Attic ventilation questions
Hi Guys,
So after getting a $310 bill from the folks at Duke Energy last month, I decided it was time to figure out a more efficient way to cool our home. I’ve since asked them for their free energy audit to identify areas where I can save energy.
I had to go into the attic recently, and noticed it was ridiculously hot. On hot days, we set our thermostat on 72, and the home is lucky to stay ten degrees above that. Now, I’m not a builder, but I walked around our home, and noticed only one gable vent – which was above the garage. We have a single family home that’s about 1600 square feet. No where else on the home did I see any vents.
I read this very informative article:
Fans in the Attic: Do They Help or Do They Hurt?
Of course, this was a few hours after ordering the $75 powered attic fan from lows, and discovering that the author was none to fond of them vs. the whole house vent fans. So I had a few questions:
1) If attic fans don’t work, why all the good reviews from people online saying they’ve lowered their homes temperatures?
2) Our house is shaped like an “L” with the front of the garage being the lower right part of the “L” – which is where the only gable vent is located. If attic fans are helpful, and we did install one, where should we put it? Should we add another gable vent to the top of the “L”, and then put it somewhere in the middle, or could we just put the attic fan on the opposite side of the house.
3) The article mentioned backdraft issues with gas fired appliances. The last thing I want to do is poison my kids with CO2. Are there ways to combat this short of installing additional gable vents?
4) We live in Charlotte, NC – so in the summers we get to 90-95 probably 30-60 days out of the year. At night, we are usually around 70-75 during those same days. Based on our location, would this make a whole house fan a better option than the attic fan?
We are trying to keep costs as low as possible, and I’ll be doing the install myself. I can do some basic home repair stuff, but have never really many true construction jobs.
Any tips are appreciated 🙂
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Replies
Jeff,
Q. "If attic fans don't work, why all the good reviews from people online saying they've lowered their homes temperatures?"
A. Attic fans may work for some buyers. But even if they work, they usually use more electricity than they save, and they may have undesirable side effects.
Q. "Our house is shaped like an L with the front of the garage being the lower right part of the L - which is where the only gable vent is located. If attic fans are helpful, and we did install one, where should we put it?"
A. Attic fans are not helpful. They are likely to suck cool, air-conditioned air out of your house and raise your energy bills.
Q. "Should we add another gable vent to the top of the L and then put it somewhere in the middle?"
A. No.
Q. "Or could we just put the attic fan on the opposite side of the house?"
A. You could, but you shouldn't.
Q. "The article mentioned backdraft issues with gas-fired appliances. The last thing I want to do is poison my kids with CO2. Are there ways to combat this short of installing additional gable vents?"
A. There are two ways to avoid this. The most obvious way: Don't install a powered attic ventilator. If you have one, unplug it or (safely) cut the cord.
The other way to avoid the problem is to swap out your atmospherically vented appliances for new sealed-combustion appliances.
Q. "We live in Charlotte, NC - so in the summers we get to 90-95 probably 30-60 days out of the year. At night, we are usually around 70-75 during those same days. Based on our location, would this make a whole house fan a better option than the attic fan?"
A. A powered attic ventilator never makes sense. As I explained in my article, a whole-house fan often makes sense, but only if you are willing to leave your downstairs windows open all night long, and only during times of the year when nighttime temperatures are cool. In your climate, you might be happier with air conditioning, but every family is different.
If your air conditioner is struggling, the two most important things you can do are: seal air leaks in your ceiling, and add insulation to your attic floor. You may also need to find ways to reduce solar gain through west-facing windows.
Finally, if you have any ductwork or HVAC equipment in your attic, you have a problem. There are many articles on the GBA site that discuss ways to move ducts from an attic to the interior of your home, or to transform a ventilated unconditioned attic into a sealed conditioned attic. If you have trouble finding these articles, just post another comment, and I'll give you the links.
