Most people think of condensation and high humidity as a summertime phenomenon. We hear forecasters talk about hot sticky days with a high dew point while we sip a cold refreshment with beads of moisture running down the outside of the glass. The water vapor in the air has cooled to the dew point temperature on the cool glass and turned into droplets of moisture. No big deal, put a coaster under your glass and enjoy a typical Minnesota summer day.
Now, think of your attic in the middle of a very cold Minnesota winter. All of us add moisture to our living space and not just intentionally with a humidifier. Bathing, cooking, watering indoor plants, fountains, drying clothes and vaporizers can add gallons of water vapor to the warm air. It’s typically not a problem until it escapes into a cooler environment (attic) by migrating with the rising warm air. Insulation and a vapor retarder will help but many homes have enough leakage for warm air laden with moisture to escape into the attic. Improperly vented bathroom and kitchen exhaust fans are a particular concern. When this vapor comes in contact with a cold surface (below the dew point) it condenses and if the surface is cold enough like the underside of your roof deck, it turns into frost. On the first warm day when the sun heats up the attic, the frost melts and if there’s enough moisture, it can easily be confused with a roof leak.