Is Venting an Unconditioned Attic Necessary?
Let’s talk about the controversial issue of venting an unconditioned attic with outdoor air. It’s easy to confuse causes and symptoms sometimes, and that happens a lot with attic ventilation. I’ll say upfront, though, that the International Residential Code requires unconditioned attics to be vented. But let’s look at the building science side of the question here.
Powered attic ventilators
To begin, powered attic ventilators are almost never a good idea. Yes, they can keep the attic cooler in summer than a passively-vented or unvented attic. One problem, though, is they sometimes do that by stealing conditioned air from the living space. These fans, mounted in the roof deck or a gable end, typically move 1,000 to 2,000 cubic feet of air per minute. That amount of air flow can easily pull air from the living space through an unsealed attic floor. In some cases, they can present a safety hazard by backdrafting a natural draft gas water heater and putting carbon monoxide into the home’s air.
I looked at a house once that had mold growing near a bathroom exhaust fan because the powered attic ventilators were sucking humid outdoor air through the bath fan duct. Even in the best-case scenario, the energy used to run the fans often ends up being higher than any savings on cooling the living space. The best way to keep your attic cooler, invest in a reflective or light-colored roof. Or just let the attic get hot and keep the heat out of the house with good insulation and air sealing.
Venting an attic in heating season
Even passive ventilation, although required by building codes, doesn’t always do what its proponents claim: prevent condensation in the attic, minimize the chances of getting ice dams, extend shingle life, and reduce cooling costs. The issue that led to ventilation requirements in building codes is preventing condensation. But does it work? Building science researchers at the University of Illinois believe there’s not much data to support the idea that ventilated attics don’t have moisture problems. Bill Rose, one of those researchers, wrote in his book, Water in Buildings, “It is always preferable to reduce an excess moisture source than to presume its existence and hope to dilute it with outdoor air.”
One reason venting an unconditioned attic doesn’t always help with condensation is that wind is the main driver for passive ventilation. Attics usually don’t have enough height to generate much stack effect. The main reason attic ventilation doesn’t always work, though, is that the moisture that causes problems in attics comes from the living space below. That occurs either because the house has excessive humidity in the winter, the ceiling isn’t air sealed properly, or both. Thus, air sealing is the best way to prevent attic condensation in winter.
Similarly, excessive heat entering the attic is what causes ice dams. Air sealing and insulation are much more effective at preventing ice dams than attic ventilation is. And they’re definitely better than the quick-fix solution of putting electric resistance heat on the roof.
Having said that, sometimes attic ventilation does help with moisture problems. Remember, cold air is dry air, so diluting the attic with outdoor air can dry it out. Engineer Kohta Ueno of Building Science Corporation says enough marginal cases are helped by venting that he wouldn’t recommend skipping it.
Venting an attic in cooling season
The warm climate, summer problems of shingle life and cooling costs also show little improvement from attic ventilation. Shingle life can be shortened by higher temperatures, and asphalt shingles do stay cooler with a ventilated attic. But the effect is small, with only about 6 percent reduction in temperature. Changing the color of the shingles from black to white drops the shingle temperature by about 20 °F, on the other hand. As Bill Rose wrote, “Attic ventilation does not deserve the attention it has received in relation to shingle durability.”For the second problem, cooling costs, attic ventilation supposedly reduces cooling costs in the house. Yes, ventilation can keep the attic cooler, but that doesn’t translate to lower air-conditioning costs. In days of yore, when insulation wasn’t required and no one had ever heard of thermal bypasses, keeping the attic cooler certainly could keep the house cooler. With modern building codes requiring R-38 or higher for nearly the entire United States in addition to verified airtightness, there’s little heat flowing into the house even from the hottest attics. Powered attic ventilators are almost never a good idea.
The sum and substance
In a cold climate, venting an unconditioned attic is probably the safest way to build. It’s often the difference between success and failure when an attic is on the edge. But it’s a method that treats the symptoms, not the cause. An attic doesn’t end up with moisture problems because of a lack of ventilation. It has problems because of moisture and heat from the living space below. The real solution is air sealing and insulation.
