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Q. We had a new roof put on about five years ago with ridge vents. What is your opinion regarding attic fans? We have an attic in the house with a fan and an attic area in the attached garage with no fan. Should we have fans or not? I have heard answers pro and con.
A. Well, here comes another con. When your house was built the common means of venting attic spaces was via gable venting and maybe soffit venting along with the gable vents. Gable vents are those rectangular or triangular permanently open vents placed near the roof peak at each end wall of a standard A-frame pitched roof system. Soffit vents are vents placed in the roof overhang at the front and rear of the house along the entire length of the roof edge.
Gable venting doesn’t rely upon soffit venting but ridge venting-- a long continuous vent cap cut into the ridge, the very peak of the roof-- does. When your roofer cut in and installed the ridge vent he should have either determined that you had soffit venting or offered to install it if you didn’t.
Roofers like to install ridge vents because it relieves them of the chore of cutting and installing, shingle by shingle, the traditional ridge cap shingles. So for them ridge vent is a real time saver. Low profile mat type ridge venting (such as Cobra® ) does require cap shingles to blend in with the rest of the roof system and that does require more work.
However if you install ridge and its partner soffit venting on a house that originally was set up with gable venting you need to go up in the attic and block off the old gable vents. I know it sounds counter intuitive. One would think the more venting t he merrier but studies have shown that left together they tend to work against one another and areas of dead air set up in the attic space which brings us to the question of why we vent in the first place.
Most homeowners think that attic ventilation is primarily a means to control heat in summertime and that’s when I get the most inquiries about ventilation but the reality is that the most important aspect of ventilation for the health of the building is to vent out moisture vapor and winter is the time of year that this becomes critical. Moisture vapor from the warm living spaces below the attic will migrate up into the attic space and with no means of leaving the structure will condense on surfaces su ch as the underside of the roof sheathing with eventual damage taking place.
I was once called to house where the homeowner had recently had a roof installed and thought the job was bad and the roof was leaking. I climbed up in his attic and saw moldy possessions and wet framing lumber. I also saw that he had very carefully sea led every vent he had with plastic to preserve heat. When I told him what he’d done he didn’t want to believe me. Malfunctioning or humidifiers set too high can be real attic soakers.
Add an electric fan to your attic ventilation program and what you are doing is super-charging a system that doesn’t need that kind of boosting. Most automatic attic vent fans are aimed at heat reduction and have thermostatically controlled limit switche s set to turn on at a certain high temperature and off once the attic space gets to a preset lower temperature. They have reputation of pulling conditioned air out of the house-- a counter productive outcome-- and they have a nasty habit of getting old and burning up while consuming a fair bit of electricity over their lifespan. I know there is a whole industry out there surrounding the manufacture, sale and installation of powered attic ventilators and they work well in dairy barns but a well design ed, properly installed, passive attic ventilation system, such as ridge and soffit venting, over a properly insulated house is all you need. When I see attic fans in a house I inspect I tell the prospective homeowner to disconnect the power to them and j ust leave them in place until the next roof job, then pull them out.