Winter weather can find every weak link in a house to create drafts and raise utility bills. So how dumb is it to purposely let cold air into an attic or crawl space? Not dumb at all -- in fact, it's essential, and a component of each of the four building code systems in the country.
Ventilation makes obvious sense in summer when temperatures soar in attics, and sunstruck shingles become so gooey you can't walk on them. Thorough ventilation is the answer. It keeps temperatures in check, reduces air conditioning cost, and also extends the life of asphalt shingles.
It may seem counter-intuitive, but venting attics is just as important in winter. Seal up roof vents the way you seal up windows, and moisture created in the house will be trapped and condense in the attic. In older homes without vapor barriers, several gallons of moisture a day can condense, soak insulation, erode wood framing, stimulate mold growth, and sometimes freeze up so the attic looks like an ice palace. It's an interesting view by flashlight some night when little icicles start falling and it sounds as though every squirrel in the neighborhood has moved in. Worse yet, on the next sunny day when the shingles heat up all that ice turns to water.
Winter ventilation is also an important factor in preventing destructive ice dams that can lift shingles, damage the roof structure underneath and create major leaks.
Energy codes vary widely, so the best bet is to contact your local building department. They can tell you how much insulation (rated by R-value) to put in the attic floor and how much ventilation you should have in the space above it.
Generally, building codes require a minimum ventilation ratio of one to 150, which means one square foot of vent area per 150 square feet of attic floor space. The ratio changes to one per 300 if the attic floor has a vapor barrier, generally foil-faced insulation or sheet plastic installed just under drywall on the ceiling below.
The reasoning is simple. Without a vapor barrier, more living-space moisture seeps up into the attic and you'll need more vent area to remove it. But limit moisture seepage with a vapor barrier and you'll need only half as much vent area to remove it.
There are two other twists to consider. One is the sometimes confusing requirement of net free vent area -- the rating you or your contractor should check when planning for attic ventilation. It accounts for the reduced airflow due to insect screening, commonly built into all vents. The other twist is that there are many types and possible locations of vents that can meet the required ratio. But to be effective you need about half the total area to let air in and the other half to let air out.
Let air into unfinished attics with plug vents, strip grills, or perforated vinyl panels in the soffit -- the narrow underbelly of the roof overhang. Let air out with continuous vents along the ridge, large louver vents on the gable end walls of the house just under the ridge, and, in some cases, individual vents set into the roof.
The best systems bring in air all along the lowest parts of the roof (on both sides of a cape-style roof), and exhaust it all along the highest part, the ridge. It doesn't make sense to install only a couple of huge entry and exit grills. That would leave stagnant, unventilated areas, particularly in corners.
When existing soffits have few or no inlets, plug vents are a good option. Just drill a hole (typically one between each set of rafters) and push a vent into place. These circular grills (and any vents) should be backed with screening, if it's not built in, to keep out insects.
Strip-grill vents are long, narrow grills that form a continuous vent channel along the overhang. This is a good type to install if you're replacing damaged plywood soffits. Instead of cutting and fitting single sheets of new material, you can more easily install two, narrower sections, leaving space for the strip grill between them.
Perforated vinyl panels provide thorough inlet venting, but you have to remove existing soffits to install them.
It may seem strange to have swaths of vinyl on a basically wood or brick house. But the long-lasting, low-maintenance material is a great fit for soffits.