Why is there frost in my attic?

Why is there Ice in my attic?

Winter in the Midwest it’s not uncommon to see ice in the attic on the underside of your roof. Frost and ice forms in attics when warm, humid air leaking from inside the home accumulates and condenses on the underside of your roof sheathing. A little bit of frost forming is not unusual after really low nighttime temperatures, and it is not a huge problem if it is able to melt and evaporate so that the wood can dry. However, most homeowners are unaware of this problem until watermarks are seen on the ceiling below the attic.


The solution is two pronged. First, stop the warm, relatively moist air inside the home from entering the attic as much as possible. This can be achieved by adding a vapor barrier and sealing holes and gaps such as canned lights, vents, etc… Spray foam is an effective product for this application. Make sure all bathroom, dryer and cooking vents are directed to the exterior and not into the attic.


Which brings us to ventilation. Any remaining moist air must be allowed to escape the attic. Most codes use the 1/300 rule for minimum residential attic ventilation recommendations. This means that for every 300 square feet of enclosed attic space, 1 square foot of ventilation is required – with half at the upper portion (exhaust vents) and half in the lower portion (intake vents).


A good living space envelope and a properly ventilated attic will keep ice from forming in your attic.