Martin,
I absolutely don't agree with "a powered attic ventilator never makes sense." Millions of people don't have air conditioning, and they cool their homes with ambient air. Powered ventilation is one of their tools for cooling, and powered attic ventilation can be an important part of this puzzle. It's this simple: an attic without powered ventilation remains warm overnight, which causes a warm ceiling in the house.
So maybe you meant to say: "It almost never makes sense to use powered attic ventilation if the house has air conditioning".
Note, we've been discussing this at another spot: https://www.greenbuildingadvisor.com/blogs/dept/building-science/top-two-reasons-powered-attic-ventilators-are-waste-money#post_comments
Kevin,
The original poster (Jeff Lever) is complaining that his electric bill is $310 a month. He also informed us that he sets his thermostat at 72 degrees. From these two clues, I concluded that Jeff, like most homeowners in North Carolina, lives in a house with air conditioning.
I agree with your main point: it almost never makes sense to install a powered attic ventilator in a house with air conditioning.
Even if the house lacks air conditioning, a powered attic ventilator can be dangerous, because it can cause backdrafting in atmospherically vented combustion appliances.
Finally, if you want to cool off your house with an attic-mounted fan, and you don't have air conditioning, what you want is a whole-house fan, not a powered attic ventilator. That was the point of my original article.
One final point: a whole-house fan can also cause backdrafting problems with atmospherically vented appliances, although the risk is reduced if all of your downstairs windows are open.
What is amazing is that the ventilation hardware manufacturing industry and salespeople are still flim-flamming home owners and contractors with the promises of cooler houses using attic fans.
I grew up in Zone 6, heavy damp snowy winters, -20 F degree Februaries, and hot humid summer nights, about 50 miles from the Atlantic ocean, on the bank of a river about 1 mile wide. Houses were sieves of drafts all year long. My father installed an attic fan and we enjoyed great breezes though every designed and fugitive opening and crack. It brought that cool basement air to the 3rd floor. In fact it was best to close windows and doors on the main and second floors so that cool basement air was not affected by incoming outdoor air.
Now-a-days, the air sealing details should be such that no basement air should be able to get to a main, second or third floor and no conditioned main and second floor air should be able to get to the attic. Put a bullet proof air barrier on the attic floor and the basement ceiling; weather strip the doors between these spaces. Then put a ventilation fan in the attic that cannot waste the conditioned air; pulling in cooler evening outside air and exhausting the heated inside air back to the outdoors. Disconnect this system and the attic from the conditioned living space.
Flitch,
I disagree with your advice: "Put a ventilation fan in the attic." In the house you have described, the electricity needed to run the fan is being wasted.
Use a whole-house fan at night if you want. But if you have air conditioning, and you have a vented unconditioned attic, do three things:
1. Keep your ducts indoors.
2. Make your ceiling airtight.
3. Install plenty of attic insulation.
You're done. Don't waste your precious electricity running a fan in your attic.
I agreed with practically everything Martin said with the exception of this one thing:
Q. "We live in Charlotte, NC - so in the summers we get to 90-95 probably 30-60 days out of the year. At night, we are usually around 70-75 during those same days. Based on our location, would this make a whole house fan a better option than the attic fan?
A. A powered attic ventilator never makes sense. As I explained in my article, a whole-house fan often makes sense, but only if you are willing to leave your downstairs windows open all night long, and only during times of the year when nighttime temperatures are cool. In your climate, you might be happier with air conditioning, but every family is different. "
I've been thinking about this exact same question as I'm located in a state with high summer day temperatures and mid 60s temps at night. I agree that opening windows and closing them every day can be a hassle. So while I'm doing a major rehab on my home I've started to install backdraft dampers (makeup air dampers) in the two bedrooms and the living room that bring in cool air from outside automatically whenever the whole house fan is on. To make it as simple as possible I'm planning to hook the whf up to a differential temperature sensor system so that it automatically turns on and off as needed. I anticipate that programming that function will be much more complicated than installing the back draft dampers has been, so if it turns into a hassle I have the fallback position of just manually turning the fan on or off. That's certainly more convenient than going around the house opening and closing windows every day.
