Special Issues

Using WUFI 2D to validate WUFI 1D results for stud walls

The WUFI hygrothermal modeling program in common use is one dimensional. The results assume the assembly is homogeneous.  A  one dimensional (1D) section taken through the cavity insulation of a stud wall does not account for the effects of the studs.

How do the studs affect the moisture content of the sheathing? Is the section at the cavity insulation the worst case or is it somewhere else?

WUFI 2D can model the entire wall section with stud effects included and provide some insight into these questions. Here is a comparison of 1D and 2D results for a wall with 16” on center wood studs:

Observations:

This is a 3 year simulation. Materials are assumed to start with construction moisture at 80% relative humidity. By the winter of the first year, construction moisture from the studs has increased the moisture content of the sheathing significantly higher than  shown in the WUFI 1D simulation.

By year 3, the construction moisture from the studs has dried and the WUFI 1D results are in good agreement with the 2D results.  So WUFI 1D is applicable in this case.

The wettest part of the sheathing occurs at the center of the stud cavity.

 

WUFI 2D and metal stud walls

Here is a comparison of 1D and 2D results for a wall with 16” on center metal studs:

Observations:

By year 3, after construction moisture drying, the peak sheathing moisture for the WUFI 1D results are about 3% higher than the 2D results.  I presume that the WUFI 2D simulation is the more realistic representation of actual  performance . To the extent that this case is representative of typical cold climate stud wall behavior, using WUFI 1D instead of 2D would lead to slightly conservative design strategies for metal stud walls.  So the use of WUFI 1D for metal stud wall assemblies seems valid.

The wettest part of the sheathing occurs at the center of the stud cavity.

 

PRESENTATIONS