FAQ:General:CriteriaforEvaluatingHygrothermalPerformance

(18): Criteria for Evaluating Hygrohtermal Performance

I used WUFI to compute the water content in a variety of wall assemblies. In order to evaluate their hygrothermal performance, I now need appropriate criteria, e.g. standards that should not be exceeded.

There are no general criteria which are applicable for every case. Different materials and different applications require different criteria. Here are some general hints:

• The most important criterion: the moisture must not accumulate over time. Water condensing in the building component must be able to dry out again. If the moisture content in your component keeps increasing - even slowly - you'll run into problems sooner or later.
• The building materials which come into contact with moisture must not be damaged (e.g. by corrosion or mould growth).
  Mineral building materials are usually not at risk; some of them may be susceptible to frost damage if they contain a lot of moisture.
  Wood should not exceed 20 mass-% of moisture during a prolonged period; otherwise mould growth may result (possible exception: increased moisture while temperatures are low).

German standard DIN 4108-3 adds the following criteria:

• The amount of condensing moisture in roof or wall assemblies must not exceed a total of 1.0 kg/m².
  This is a more or less arbitrary criterion. In order to test it with WUFI, start

the calculation with the normal equilibrium moisture (corresponding to 80% RH) and see if the total water content exceeds the starting value by more than 1 kg/m².

• At interfaces between materials that are not capillary-active, no moisture increase exceeding 0.5 kg/m² is permissible.
  This is meant to avoid moisture running or dripping off, which could accumulate elsewhere and cause damage.
• The moisture increase in wood must not exceed 5 mass-%, the moisture increase in materials made of processed wood must not exceed 3 mass-%.
  These are more or less arbitrary numbers.

In addition, special criteria may be applicable in specific cases, for example:

• Are there any materials which are particularly sensitive to moisture damage?
• Does increased heat loss by moist insulation exceed any energy conservation requirements?
• Is the building material at this moisture level sensitive to frost damage?
• Is there salt in the wall which must be kept from crystallizing or from moving around?
• Etc.

Even if you don't have clear criteria which fit your case, you may still perform a ranking of your assemblies by comparing them with each other or with a standard case.