Sidenote: if anyone is thinking about this it will certainly pay off in spades to completely air seal the ceiling from the attic and having the attic floor well insulated, as Martin mentioned. I may have rose colored glasses on but I'm hoping to not have to use air conditioning at all, something unheard of in these parts. If I can just get the inside house temperature into the high 60s at night it seems to me my soon to be well insulated, air tight house will stay below the high 70s until the next nights cooling cycle begins anew. A guy can dream can't he?
Eric,
Good luck with your plan. I admire anyone who is willing to design a new system to save energy.
I assume that the backdraft dampers you are installing are connected to some type of exterior air intake grille. Your plan will work, as long as the exterior intake grilles are properly sized. They will need to be fairly big to work.
That introduces another topic: these large intake grilles represent big holes in your thermal envelope, and these holes will tend to leak air during the winter as well as the summer. If you want to go ahead with your plan, you should design some type of well-gasketed insulated shutter for each of these intake grilles, so that they can all be properly sealed in October.
Martin,
Yes, I've thought about that. I've installed regular wall vent hoods on the exterior that come with a round short galvanized tube that goes through the wall. The tube is 8" in diameter. I then have purchased some 8" backdraft dampers to fit in the tube (American Aldes). It's a very nice fit. I think this is the way to go rather than purchasing backdraft dampers with their own ducting that attaches to other ducting. You want to be able to replace the damper itself if need be without replacing the whole installation. The hardest part was locating inside wall vents that attach to round fittings, not square or rectangular. I finally located some.
On the question of sizing: my back of the envelope calculation was that if you have a 12" whf fan blade, 112 square inches, then you need at least that much area for the makeup air vents. And yes, I expect to have to make some well fitted insulated round plugs to fit in the 8" tubes. Putting those in and out twice a year seems like a good trade off for opening and closing windows every day in summer.
All too often attic ventilators cool the attic by sucking cooler conditoined space air into the attic. In an air conditioned house the power use by fan is not merely wasted, it signficantly increases the overall power use by increasing the air conditioning load.
Whole house fans and nighttime ventilation schemes aren't always the best option, particularly if you are air conditioning during the day. In the southeastern US that 70F night time air is often nearly saturated, and ventilating on nights with outdoor dew points north of ~55F introduces a latent load. (Even in New England I check the local dew point projections on Weatherspark.com before throwing open the windows a night for sensible load cooling.)
Eric,
I would use the same guidelines as those for net free area in attics equipped with whole-house fans: namely, one square foot of net free vent area for every 750 cfm of fan capacity. Most whole-house fans are rated at 2,000 cfm and 6,000 cfm; you should check yours.
A 2,000 cfm fan needs a net free vent area of 2.66 square feet. That's equivalent to 384 square inches, or about 8 inlets that measure 8 inches in diameter.
A 6,000 cfm fan needs a net free vent area of 8 square feet. That's equivalent to 1,152 square inches, or about 23 inlets that measure 8 inches in diameter.
So get ready to install between 8 and 23 new holes in your house.
This discussion in this thread has been ping-ponging back and forth between discussions of the use of powered attic ventilators in air-conditioned homes -- a practice that is never recommended -- and the use of whole-house fans for night flushing in a home without air conditioning.
Dana is correct, of course: whole-house fans are intended to be used in houses that are not air conditioned. No one should attempt to combine these two incompatible cooling strategies (air conditioning during the day, and the use of a whole-house fan at night).
Martin,
"A 2,000 cfm fan needs a net free vent area of 2.66 square feet. That's equivalent to 384 square inches, or about 8 inlets that measure 8 inches in diameter."
There's no way that's happening. Besides, the net free vent area in an attic is evacuating up 150 degree air. You need a lot of vent area to naturally vent that area without fans. I'll let you know in any case if my plan does not work out. As Frank Zappa said, "there can be no progress without going outside the norm."
