It’s a sunny winter day. Is it raining in your attic?

Most homeowners never go into their attics and, short of a roof leak, have no idea how to look for problems. After more than 35 years as a home inspector, I know that the most frequent problem with attics is substandard ventilation that allows moisture to condense on the nails in the attic and drip water onto the insulation.

In cold weather, any house or building, no matter how tall or well built, experiences what we call the “stack effect,” in which the building breathes in air at the lowest level and exhales it at the top. You can see a good “Stack Effect Demonstration” by the Denver Fire Department on YouTube:

When skyscrapers were first developed many people found it difficult, if not impossible, to open the ground floor entry doors in cold weather because of the stack effect. As we learned in science class, hot air rises. So indoor air, heated to a comfortable temperature on a cold day, will try to rise. As it does, the building will try to draw in cold air from outside. Thus, the “stack effect” – and that annoying suction that makes it so hard to open the ground floor doors. That’s why architects developed revolving doors.

In a private house, the stack effect will continually move that comfortably heated 70° air with its 50% humidity up into your attic. If there is not enough wind and ventilation above the insulation on the attic floor, the air will cool, reach the dew point and condense on the tips of the nails that hold the shingles on the sheathing. The water will then drip onto your insulation and drastically reduce its R-value, its ability to actually insulate.

It is a fairly simple DIY inspection for a homeowner to check the heads of these nails for any signs of rust. There shouldn’t be any rust at all. But if there is, then you should check for additional signs of moisture damage, like any darkening of the rafters, the roof sheathing, or the trusses. These signs could indicate the house has a major problem. I have even inspected two houses where a contractor installing vinyl siding or aluminum siding decided the old-fashioned gable vents were unattractive and drastically reduced their size. In both cases, this resulted in the plywood roof sheathing delaminating and
failing, so that all of it had to be replaced before a new roof could be installed.

By far the best ventilation system for a home with an attic is to have a ridge vent at the peak and some type of ventilation at the bottom near the eaves and the gutters. If the house has soffits (those odd ceiling areas between the exterior walls and the gutters), these can be replaced with soffits that are vented to provide maximum ventilation. An excellent example is the Invisivent by Certainteed.

Good soffit vents and ridge vents create good ventilation, and this will lower heating bills and air-conditioning bills, while also prolonging the life of the shingles.

In making such changes, it is also important to ensure that the wind coming through the soffit vents does not blow the loose insulation away from the edges of the attic into a pile in the center. Baffles of either cardboard or Styrofoam should be installed between the soffit vents and the insulation.