BUILDING ENVELOPE: Ventilation
Questions:
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.
