Frost in the attic - a sign of trouble?

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.

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.