Ask the Carpenter: The best way to ventilate an attic, save on cooling costs
Carpenter and contractor Rob Robillard explains why he doesn't like attic ventilation fans.
Q. We’ve received two bids to re-do our roof and add much needed ventilation to our attic, which is not ventilated at all. One roofer told us that ridge vents are required by code and that he would add one, plus gable vents for air intake and an exhaust fan. The other told us that ridge vents were required only on new construction and should not be combined with gable vents or fans. He wants to add gable vents and a solar-powered exhaust fan.
I have read that gable vents shouldn’t be used with ridge and soffit systems, but soffit vents aren’t an option on our roof. The ceilings on our upper floor are sloped, so the roof line is right on top of those ceilings for the first 3 to 4 feet. There are no soffits to speak of on one side of the roof. What are your thoughts? Can ridge and gable vents be used together where there are no soffit vents? Can soffits be added, and can they be ventilated if a portion of the roof is directly over a ceiling? Any help untangling this would be greatly appreciated.
BETH
A. I hate when builders construct homes without soffits. Ridge vents (which run along a roof’s peak) and gable vents (installed at the gable ends of the attic) cancel each other out. Maybe it’s possible to install a gable vent lower in the attic space?
In general, you should put more entry vents (soffit and gable) down low than exit vents up high. Why? If you construct a house with an attic ceiling with leak penetrations from pipes, wiring, etc., and you have lots of ridge vents, the makeup air is going to be pulled from the house rather than from the outside. Unbalanced ventilation should be in favor of the lower vents (gable and soffit), because you don’t want to depressurize the attic.
I suggest you read some of the well-written and informative articles by Joseph Lstiburek, founding principal of Building Science Corp., for guidance on ventilation and insulation issues.
I’m not a big fan of attic ventilation fans. They are usually mounted in the upper slope of the roof and are designed to cool the attic, save energy on cooling costs, and extend the life of the roof shingles. These thermostatically controlled fans are supposed to push out the upper attic hot air and draw in cooler outside air, usually through the soffit vents. The unmentioned evil of attic ventilation fans is that they often pull interior air-conditioned air up from the home into the attic through the ceiling (via wires, pipes, and other air leak penetrations). Once this happens, the negative pressure that is created in the house pulls hot outside air in, causing the air conditioner to work harder.
I’ve also heard that attic ventilation fans can create backdraft issues with gas appliances like water heaters, which can cause health and safety problems. The fan basically causes makeup air to come down the flues. This often results in a water heater or boiler not venting flue gases properly, and the fumes are actually released inside the house. I’d consult your plumber or heating, ventilation, and air conditioning technician on this one.
There’s a lot to digest here, but it sounds as if the second roofer has a better understanding of your issue.
Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to [email protected] or tweet them to @robertrobillard. Subscribe to the Globe’s free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp.
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