I was up in the unfinished attic of my old house and noticed frost on the underside of the roof sheathing, and also on some of the rafters. The rafters are exposed. Attic insulation is in the floor of the attic rather than the roof. What causes this frost? Will it cause any harm? Can I cure it?
What you’re looking at is evidence of excessive moisture in your house. Water vapor is migrating upward and eventually filtering into the attic. When it reaches the cold surface of the roof sheathing and framing it turns to frost.
In the parlance of building scientists, these cold wooden house parts make up a “condensing surface,†much like a cold glass of ice tea beads with water on a hot summer day.
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| Left unchecked, frost in an attic can lead to serious mold and decay |
A patch of frost here and there – on the tips of roofing nails poking through the sheathing, for instance -- probably isn’t a cause for alarm. When the weather warms up, the frost will become water droplets and evaporate harmlessly. But anything more than an occasional patch is the symptom of a larger moisture problem.
Left unchecked, moisture in the attic can lead to serious mold and decay, expensive to repair and potentially unhealthy for anyone in the house.
So the short answer is, yes, this is something you should fix.
Where the water comes from
What ends up as frost in the attic starts as water somewhere else in the house. There are many potential sources, but why not start in your basement.
Old houses are especially susceptible to damp basements, either because they have dirt floors through which moisture freely passes or because their foundations aren’t sealed against water leaks from the outside.
When you’ve got a lot of water that seems to seep in regularly, repair any cracks or gaps in the foundation walls and install a sump pump and interior perimeter drains. Yes, it’s a lot of work -- but it's work that helps make sure your old house stays healthy.
In addition, make sure the ground on the outside of the foundation is pitched away from the house so rainwater is directed away from the basement.
In an old house with a dirt floor in the basement, add a layer of polyethylene sheeting. Overlap and tape joints and lap the poly up the foundation walls to prevent moisture from getting inside.
Other sources of moisture
Once you’ve got your basement dried out, check the rest of the house for sources of unwanted moisture -- especially these potential trouble spots:
- Malfunctioning or non-existent exhaust fans in the bathroom and kitchen. Cooking and bathing both produce water vapor. Fans should be ducted to the outside.
- Leaky heating and cooling ducts, especially those that pass through an unheated space (like the attic). Joints should be sealed with something other than duct tape.
- Unvented gas or kerosene heating devices. Much of the fuel that’s burned in these fireplaces and heaters turns into water vapor.
- Improperly ducted dryer vents. Dryers should never be vented to a basement or an attic, only to the outside.
Stop air leaks to the attic
Water vapor naturally wants to move upward, right into the attic. In addition to stopping water leaks and examining ductwork you also should seal up air leaks in the attic floor. Use expanding polyurethane foam or other weather-proofing supplies to seal gaps.
Even if you have to pry up a few floor boards to do it, check around light fixtures and other penetrations in the ceiling. If your house has been “balloon framed,†meaning that wall studs run all the way from foundation to roof eave, you may want to add blocking in stud bays to keep these cavities from acting like little chimneys.
Once you get all that warm, moist air out of your attic, the frost will disappear, too.
One more thing – you may not have enough insulation in the attic floor. Check the Department of Energy website (www.energycodes.gov) for what’s recommended in your area. Adding another layer of insulation also will help.
