Buildings

Attic Ventilation

Q: A house we've looked at has extra attic ventilation. In addition to the regular gable-end vents, it has soffit and ridge vents and two power ventilation fans. The builder says that this will help keep the attic cooler in summer, reduce the air conditioning bills, and make the roof shingles last longer. My friend Lou, who's from the old school (still doesn't own a microwave), says it's all a bunch of hype — houses have lasted for hundreds of years without that stuff. Why do we need it now?

A: The fact that houses didn't have certain things for hundreds of years would be a good reason not to do something if we were building houses the same way we built them hundreds of years ago. But we're not building them the same way. Houses today are tighter, better insulated, heated and cooled, wired, and filled with a chemical stew of substances that didn't even exist 100 years ago. So, if we look at Lou's logic, I'd have to say Lou is all wet or, more appropriately, full of hot air.

In this case, however, Lou is probably closer to the truth than he realizes. The original purpose of venting the attic was to remove moisture from damp crawl spaces under the house. Years ago, most buildings were "balloon framed," meaning there were lots of direct connections between the crawl space and the attic. Opening some holes up in the attic had the effect of drawing moist air up from the crawl space, and could help dry out the area under the house.

Over the years, a number of other reasons have "evolved" for venting the attic, including reducing air conditioning bills in the summer, reducing attic moisture problems in the winter, and extending shingle life. Interestingly, the amount of ventilation required by most building codes is still the same as it was 50 years ago — when the purpose was to help dry out the basement!

Let's look at the air conditioning issue. Fifty years ago, when we had little or no insulation in the attic, the temperature of the attic had a big impact on the temperature of the house. The attic floor would heat up and act as a big radiator, radiating heat into the house. Insulation slows down the movement of heat from the attic into the ceiling; and the more insulation we have, the slower the heat moves through, and the less the ceiling will act like a radiator.

New houses today are required to have at least R-30 in the attic. If this insulation is installed properly, heat will move through it very slowly, and once the sun goes down, most of the heat will be given off to the outside, unless, of course, you've got air moving between the attic and the house.

One source of this air movement is air leaks — around the pull-down staircase, around and through recessed light fixtures, and around holes drilled for plumbing, wiring, and ductwork. If you have these kinds of leaks, then air that you paid to cool is leaking into the attic, pulling hot, humid air into the house to replace it.

What happens if we have a powerful exhaust fan running in the attic? While the diagrams that come with the fans always show them pulling the air in through the soffit vents or gable-end vents at the other end of the attic, the fact is that the air didn't go to the same schools as the people who drew the diagrams. So, when the fan comes on, it pulls air through the light fixtures, around the attic stairway, and through any other holes available.

The attic is, in fact, cooled by your air-conditioner. And while that attic fan is pulling air out of the house, it is also pulling make-up air into the house from outside or from the crawl space. Some of these attic fans are powerful enough to create a negative pressure in the house. If you have a gas water heater, this negative pressure can cause it to back-draft, putting poisonous combustion gasses into your house instead of up the flue.

The bottom line? Code-required attic ventilation levels are adequate. Putting in extra ventilation is probably a waste of money. Finally, powered attic vent fans are more likely to increase energy bills than to reduce them, and they can create dangerous conditions in your house. A little extra money to really seal the ceiling from the attic will go a long way toward providing comfort and lower bills.