I’ve always thought these were a no brainer, and that you definitely saved money in cooling costs by keeping them running in the summer. But in looking around a bit (my fan is somewhat broken and I’m trying to figure out how big of a deal it is) and I see a contrarian opinion out there.
This opinion maintains that attic fans waste energy, because while they do draw some cooler outside air from outside the house, they also suck in some air conditioned air from the floor beneath them, since the ceiling/floor is not airtight. So net-net, these people argue, the attic fan is worse than nothing.
Who is right?
[If there was no air conditioning, it would appear that everyone would agree that attic fans are worthwhile.]
an attic can be about 90F to above an most summer days. that heat is traveling into your cooler conditioned space.
your attic should be vented with enough air (through its air intakes) that a fan don’t create low enough pressure to suck air out of your house. a cold attic is also good for winter to prevent ice dams, so you want lots of ventilation passively.
Maybe it’s just me, but I would never run my attic fan while the central AC is on. My attic fan is the whole-house variety and it pushes air from the inside of the house out through the attic soffits.
I have a weather display that shows the temp inside and the temp outside - when the temp outside drops to or below the temp inside, I would shut the AC down, open some windows and turn on the attic fan.
I usually do this around 9 or 10 at night, and leave the attic fan on until morning.
There is also the type of fan that is installed in a roof gable and pulls air from the soffits and pushes it out of a gable vent. Useful for venting heat in the attic only. I don’t have one of those yet.
Sorry, I can’t say who is right or not, but the way that I use my fan makes sense to me.
They can be a lot hotter than 90F. I’ve been in attics that are 130F+, working for 30 minutes at a time installing HVAC ducting.
Attics fans, whether powered or a roof turbine, are a good way to reduce the heat above the living space. They will not pull air out of your air conditioned space as long as you have proper attic venting.
AC cooled room air shouldn’t be getting into the attic that much, and if it is it’s the warmest air at the top of the room. When you use an attic fan with AC in rooms below there should be enough ventilation in the attic that it wouldn’t draw significantly from below. And it will keep your house cooler, reducing the heat that gets to the rooms below that will need to be air conditioned. So unless you have a wide open entry way into your attic you should come out ahead of the game.
I’m in my first summer since putting a new roof on the house…part of the installation included two solar powered attic fans. Living in South Florida, I know my attic gets really, really hot. In a few months, hopefully I’ll remember to look at my electric bill and compare from the last few years.
Weird. I’ve never even seen an attic fan in Central Florida. I measured the temperature in our attic at 139 degrees when we were moving into the house (it was 97 degrees outside.)
Back in the 50’s, they called whole house fans “attic fans.” At least in Texas. We had one in the ceiling of the hallway all the bedrooms opened onto. We used it at night–making things a* little* cooler. It was very loud & didn’t do much for the humidity. The main living area had a breezeway, with jalousie windows–and floor fans. For all those Gulf breezes…
Then we got our first window unit. Bit by bit, AC took over. The attic fan was removed & the hole was sheetrocked.
so are we talking old-fashioned house with a stairway to a room above the insulation inside the roof trusses?
Or are we talking relatively modern house, where the attic is generally a no-go space with open insulation on the “floor”?
In the former, you just need to seal the door well between the attic and the rest of the house.
In a more modern house, the attic is basically a dry covered space between the insulated box of the house interior; and a cap of waterproof plywood and shingles. Inside that fairly closed space, on a hot day, with nice dark shingles, it can get very, very hot. Many houses in sunnier more southerly climates will have thermocouple-activate fans that will vent that air, suck it out to help reduce the heat.
The ceiling should be sealed, usually with a plastic vapour barrier (Heck, the whole heated house should be sealed). This prevents hot air seeping in during the summer, and cold air seeping in during winter. The vapour barrier also prevents condensation issues, where moist warm air goes either up or down and the moisture condenses causing water and mold. (In extreme cases, houses in Canada are known to get large ice build-up in the winter if warm moist air leaks into the attic or walls.)
The fan will suck air through soffit vents and roof vents. If the house is so badly sealed or those vents so lacking, that instead it is sucking air out of the house, cost of lost air-conditioning air will be the very least of your problems. Today builders even seal carefully around lighting electrical boxes.
There will also be insulation across the top of the ceiling, the “floor” of the attic. This prevents summer heat or winter cold from coming into the house through the ceiling.
Having said that - why would you deliberately “bake” the top of the house when you could ventilate it? When you’re trying to air condition a house, having much of the exterior environment at 139F instead of 90F certainly has a detrimental effect on power consumption.
I assume the OP is referring to a powered roof vent, on top of the roof or at one end of an attic.
I’ve seen arguments and measurements on both sides of the coin.