For cooling, though, passive venting may or may not help reduce costs. If it does reduce costs, that probably means you need more insulation and air sealing, not a cooler attic. The main exception here is if you have ductwork in the attic. Then, a cooler attic can save on cooling costs even with a sufficiently airtight, insulated ceiling. If keeping the attic cooler is important to you for other reasons, changing the roof color can make the biggest difference.
The big takeaway is that for both heating and cooling, attic ventilation treats symptoms, not root causes. Airtightness, insulation, and roof color are methods for treating the causes. And they do a better job of reducing or eliminating the symptoms.
Note: This is a (slightly revised) excerpt from chapter 11 of my book, “A House Needs to Breathe…Or Does It?“
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science. He also writes the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
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This Post Has 26 Comments
In hot/dry climates I think good attic ventilation is very important for cooling, especially with ducts in the attic. I have been in many homes where one of the main comfort complaints was that it “gets hot as soon as the AC system turns off”. In many cases this turns out to be from radiant heat coming from the ceiling. In 9 out of 10 cases, the passive attic ventilation was blocked and/or seriously undersized. I have data logger graphs showing houses where the AC cycles on and off at night when the outdoor temperature was below 70 degrees. Again, inadequate attic ventilation causing a hot attic all night (it hit over 100 degree during the day). These houses all had good attic insulation, R-30+. I’ve suggested small solar attic fans with good results. They are not strong enough to create a measurable pressure difference between the house and the attic and they have not energy cost. Strategically placed, they can really boost the passive flows.
They turn on when the sun comes up and turn off when it goes down. There are probably some new ones with a battery so they run a couple hours after the sun goes down. California’s energy code prescriptively requires insulation under the roof deck of *properly ventilated* attics in the hotter CA climate zones. It’s proven to be a cost effective cooling measure. I call it “super duper radiant barrier”.
Russ: If it “gets hot as soon as the AC system turns off,” then I have a hard time believing the attic really is adequately insulated and air sealed. If the AC is cooling the house properly and the ceiling is well insulated and air sealed, why would it get hot so quickly? Also, are you worried about having vents at all in wildfire country?
It’s a radiant heat issue. As soon as the air stops moving in the house it’s like a heat lamp is turned on. Once you notice it, you can feel it on your arms. These houses had R-30 batts, but they were not well installed. The interesting thing is that the thermostat does not sense this at all. You feel hot, but it’s not the air temperature. Improving the ceiling insulation and attic venting helped fix the problem.
The houses where the AC cycled on and off at night had “good” insulation. They were less than 2 years old at the time.
Wildfire hazard zones are a completely different beast. My comments do not apply there.
This report is bogus and you are 100 percent right. You need plenty of attic ventilation and the idea of power attic vents sucking air from inside the house is Ludacris unless your house is falling apart. I’ve been in the construction field for over 40 years and there is zero doubt that heading insulation to your headache is a plus. And also making sure your attic as well vanilla get to get rid of the build-up of heat as a complete necessity.
Paul: I do love a vanilla attic!
I agree that’s the most likely cause, but poor duct insulation could also be to blame. If the ducts are in the attic and poorly insulated they will be subject to immediate heat gain once the system turns off, and that heat can pour into the home. It would no doubt come through the ceiling as well if not air sealed and insulated, but the ducts are often forgotten and made the weak link in the ceiling insulation system.
Another great article, Allison. I would love to read a followup article on the use of vapor diffusion ports in conditioned attics.
Something that I’ve wondered about is whether changing roof color from darker to lighter could in some cases inadvertently increase relative humidity in the attic. If the attic temperature decreases (due to cooler shingles) but the amount of moisture entering the attic stays the same, relative humidity in the attic would increase (possibly to a harmful degree). It seems like this could present issues in both vented and conditioned attics. What are your thoughts?
When I purchased my current home the air handler closet was purposely (?) left open to the vented attic. My guess is that this was an innovative contractor’s idea intended to keep the attic cooler and/or decrease the humidity in the home. The air conditioner was more than twice the necessary size for the home.
