What’s causing water to drip from a light fixture in our house?

Q: What's causing water to drip from a light fixture in our house? It happened after we had some very cold weather, followed by a warming trend that pushed temperatures above freezing. There was no rain. I went into the attic and found melting frost on the underside of the roof and dripping down.

A: You’d better deal with this quickly because wiring is involved. There's no question that the cause of your problems involves air leakage and, perhaps, insufficient attic ventilation. With the wide swings in temperature this winter, conditions have been ideal for this sort of trouble. As warm, moist air migrates into the attic from the living space in your home, it cools, condenses and causes frost to build up on the underside of your roof.

When outdoor temperatures rise above freezing, this frost melts and water runs down into insulation and wiring. Attic ventilation can allow some of this moisture to escape harmlessly before it ever forms frost, but if the flow of moist air into your attic is large, no amount of ventilation is going to be sufficient. Few people realize how much warm air sneaks up past the hatch leading into their attic, and this is the first place you should examine. In addition to making sure your attic vents are clear and functional, take steps to positively seal your attic hatch.

If this doesn't solve your problem, consider increasing the amount of attic ventilation you have. Building codes require a minimum vent area equal to 1/300th of the floor area of your attic, but that’s often not enough. In some cases, doubling this amount of vent area is necessary to solve attic moisture problems.

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