Eric,
You misunderstood the rule of thumb. The rule dictates that attics that have a whole-house fan -- not attics that have passive ventilation without fans -- should have 1 square foot of net free vent area for every 750 cfm of whole-house fan rating. So the rule applies in your case.
It will be interesting to find out whether Frank Zappa's approach works as well for cooling system design as it does for dental-floss farming.
Jeff,
There seems to be some suggestion from previous responses that a whole-house fan may work for you at night. Unless you are much more heat/humidity tolerant than the average human, there's no way you want a whole-house fan around Charlotte. Sure, it would help to cool things down a little, but you would feel like you are sleeping under a damp beach towell all night, all summer, in your climate. You might feel comfortable with it at night for a few weeks during Spring and Fall, but having to seal it up for Winter and Summer just is not worth it.
You mentioned 70 degree nights. You know as well as I do that many of those nights can have temperatures near 80 degrees, lasting far in to the night, toward midnight. By the time you cool off, it's time to wake up and dry off---no sleep. Perhaps Eric can get away with a whole-house fan (he talked about temperatures at night in the 60's--like the Spring/Fall seasons around Charlotte), but I believe you will be taking a whole-house fan back out , if you install one. Some people will say, "Well, people used to sleep without air conditioning, in days of old when there was none." Sure, that's true, and you could condition yourself to do that--but most people that have access to air conditioning won't be doing that.
Perhaps your best bet is to air-tighten/insulate and investigate whether or not you can use high-efficiency minisplits for AC; they are pretty nice, with SEER up around 26.
Sonny,
Agreed. I live in California where it's dry, dry, dry. Even more lately. That's an important differentiation.
Eric,
In a dry climate, a cheap swamp cooler costs about the same as a whole house fan, performs the same function, but also drops the incoming air temperature by 10-20 deg. F. Depending on how you install it, it will cool the 2nd floor much better and faster than a whole house fan.
But I recommend spending the extra $800 for a GREAT swamp cooler: http://www.homedepot.com/p/Champion-Cooler-5000CFM-120V-Down-Draft-Roof-Wall-12-in-Media-Evaporative-Cooler-for-1650-sq-ft-with-Motor-AD1C5112/100662139
If you are familiar with typical swamp coolers, this is a different beast that works well even up to 110F and 25% RH
I think I probably did not make it clear enough, though I implied it. We're in a drought here. A common estimate is that an average swamp cooler uses 7 to 11 gallons of water per hour when working. We're already starting to be restricted on water usage and if things don't improve it will only get worse. For me it makes much more sense to go the WHF route than install something additional that consumes water. Everyone here has swamp coolers so it isn't like it's an unknown thing to those in dry climates. There's much more potential efficiency in WHF's than swamp coolers, provided:
1. Water usage is included, and
2. The whole house fan is intelligently controlled (read "automated") using dual outside and inside temp sensors.
3.[ EDITED, Very important] You have a tight and well insulated home.
In other words, the fan shuts off when the inside temperature is within 2 or 3 degrees of outside temperature at night and then turns on again if the delta gets bigger again. This should be automated because most people are sleeping when this would normally kick in..
Kevin,
Swamp coolers were big in Phoenix but they are now obsolete because they were great during the dry months but are a boat anchor and basically worthless from July through September during the monsoon season. The dew points during monsoon season hoover in the 60's and the ambient air temperature is about 110F-115F. So you can only imagine the miserable conditions a swamp cooler will bring when it's 110F outside with a 65F Dew Point.
Nobody installs swamp coolers in Phoenix anymore. It's an antiquated technology that has been superseded by mini splits and HVAC units.
Jeff, your real issue is "why does it cost $310/month to cool a 1600 s.f. house and what should I do about it?". That's a hugely excessive number and any discussion of mitigating it with attic ventilators, whole house fans, swamp coolers is diversionary, irrelevant or worse to your actual needs. Your home clearly has major enclosure deficiencies and/or mechanical/air distribution issues. Get a professional energy audit - that's your first step. Then address the problems in order of cost-effectiveness. The good news is that you'll soon be saving $200 or more each month in your energy bill.