However, given a dark roof, a typically insulated house where the insulation sits atop a flat ceiling layer, well-ventilated eaves, a properly sealed ceiling, and bathroom ducts that run all the way to the roof, I vote for a powered roof vent over the natural convection delivered by passive roof venting such as a ridge vent or a collection of roof vents. I don’t see any significant convective cool air loss from a well-sealed house due to negative pressure in the attic.
My attic gets very very hot–certainly over 130 degrees–on a hot day. So if the ambient temp is 95, my notion is that this cooler air is replacing the hot air in the attic and diminishing the conductive transfer of heat through the ceiling insulation.
My roofer tried to convince me otherwise, arguing that a powered roof vent would simply suck air backward through the ridge vent and other (passive) roof vents. For all I know, he’s right.
If a Doper has a really solid reference, I’d love to read it. Otherwise my powered roof vent is staying on, controlled by a thermostat switch.
Here’s a review of some studies. I did not read through the whole thing, but they quote one study as saying there is a lessening of a/c use, but by less than the cost of running the attic fan itself. [Should be noted that this study dates from the late 70s.]
And there are any number of other ostensible experts like this guy making similar arguments.
The studies seem to show that sealing and insulating is more cost effective than an attic fan. But sealing and insulating may not be easy to do in existing attics so it sounds like an attic fan can be minimally effective in those circumstances. If you’re actually storing things in your attic and you want to keep heat and humidity down then an attic fan sounds like a reasonable alternative.
I don’t have a cite, but I would suspect that ridge vents might change the equation. If your only airflow is from vents under the eaves (like my house was when I bought it) then there’s no way for the attic to set up an efficient airflow that gets hot air out. In that case, I can’t imagine a powered vent not helping (again, provided that the attic properly sealed/vented and not sucking air from inside the house). On the other hand, a ridge vent will also help, and it’s an improvement I went with when my roof needed some other work done as well.
Is there a consensus if gable vent fans are good or not? Provided there is sufficient soffit ventilation? I live in NM and use a swamp cooler to cool my home. All of the duct for the swamp cooler are run through the attic and my attic stays well above 95 degrees.
Second question, is there a way to calculate how much vent square inches are required depending on the CFM size of the fan?
Since I last posted in this thread, I’ve researched the powered gable vent fans (either solar powered or line powered) and have found more negative opinions on this than anything else.
It seems to me that removing the very hot air in the attic any way possible (without drawing or affecting conditioned air from below) makes sense to me. All that high heat in the attic will eventually radiate through the insulation and structure to begin affecting the conditioned space below. I’ve experienced this directly.
The negative opinions that I’ve seen suggest that powered gable fans can cause more problems than they solve, but it’s been quite some time since I looked at this and don’t remember specifics.
The difficulty of installing these myself, and the contrary opinions that I have found about their usefulness have put me off this project for now.
The concept of keeping the air in the attic at a reasonable ambient temperature still makes sense to me, though.
I have no cites … just experience in the industry …
First and foremost … attic venting extends the lifespan of asphalt shingles … if there’s only 1/2" of plywood between the shingles and 190ºF air, them shingles are going to be 190ºF in short order on the underside … 120ºF on the top side … bad things happen with the different expansion rates … so the main reason to vent your attic is to keep the temperatures closer to equal on both top and bottom of the shingle … the symptom of this venting being inadequate is the little rocks in the shingles popping out … those little rocks are there to protect the asphalt from solar radiation, which is surprisingly damaging to roofing … in all my decades of roof patching, far more leaks develop during the summer months than winter …
In theory, a soffit vent every other bay and a full length ridge vent should be good for most places … cooler air comes in at the soffits and exits out the ridge … places like the Desert Southwest in the USA may need more venting at the top …
With that in mind … we can see where a small attic fan help in a big way … small like 25 Watts or so, that’s 18 kW-hrs per month … chump change compared to having to re-roof in 20 years rather than 30 years …
Your ceilings should be air-tight … or go pay $10 for chalking and make it air tight … 14" of fiberglass insulation is code around here … R-39? … just make sure the insulation doesn’t block the soffit vents …
Generally speaking, a house’s attic should have one of two venting systems; either soffit and ridge, or gable and ridge. Having all three is counter productive to good attic ventilation. If you have soffit and ridge ventilation and want to increase air flow, then roof vents (either passive or powered) would be the way to go. If you have the other, then gable fans would be the way to go.
On the topics of attic and crawlspace ventilation and insulation, the internet is full of conflicting “expert” opinions based on experience and theory but empirical studies are rare.
I propose that the Chief Pedant conduct an experiment. On several hot days, record the temperature of the living space, the ceiling above the living space, the attic above the living space, and the outside air both with power supplied to the vent fan and without power supplied to the fan. See if the relationships between the variables changes systematically depending on whether the vent fan is powered or not. My bet is that so little heat is transferred from the attic to the living space that any cooling of the attic by the fan will have a negligible effect on the temperature of the ceiling and hence a negligible effect on the temperature of the living space.