contractor sprays interior walls with low-density polyurethane foam
We’re building a new set of walls in a testing trailer to see how well open-cell spray foam performs in the extreme cold.
The Mobile Test Lab is a trailer with nine wall sections, each a different combination of studs and interior and exterior insulation. Last year we tested wall systems with interior fiberglass insulation and exterior EPS foam board to see how they handled moisture. We were wondering how much exterior insulation would be needed to prevent the sheathing from reaching dew point (the temperature at which vapor condenses into water). Researchers found that as long as a wall has 65 percent of the insulation on the cold side, the wall cavity stays pretty dry and no mold growth occurs. All of the walls with less than 65 percent on the outside had some degree of mold on the sheathing. We also found vapor barriers made a big difference, as walls with vapor barriers were much less humid and featured only small areas of mold near holes in the plastic. (For more results, read the project Snapshot here.) 
This time we’re testing open cell spray foam to see how it handles cold weather and indoor humidity. This winter we’ll set indoor conditions at 70 degrees, 40 percent relative humidity and positive pressure (a rather exaggerated condition for homes in Fairbanks, which will force moisture into the walls).