The other day I was working at my brother’s project—a small house that will serve guests at his resort in Northern, Minn. The outdoor temperature was 0°F and dropping, cold enough for the window glass and the insulated steel doors to become condensing surfaces. Relative humidity was elevated because of the new construction components drying inside the structure, and humidity was running about 50%. The enclosure is very tight (0.30 ACH50; 60 CFM50).

We’ve installed R-10 continuous exterior insulation (CEI) around 80% of the structure, while the last 20% is waiting for several mechanical and electrical penetrations to be located. My brother added a temporary electric heat source and installed cavity insulation in the areas without CEI to reduce the heat flow through those locations. There isn’t a vapor retarder installed yet.

If you’ve worked construction in a cold climate long enough, you can probably guess the outcome: The cavity insulation was frozen to the exterior sheathing. This condition was created by the cavity insulation reducing heat flow through the wall assembly, causing the sheathing to cool. The wintertime flow of interior water vapor trying to move from inside to out will contact this cold sheathing, condense, and freeze, even in structures that are air-sealed well.

There are three ways to solve this problem. The first, and best, is to warm the wall sheathing above the condensing temperature. We had done that for 80% of the structure already with the CEI. The next option is to install a vapor retarder (preferably not polyethylene sheeting). This helps to keep any interior moisture from moving into the wall cavities, where it can condense. This project meets the required continuous exterior to cavity insulation ratio for climate zone 7, and we are planning a class III vapor retarder—painted drywall. The third solution…