Gable vent exhaust fan or no?
May 9, 2013 9:29 AM   Subscribe

Whether to install a gable exhaust fan in my attic and which of 3 gable vents to install it on?

Got a 140+ year-old one-story, 1250 sq.ft. house with a pretty hot attic during Texas summers. If you imagine looking down at my house from above, the roof's ridges are shaped roughly like the letter T, with a gable at each end, where the 'bottom' of the T faces south, and 'top' runs east and west. Assume the prevailing winds in my part of Texas are mostly south/southeast during the summer. At the tip of each 'point' of the T, there is a roughly 2' x 3' gable vent with wood louvers angled down, spaced roughly 3-4" apart, totaling 3 vents.

The roof has ridge vents along the length of the T, but has no soffit vents at all (my understanding is that the inflow updraft from soffits is how a ridge vent works--so it may not be fully efficient...or is it getting air through the gable vents?) and soffits cannot be added owing to the design of the house. So the only ventilation is the 3 gable vents and the ridge vent along the roof's ridges.

The roof surface is a gray composite shingle, and the decking is the old lathe-style (it used to have wood shingles--most of which were lazily piled up in my attic and had insulation blown on top of them, wtf). Inside the attic, someone sprayed on a silver radiant barrier paint on the underside of the decking at some point.

Nothing blocks the sun to the west side of the house. As mentioned above, it's a pretty hot attic, but not ridiculously out of the ordinary for Texas. My HVAC and corresponding ductwork are in the attic but no other appliances are up there.

Two things I've been wondering, with the second question being more important than the first:

(1) I am probably going to do it anyway, but is it worth* the money (~ $150) to install a gable exhaust fan in one or more of the three gable vents? (Assume I'll do it myself, attaching it directly to the wooden gable, so we're just talking the cost of a decent fan.) *not expecting to make back $150 in utility bills, just want a cooler attic so my HVAC lasts longer and so bills go down and attic's not so hot

(2) If yes to (1), which of the three gable vents should I install the exhaust fan on? The south-facing one, the west-facing one, or the east-facing one?
posted by resurrexit to Home & Garden (8 answers total)
I've installed gable vent fans a few times (once initially, then once more to replace it after the long Georgia-summer duty cycle). I'm not sure that it matters, but I'd probably do it on the east side, since it'd hopefully be in a bit of shade during the afternoon.

It's a pretty simple installation, and we noticed a marked difference after it was in and running during the summer. The upstairs was unbearably hot, and then it wasn't. That was enough for me.

Later on when we had the roof replaced, the roofers installed a ridge vent (our attic had soffit vents) and we were told that the fan would no longer be necessary. I adjusted the temperature up on it a bit, but left it in place and could still hear it kicking on occasionally when it was really hot out.
posted by jquinby at 10:09 AM on May 9, 2013 [1 favorite]

I'd go for the east side too, because the prevailing winds are likely to be from the west, so you'll be less likely to have the fan fighting higher pressure.

And, yes, I'd put in a fan. We put two in our attic, one of those solar roof vent fans, one a gable fan, and it does wonders for the attic temperature.
posted by straw at 10:18 AM on May 9, 2013 [1 favorite]

Actually, I need to specify - the fan was mounted in the west-facing vent facing outward, so as to draw in from the opposite (hopefully cooler) side of the house.
posted by jquinby at 10:20 AM on May 9, 2013

You are correct. The proximity of the louvered vents to the ridge vent and the fact that you have no soffit vents means that all the air is being circulated at the top of the attic. You need to install soffit vents, then perhaps you won't need a fan. Since as you say you cannot install soffit vents you need to create an opening lower on all of the gable ends. The home supply will have a selection of round and rectangular vents that can be installed on a vertical wall.
posted by Gungho at 10:43 AM on May 9, 2013 [1 favorite]

I wouldn't put a gable fan in. I wouldn't put an attic fan in at all. I'm in Texas as well, and the #1 cause of fatal single-family home fires in our area during the summer (besides stupidity) seems to be poorly maintained attic fans. You stick them up there and wire them in, and then they're out there in the dust and grit and wind and rain until they throw a (cheap, asia-made) bearing and start overheating or develop an electrical short in the (cheap, asia-made) wiring that's not properly grounded to anything with ground fault or arc fault detection, and suddenly you've got a fire in a nice dry part of your house with lots of air availability.

