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Q. I am thinking of installing a thermostat controlled roof vent fan. I have a two story contemporary 24 year old well insulated attic. Do you think it would be worth the investment? I plan on s taying in the home.
A. I’m on record for thinking attic fans are not worth any investment. In fact to the contrary: they’ll cost you more in the long run. My complaints about powered attic ventilators--as attic vent fans are called-- remains that they can fool you by pulling conditioned air from the house giving you a false impression that the fan is in fact assisting in cooling. From a pure comfort sense you can argue that they are but at great and wasteful expense. My other complaint about attic fans, and a position from which I only reluctantly retreat and only under special circumstances, is that they have a reputation for burning up as they wear out and taking the roof-- and house-- with them as they blaze into oblivion. I have personally known folks who have lost homes to them and I have gone into attics and found seized attic fans that were minutes away from starting a fire. I argue for proper convective attic venting and nothing more. Ridge and soffit venting working in concert is the state of the art in today’s homebuilding world.
You could have someone over to the house on a hot day to examine the ceiling just under your attic with an infrared thermometer to see just how hot the drywall is radiating attic heat into the upper rooms. As for finding actual air leakage from the attic, that might be hard to do if you can’t identify hot spots.
If you really think the attic is just too hot with what ventilation you now have consider adding passive roof vents along the rear face to reduce the obviousness of them. That should lower the attic temperature somewhat. It’s not uncommon to find air in attics-- ventilated-- at 140ºF. I entered an attic last summer that was an unvented hip roof with metal roofing that was above 160ºF. I didn’t stay long.
The real function of attic ventilation is to allow moisture vapor from the house below to exit the building and not condense creating problems. Venting hot summer attic air is good and helps asphalt roofing material last longer but the thermal barrier of the insulation envelope is the key.
You say you have good attic insulation. If it hasn’t been upgraded in the 24 years of your home’s life then I’ll bet it has settled and the R-value has diminished somewhat. That’s called R-value drift. It might be wise to have a professional check it out. That’s a better investment.
I’ll bet your thermostat for the heating and air is located about five feet off the floor of the first level. The thermostat is only sensing the temperature of the air just around it on the first floor. When that air gets to the set temperature it shuts the system off-- including the circulating fan. Try turning the little thermostat selector switch that says OFF ON AUTO to the ON position and leave it there for a day to see how the air temperature balance changes in the house. The fan will keep circulating air throughout the house and will only turn on cooling when the temperature rises above the set point. The desired result would be that the difference between upper and lower level would vary about three degrees.
I have another reader who reached the point where he feels the best solution is to install a separate zone of coolin g for the second level. Expensive, but it would certainly give him greater control throughout all seasons especially if he uses a heat pump. However the installer he consulted told him he would have to cut a hole in the side of his house to get the equipment in and he balked at the notion. My feeling is that there is another way into that attic-- it just hasn’t been installed yet. He would want an access from a bedroom or hall to the attic that an adult could enter upright to service the unit. His duct-work would have to be reworked and I told him without looking at it to think in the eight thousand dollar range by the time he’s done.