Ventilation

Can I close some of my attic vents during the winter?

Yes. However, assuming that an attic is properly insulated, there isn't much advantage in closing the vents in winter.

Because insulation is typically in the floor of the attic, the attic temperature will be close to that of the outdoor surroundings. Closing some vents won't significantly change this temperature.

An attic requires a certain amount of ventilation during the winter for moisture removal. This ventilation area is about half that required during the summer.

An attic will require more ventilation if significant moisture sources exist, such as kitchen or bathroom vents.

How important is crawl space venting?

Crawl space ventilation is required by code in many areas. However, a growing body of research indicates that it often is not effective in reducing moisture levels in crawl spaces.

The two most important methods to deter moisture accumulation in crawl spaces are adequate drainage away from the foundation, and a moisture barrier over the soil in the crawl space.

Drainage can be created away from the foundation walls with a minimum 5 percent slope for a distance of 15 feet from the foundation. This is the ideal. Dense ground covers like healthy turf grass also help surface run-off to drain away from the foundation.

However, even soil that feels dry inside a crawl space can be a significant source of moisture. Keep this moisture in the soil and out of the crawl space by covering the ground with a six-mil plastic vapor barrier. Overlap seams in the plastic a minimum of six inches and extend the plastic up the foundation walls six to 12 inches. Use soil, sand or rocks to weight the plastic down around the perimeter and over seams.

Is there a simple rule for sizing a kitchen or bathroom exhaust fan?

Yes, but the rule for the kitchen is different than for the bathroom.

Exhaust fans are rated by their air-moving capacity in cubic feet per minute, or CFM. The rules of thumb relate the required CFM to the volume of the space to be ventilated.

To size an exhaust fan for a kitchen, multiply the volume in a kitchen (length by width by ceiling height) by 0.20. A 12 by 12 foot kitchen with an 8-foot ceiling would require an exhaust fan rated at 230 CFM.

Kitchen exhaust fans should move at least 200 CFM as a practical minimum. A bathroom exhaust fan should move 0.13 CFM per cubic foot of space, with a minimum of 50 CFM.

Exhaust fans must be vented to the outside, not into an attic or crawl space. The venting duct should be as short as possible and have few right-angle bends. Using a flexible ducting material, pull it tight between the exhaust fan and the vent terminal while avoiding sharp bends. Then cut off any excess material. Improper ducting can reduce exhaust fan air flow by 50 percent or more.

A backdraft damper is recommended to prevent cold air from entering through the exhaust fan when it is shut off.

How do I select a quiet exhaust fan?

A sone is a subjective unit of loudness. Sone ratings for exhaust fans typically range from a low of one to a high of seven. The smaller the number, the quieter the fan.

However, the quietest fans move the least amount of air.

Don't sacrifice adequate air-moving capacity for quietness. Choose a fan that can do the job.

Once the capacity of fan needed has been determined, compare sone ratings on fans of equal capacity and choose the fan.

Can I vent my bathroom and kitchen exhausts into the attic?

Although the practice is quite common, direct venting to the outside is the recommended method. A well-ventilated attic can easily handle the moisture diffused through the ceiling, but it may be overwhelmed by the moisture from a steamy bathroom or busy kitchen.

The greatest danger is that moisture will condense and freeze on the cold underside of the roof deck near the exhaust outlet. If frost accumulates, it can result in enough water to drip down onto the insulation and ceiling.

Through-the-roof vent kits are available, and they are relatively easy to install in composition shingle roofs. Carefully installed, they are not likely to leak.