The article came down against powered ventilation. What about passive vents like the ridge vent in the image accompanying the text?
We are in the process of having a new roof installed because of hail damage. I wanted to purchase “cool roof” shingles. It seems that they are not available anywhere but California.
The energy star shingle ( Shasta White by Owens Corning) is not acceptable to the HOA rules. We went with the impact resistant shingle that’s available for a small upcharge in a light tan color. With a modern synthetic underlay I’m hoping that we get better performance than a standard shingle with 30lb felt. I have monitors in the attic so it will be interesting to see how they perform.
This is timely! I’m just finishing up a retrofit attic insulation and airsealing project in the attic of my circa 1938 masonry home. It’s got 300 +/- square feet of living space with sloping drywall installed directly against the underside of the sloping roof rafters, then knee walls. Behind the knee walls on both sides is accessible storage. The attic floor is 2×6’s and the existing insulation was roughly 3″ of loose fill which didn’t extend out to cover the sill plate at the top of the masonry walls. The house is finished with plaster on plasterboard that’s padded in from the masonry wall with 5/4 × 3 framing lumber. The top corners of the plaster walls were open and visible from inside the attic. My process was to vacuum out voids between the plasterboard and masonry and seal with spray foam. I also sealed the sill plate and the roughly 2″ of fascia with spray foam. Then I compacted and added insulation to get a full 5 & 1/2″ of depth, then I installed 2″ foam board r 13 over the everything, foam sealing the entire parameter and between boards before reinstalling the old floor decking. I did the same on the unconditioned side of the kneewalls. The only existing venting is two small fascia vents of only a few square inches each, and covered by the roof gutters. The original roof was slate which may be self-venting I don’t know. I was considering adding gable vents but now maybe not 🤷
I have been thinking about this for years. I have changed my roof to white shingles (which mildewed to a much darker color. I added blown in insulation to my attic and I added a solar powered roof ventilator- which doesn’t do much. My attic gets hot in Atlanta even in the springtime.
I think another consideration about attic temp. is damage to things stored up there and the effect the heat has on the wood rafters and plywood maybe drying them out too much?
I have read the studies showing that the powered roof attic fans barely cover the cost of the electricity to run them.
I am trying out a cheap idea this year – I bought a $22.00 box fan and strapped it to the gable end vent inside the attic and then hooked up a smart switch ($10.00) to it to run the fan during daylight hours.
I’m not sure it is that helpful but it is a cheap experiment. I need to get measurements of temp. difference. I did check the wattage of the fan it is 32 watts on low so at .16 per KW running 12 hours a day X 30 days =$ 1.84 a month in electrical cost — I am hopeful it can be a worthwhile investment of $33.00. One of my friends thinks the fan will melt in the attic- yikes! ( box fans are now completely plastic)
I also built a 2″ thick insulation box over my attic stairwell, sealed all the electric ceiling outlet boxes and ran the bath exhaust vent hose thru outside gable wall.
I don’t see mildew in many attics in Atlanta ( I’m a HVAC Tech). I hate working on air handlers and furnaces in the attics in the summer and it must kill the efficiency in so many ways but it is common
almost as common as black asphalt shingled roofs.
We just replaced a solar attic vent fan, it didn’t move enough air to make any difference, so we replaced it with a thermostatic controlled powered vent fan and oh boy the attic was a lot cooler on her AC ductwork and storage items, made a happy customer
Is there any data on what happens when a reflective metal roof gets dirty? While you were in Austin (sorry to miss you!) you might have noticed that many of the silver metal roofs get very dirty. I think this happens primarily due to sticky stuff that comes from aphid bugs (“honeydew”). Getting that off doesn’t take much, but it does require some physical brushing. Annoying with cars, but almost never done with roofs.
See comparison pic here:
My theory is that the dust blows into it, and creates a layer of dirt. It’s unslightly (but not too bad). I wonder if it noticeably impacts reflectivity/emissivity?
