Trained Eye


Frost in Attic


Today we have two questions about a common problem that many homeowners, in older homes, may be currently experiencing. They are almost identical in nature and will be answered together.

My mother in law lives in an older home of about 70 years. It has a gable roof with a 12-inch vent at each end of the roof. The attic has wood shavings with fibreglass insulation on top. About two years ago in the spring, a crack first appeared in the living room ceiling and drops of water have seeped through it. This has occurred again. Upon entering the attic, it was noted that the insulation was wet, and the rafters were too. There was a fairly extensive frost buildup on the ceiling of the attic.

Would this be caused by heat escaping into the attic from the rooms below? Do you have any suggestions as to how to locate the source of the problem? Would it be necessary to remove and replace all the insulation?

This had never happened before. The only thing that has occurred was that the brick chimney was repaired in the attic and outside, and a liner was put into the chimney.

Any suggestions would be appreciated.

Thank you.

I have recently discovered moisture damage to my living room ceiling. The areas damaged are where the wall meets the ceiling by the front door of the house. It also appears close to a light fixture. The home is about 1940 vintage. The attic insulation consists of wood chips and a thick layer of blown in pink insulation. I noticed there is frost across the roof and gable ends when I peered thru the attic hatch. The house has gable vents and 2 plastic roof vents. There really is no soffit area on this house as there is not much of a roof overhang. What can I do to prevent the moisture from ruining my ceiling? Would a "whirlybird" vent on the roof help?

Thanks for your help.


Mr. Davis: You have the source of the problem in your mother-in-law’s attic, and Mr. Elaschuk’s as well, almost nailed down. The true nature of the problem is heat and, more precisely, heated air leakage into the attic from the home. I very much doubt that this hasn’t happened before, it just never caused damage to the ceiling below, so there was little evidence of the melted frost. I have discussed this topic several times, but your common problem has become so prevalent in recent years that it bears revisiting, especially with Spring approaching.

Air from a heated home will leak into the attic of a home through any small openings or gaps in the ceiling by natural convective forces. This phenomenon is often called the “Stack Effect” and is named after the forces that cause hot air to move up a chimney, plumbing vent or stack. One simplistic way to think of this is by recalling one of the first physical laws that we learn in elementary school; heat rises. In this heated air there is often a large amount of moisture in the form of water vapour. One fundamental scientific principal that we must know, for this discussion, is that warm air can hold much more moisture than cool air. When this heated, moist air hits the cold underside of the roof sheathing and rafters it condenses. This condensation freezes and forms frost when the temperature drops, normally at night. When the weather warms up outside, the frost melts and drips into the insulation below.

This may have been occurring for many years, but the insulation has absorbed the majority of this water and dried out in the summer, due to warm air movement in the attic through the attic vents. At some point, the insulation has lost its ability to absorb this moisture and sufficiently dry in the warm months and the ceiling plaster has become wet, damaged, and in the worst cases, dripping. This may have become progressively worse over time, as improvements to air tightness in the main floor of the home actually made the problem worse. If older windows and doors have been replaced or caulked, wall insulation increased, or other improvements for energy efficiency, less air leakage is occurring in the home, increasing the amount of air forced up into the attic.

The conventional method to deal with attic moisture was to ensure enough ventilation to allow warm air leaking in to escape through the vents, before it condensed. Adding more insulation to the top of your ceilings caused the attic temperature to drop, actually increasing the chance of condensation. Fibreglass insulation has a good insulation R-value, but does little to stop air movement. The existing attic venting is not sufficient anymore. What has to be done is minimize the pathways for air to leak into the attic. This can be done by sealing around openings, light boxes, wall plates, vents, chimneys and other protrusions through the ceiling in the home.

Research has shown that stack effect is often increased if there is significant cold air leakage into the basement. Cold air leaking in through gaps in and around foundation walls and basement windows will cause higher amounts of heated air to rise into the attic.

The first thing to do for remedial action is to check basement windows or openings and seal them or cover them with plastic for the heating season. Proper insulation in the basement and vapour barrier between the floor joists may also help. Running exhaust fans in bathrooms and kitchen may also help to reduce the amount of moisture going into the attic. Unfortunately professional air sealing and insulation in the attic may be required to totally eliminate the problem.