potential demands apparently different properties. Anyhow, when digging into the physics and confronting the theory with what is measurable, then an array of eight basic material characteristics appear, all backed by an experimental rationale. Adapting that array to the potentials in use is not a question of creating new properties but of implementing the basic ones into potential-adapted expressions. A different problem, however, is that these basic properties are macroscopic in nature, i.e., represent the complexity of a material at the micro-scale by one single “average” number. That introduces restrictions as to the use of the property values measured experimentally. Hence, those restrictions are typically mixed up with the inability of the actual software packages to represent reality in full detail. Too many times, assumed incorrectness of the property values are blamed for causing the differences found between the predicted and real heat, air, moisture response of envelope parts. That inability, instead, should convince researchers and building engineers that the way to gain a well balanced understanding of the heat, air, and moisture response of envelopes is not by modeling only but by combining modeling with testing and field experience.