I see that you say that you can't install soffit vents. You'd be surprised. First, if you can take a picture of it, please do, and post it or mefimail it to me. I'd be happy to help you figure out how to do it. If you can't, though, that's fine -- but look into some of the options for adding soffit ventilation for houses that have no soffits.

If you can't, that's still fine -- but add insulation instead of adding ventilation. If you get your attic up to R-60 or so, you won't have anything to worry about with heat getting into the conditioned part of your structure. Blown-in insulation is cheap compared to electricity.

Remember, your attic isn't "inside" -- it's outside, it's just covered with a shed. The only reason that the attic is there is because we seem to like having sloped roofs even in the southern US, and it's a convenient place to stash HVAC systems and insulation. We'd be just fine if we didn't ventilate attics and stuffed them with insulation instead.
posted by SpecialK at 12:03 PM on May 9, 2013 [1 favorite]

Current thinking, backed up by stringent testing, says that venting your attic and insulating your ceiling are more important. In fact, powered attic vent fans seem to actually draw cool air-conditioned air from inside the building envelope through the many cracks and gaps in anything but "superinsulated" construction. Not online, but a question in the latest Fine Homebuilding.
posted by dhartung at 3:28 AM on May 10, 2013 [1 favorite]

Response by poster: Everyone--thanks for the thoughtful responses. As to the possibility of installing soffit vents, the only option that could work for me is the under-the-shingle variety. Because those entail peeling or even cutting the shingles back for a couple of feet all the way around (and honestly I'm not convinced that they would make that much difference anyway) on a pretty new roof, I'm not going that route. I also don't know about adding any additional gable-end vents, no matter what size, because my historic district is pretty stingy on stuff like that--and I'll take a hot attic any day if it means there aren't four three-story townhouses crammed onto the lots around me.

I appreciate the suggestions to insulate better. I really wish the previous owners hadn't blown in insulation on top of all the trash that was up there and that I had gotten the first crack at it. The attic looks really messy, which drives me crazy. Clean-up may be a long, itchy winter project.

Still not sure about the fan or where to put it if so, but I thank all of you for chiming in and thinking out loud with me.
posted by resurrexit at 11:42 AM on May 10, 2013

If your roof is new, you'd be surprised at how cheap/easy it is to peel up the last couple feet of it and install those under-shingle vents. Old shingles crack; new shingles are flexible enough (on a warm summer day) to be peeled up with the help of the right tool and might even be able to be re-used. A *good* roofing company (not your average fly by night) can do it pretty easily and might not even charge that much.

Also, you'd be surprised at how much of a difference it makes. It's not just the "free air" space that makes a difference, it's the convective pattern. Having the air moving along the underside of the sheathing helps to keep the sheathing cool, which helps keep the tar paper and shingles cool, which keeps the petroleum products that make up the roof (tar paper and asphalt in the shingles) from outgassing the volatile parts that hold them together, which keeps them from becoming brittle and breaking in wind or hail.

Yeah, having the previous owners blow in insulation without having pulled out the old stuff or without having them use spray foam to seal the penetrations is a pain in the butt. But, it's better than the more common alternative: No insulation. I helped a friend in Houston insulate his attic last summer. The previously existing insulation was four inches of blown-in. Yeah, four inches. That's it. His electric bill was $500/mo.

If you want to put a gable fan in, wire it to a switch downstairs. When you turn it on, verify it's operation.
posted by SpecialK at 11:58 AM on May 13, 2013

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