Good article, Allison. On a related topic, I am wondering about a fan we had in our last house (we live in southern part of Michigan). We had a “whole house attic fan” which was essentially a 2’x2′ louvered box fan mounted horizontally in the ceiling of our second floor. We used this fan often in warmer parts of spring and fall and cooler parts of summer when temps during the day were hot but evening temps dipped below 70 or 65. An hour before bed we would open a couple windows on the first floor and let the fan run for a few minutes, pulling in cool outdoor air and exhausting hot air from attic and house out through the attic (eave and ridge vents). This would cool our house very quickly for comfortable sleeping without a need for running A/C. After reading this article I’m wondering if we were creating any air quality or air pressure issues with this arrangement. The only thing I can think of as a negative was if a next door neighbor had a fire pit burning, but that was typically put out before we ran the fan. I made sure to open enough window area to give the fan adequate air intake. For anybody wondering, I placed layers of rigid foam board above the louvers during the cold seasons when we did not run the fan. Thanks in advance for any comments.
In 1999, I built a 3500sf rancher with a non-conditioned, non-vented attic with black architectural shingles. I put all my mechanicals in an encapsulated crawlspace. I framed with 2 x 6 exterior walls on 24” oc and used cellulose wet spray. My building department and many builders in my town thought I was insane. It was my house and my goal was to find out if all the building science I had been reading and studying would work. I made sure that the ceiling was one continuous plane and I air-sealed it perfectly. I used an R75 of cellulose and I vented all my kitchen and bathroom exhaust vents in hard-ducted, short, straight runs through the roof to the exterior. The house and attic assembly performed perfectly. The attic was hotter than hell but who cares? It was air-sealed and insulated perfectly. There were no mechanicals or venting into the attic. I used jumper ducts in all the bedrooms and eliminated interior pressure issues. I used straight fresh air on the return side of my multistage heat pump. Pure performance and comfort. The roof was replaced a few years ago due to a major hailstorm and I would be willing to take a nap in the crawlspace.
This has been a vexing problem for me across the decades of building in multiple climate zones. I’m now firmly in the camp of “build tight, build right” which includes Joe’s most radical view yet, “don’t ventilate, except”. See note BSI-141 (https://buildingscience.com/documents/building-science-insights/bsi-141-shakespeare-does-roofs). I’m now in zone 4C with ping-pong vapor and night radiant cooling and struggle to think how you could ventilate.
Thank you for the great article Allison. Another problem we’ve got down in CZ1 with vented attics is the high dew point inside of them. The ventilation paths also provide a way for the water vapor to come in and hang out in the attic. Improving the chances of condensation forming on the outer duct surface. (Why is the duct surface at or below dewpoint? I’m sure there is alreadyan article to describe that in the EV blog)
Another by-product of vented attics is the exacerbated sensible and latent gains that come in through fixtures and top plates due to mechanical driven infiltration as a consequence of leaky ducts and/or umbalanced rooms. Though is not the vented attic’s fault that the house literally sucks when the HVAC is running.
Thanks again for the article!
@Genry Garcia, I would say that if the HVAC air handler and ductwork is in the vented attic, that is a prime candidate for a spray foam encapsulation. My house was also designed that way, and it is a poor design. About two years ago I had the attic spray foamed (3″ of closed cell, Central VA) and it was a great decision. A decision that I based in part on some of the articles I read on the Energy Vanguard blog. In terms of energy savings, it probably helps the most in the winter. But it also helps in the summer. I do not condition the attic, I just let the duct leakage make it a “semi-conditioned” space. I have considered a dehumidifier, but so far have not felt it was needed (I monitor the temp and humidity). I did the same thing to the crawl space, but I did put a dehumidifier there.
As a follow-up, yesterday, in Catonsville, MD (Baltimore suburb) the high temperature was 82° with humidity in the mid-30’s. Inside the poorly vented attic, the high temperature was 120° @ 67% humidity. So + 40° on a mild day in the attic. What’s the rule for too hot/humid in an attic? Is there one?
