Hot attic in Texas
I have a friend, I’ll call her “E-laine”.
Elaine owns a 1976 Ranch-like house near Dallas, Texas.
(Hot & Humid).
Elaine has high utility bills and comfort issues.
…………….
The house is a 2400 sf one story slab on grade with an attached garage.
The furnace/ac air handler and ducts are in the attic.
The ducts are not air-sealed.
The house footprint is sort of sprawling & there are many windows.
There is NO intentional air barrier at the walls or ceiling.
The attic floor has batt insulation and a layer of recently added blown-in.
The house has soffit vents and ridge vents.
……………..
The house is not-so-comfortable, especially in the summer.
Elaine is aware that the attic is very hot.
She is thinking that it would save energy if she could make the attic “cooler”.
There’s a pull-down attic stair in her garage.
She decided to leave the attic hatch open and has strapped a box fan in the opening.
When it’s hot, She runs the box fan and blows not-so-hot garage air into the hot attic.
……………..
I am thinking that Elaine is making a mistake.
What do you think?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I think a lot of people take limited data from certain attic fan situations (eg large exhaust fans) and assume it applies to others (eg supply fans). For your question, you want measurements, not guesses.
Not doing tests/repairs on duct and house air sealing is a mistake.
That's a typical "solution" around here in DFW. It does help a bit as long as there are enough vents at the top of the attic, and you actually do not need the fan, as cooler air pushes hotter air up. A hot garage, specially if its door face to the sun, it maybe a degree or two cooler than outside, but if the garage door is open a couple of inches at night, it does allow to semi-cool the attic a bit faster.
The real solution is to fix the house, and for that, she needs to hire a HERS Rater and do an analysis of her house. Usually those guys can give you some good recommendations.
John,
Whether she is making a mistake with the fan seems irrelevant. It's obviously not a permanent fix and there's some low hanging fruit to be picked like air sealing the attic floor, sealing the ducts, maybe adding a radiant barrier. However, both Jon and Armando are right, she should have a professional evaluate the home and offer recommendations so she spends her money wisely.
I would guess (these are all guesses, someone needs to evaluate the site), that the ducts in the attic are leaky and probably uninsulated. Sealing the ducts will help, insulating them will help, and more attic ventilation (ridge vent, enough soffit vents) will probably help. A radiant barrier under the rafters may make sense here too.
Power attic ventilators introduce new problems, especially in a leaky house so I don't really think of them as solutions. Have someone who specializes in this like an energy rater come out and see what would work best in your specific location.
Bill
The local fire marshal probably wouldn't like her solution. Leaving the attic hatch open allows fire from the garage to spread into the attic. Once that happens, the house is generally a total loss. Then again, most people don't have fire-rated stairs anyhow, so the issue might be moot.
It's a mistake. It's not likely to be saving any energy (it's more likely to be increasing energy use) even if it somehow manages to provides some comfort. Almost all powered attic ventilation schemes increase energy use, (though small solar powered ventilators can reduce energy use in some cases.)
A box fan pressurizing the attic with air drawn through a garage seems like a dubious "solution" no matter what it does or doesn't do for energy use, given that pressurizing the attic with potentially contaminated garage air can potentially enter the conditioned space via the leaky ceiling and leaky ducts.