I don’t believe if there is any amount of insulation in the attic, that house AC is going to be lost by using an attic vent fan, we install lots of thermostat controlled attic vent fans and they really help keep the AC from running so much and also can save an asphalt single roof from a short lifespan and if course if the attic is a storage place so I definitely a proponent of attic ventilation
In a previous home (which by today’s parlance would almost have been a tiny home, with 550 SF or so of CFA), I had set up a PV-powered gable vent attic ventilator. Was a cheap little thing, and not sure it did much other than maybe cool it down up there slightly faster in evenings. Then again, for a long time we had no hardwired AC there, and I was just as often using the “poor man’s economizer” in the evening, a box fan up against the laundry room window! The things one does with the DIY bug I tell ya!
So where’s your data? Everything you linked to, save 2 links, were to your own website. The two that weren’t, one was to the international building code, and the other didn’t show any hard data at all. Science requires data, and the plural of anecdote isn’t data.
P.S: a low wattage attic fan can be installed in a few minutes by anyone who can reach a gable vent and operate a screwdriver, and costs a few hundred dollars at most. A new roof is tens of thousands of dollars and may have been good for decades to come. How. Is it more cost effective to retrofit a roof versus install a thermostatic fan drawing 20 watts only when it’s above 90° up there?
The reason why attic fans aren’t such a great idea is due to the fact that they can depressurize the house through leakage in the ceiling. As a result, this can bring on new issues that increase humidity inside the home, degrade comfort, and increase energy consumption.
An airtight ceiling plane and adequate soffit and ridge ventilation is what will keep the attic space happy and healthy without negatively impacting the rest of the house. This is basic building science stuff. I don’t think Allison really needs to show data here. It’s simply an explanation of the physics involved.
Pages 4 & 5. But the whole thing is a good read.
This is great if you have well-sealed ceilings in a modern home. My NJ split-level home was built in 1954. The sad 2×6 ceiling joists have the thickest insulation they can take. The outside walls are 2×4. There is no “air sealing” above the ceiling sheetrock. However, my problem is not a hot second floor. My problem is the attic itself. My attic’s only real ventilation (perhaps there are soffit vents as well) are 2 gable vents at 90 degrees to each other. In the summer, going into the attic is unbearable for more than a minute or two. Other than a powered ventilator, what is going to cool down the attic to make it suitable for humans? Why the concern that air will be pulled from the lower floors if there is outside air available from the other gable vent? And if pulling humid air through a bathroom vent is a problem, wouldn’t bathroom vent fans cause the same issue?
This is for Joseph in NJ. I escaped NJ for PA and rehabbed a house that probably killed its previous owner. I battled through mold, poor ventilation and radon (still working on that one). I loved the floor plan of the house which looks like split level but is actually two boxes pressed together, one having a basement and the other having a second floor (on slab). While working in the attic over the second floor, I experienced the skin melting temps you encountered. The house has a triangular gable vent on either side. I am a fan of squirrel-cage blower fans. I have many for different uses. I installed one in the attic next to one gable and use corrugated card board to point the narrow, focused exhaust out the close-by gable and cardboard to assure the air coming into fan is coming from the interior of the attic. The fan was a three speed thing I purchased from Amazon. Total cost was about $90. It pulled outdoor air from the vent across the attic and worked amazingly well. I put a temp switch in to turn ON the fan when temps got about 75 degrees . That was about 5 years ago. I forgot about it. Had to go up into the attic recently, was amazed how comfortable it was. The fan was running. The air being pulled in was actually refreshing. Someday, I might do something more permanent. I have since replaced the roof and had a ridge vent installed. The fan sill makes the big difference. I am working or opening up the soffit vents on this side of the house. I did on the other side (the one with the basement) which keeps the attic bearable in the summer and assures the mold is not coming back. Strangely, the previous owner, a contractor, installed cement board on all wall/ceiling surfaces when he purchased/built it. It has studs, 1/2 wall board covered by 1/2 cement board. This seals up the house amazingly well and probably saved the bones from collapsing under the problems he cause while living in it. I was curious as to why he did that and have arrived at the thought that he created a “heat store”. That is, once you warm up the walls in the winter, they stay evenly warm. I installed under floor hydronic heating and the effect is wonderful. My house is a constant 70 degrees all winter. It is also cooler inside than outside all summer. Try the cheap fan on a temp sensor to fix your attic problem.