4 Ways to Control Humidity in an Encapsulated Attic
Bringing an attic within the building enclosure that defines the conditioned space has a lot of benefits. By having the insulation and air barrier at the roofline, the attic stays close to the temperature in the living space below. That helps make heating and cooling more energy efficient if you have equipment or ducts in the attic. It also makes the attic a better place for storing stuff. But you have to be careful. If you don’t control humidity in an encapsulated attic, you can create some serious—and expensive—problems.
Control humidity, you ask? Do I really need to do that? Well, maybe. If you’re in a dry climate, you probably don’t need to worry about it unless you have an aquarium obsession or cook a lot of pasta. Also, it’s not often a wintertime problem because cold air is dry air.
But if you’re in a humid climate—especially hot or mixed humid—you should be proactive and address the potential for moisture problems.
Let’s look at four ways to do that.
1. Supply air from the HVAC system
This is the most common and least expensive way to condition an encapsulated attic. With ductwork in the attic, you tap into the supply side and add a small amount of heated or cooled air to the attic. That makes the attic directly conditioned.
How much conditioned air should you supply? As with encapsulated crawl spaces, you generally want 20 to 50 cubic foot per minute (cfm) of air flow for each 1,000 square feet (sf) of attic floor area. The idea is that in summer, that provides just enough cool, dry air to control humidity in an encapsulated attic.
It’s not a bad way to control humidity in an encapsulated attic, but it’s not ideal either. The primary reason to go with a different method is that there’s not a separate thermostat for the attic. That means the HVAC system runs when the living space below needs cool air, not when the encapsulated attic needs drying. So you may not do enough drying with this method, especially on those cooler summer days when the AC doesn’t run a lot but the humidity is high.
Another potential problem with this method is that it pressurizes the attic. That can force attic air and any odors it contains down into the house. Or, if the attic isn’t well sealed to the outdoors, it can send conditioned air outside.
2. A small exhaust fan
This method takes a nearly opposite approach. Instead of supplying conditioned air directly to an encapsulated attic, you exhaust a small amount of air from the attic. Assuming the attic is well-sealed to the outdoors, this method results in indirect conditioning of the attic. The exhaust fan creates a negative pressure in the attic, and that allows conditioned air from the living space to move up into the attic.
The amount of air you want to exhaust is similar to what you’d supply with the previous method: 20 to 50 cfm per 1,000 sf.
One advantage this method has over adding supply air is that you can control it based on the needs of the attic. In my house, I have a fan that can run at 50, 80, or 110 cfm. I also have the switch on a controller that allows me to run the fun continuously (60 minutes per hour) or however many minutes per hour I want. Because it’s needed only for cooling season, I leave it off in winter. Lately I’ve been running it at ~30 minutes per hour in the summer.
I wrote about the results of using this method in my own house, and you can read that article to see how well it has performed.
3. A dehumidifier
If you’re in a hot or mixed humid climate, this one’s the best way to control humidity in an encapsulated attic. It operates based on the conditions in the attic, and it controls the humidity directly…by removing water vapor from the attic air. We specify dehumidifiers just for dehumidifying encapsulated attics in a lot of our HVAC design jobs at Energy Vanguard.
Yes, it’s another piece of mechanical equipment to buy, install, and maintain. But if you go another route and end up with mold in your attic or rotten roof decking, you’ll wish you had installed a dehumidifier.
4. A vapor diffusion vent
This final method is not like the other three. Those all rely on air movement to control humidity in an encapsulated attic. This one relies on the process of vapor diffusion.
If you follow the work of Dr. Joseph Lstiburek at Building Science Corporation, you may have seen him talk or write about vapor diffusion vents. The idea here is to take advantage of the stratification of humidity that occurs in encapsulated attics. The graph below shows that humidity peaks in the afternoon of a summer day. And it also is higher near the ridge than at the attic floor…a lot higher sometimes.
So Dr. Joe found that by providing a way for that water vapor to exit the attic at the ridge, you can get rid of a lot of the humidity. The vent at the top is, as the name says, for vapor diffusion. You don’t want air going through there because an encapsulated attic should be airtight.
But when you have a high concentration of water vapor near the ridge, putting a vapor permeable material like house wrap over an expanded ridge vent gap allows water vapor to diffuse though the vapor diffusion vent and leave the attic. For more details on this method, see his article, Venting Vapor.
With a complex roofline, installing a vapor diffusion vent can be tricky. And you may not like the look of the expanded ridge vent. (See Photograph 7 in Joe’s article.) Also, he says it’s really only for IECC climate zones 1 through 3.
Oh, and you still have to do one of the three methods above. Dr. Joe recommends 50 cfm per 1,000 sf of supply air. This combination, though, overcomes one of the disadvantages I mentioned above: supply air being controlled by the needs of the living space below.
Type of encapsulation
I haven’t said anything about the way an attic is encapsulated, so let me cover that here. First, you can encapsulate by putting the insulation on top of the roof deck or between the rafters beneath the roof deck. Second, you can do it with air-permeable insulation materials like mineral wool and fiberglass. Or you can use air impermeable insulation like rigid foam or spray foam. Spray foam comes in two types: open-cell, which is vapor permeable, and closed-cell, which is vapor semi-impermeable.
The encapsulated attics that need humidity control the most are those with air-permeable insulation or open-cell spray foam insulation beneath the roof deck. With insulation on top of the roof deck, humidity control is less of an issue. You shouldn’t see the daily swings in humidity that I showed in the graph above because the roof deck is protected from outdoor temperature swings. But it’s not a bad idea to monitor the attic conditions.
The nitty gritty
When you move the building enclosure to the roofline, you change the moisture dynamics of a house. Depending on the climate and the type of encapsulation, you may need to control humidity in an encapsulated attic. If you do, here’s how I rank the methods above in terms of how well they’ll do the job:
- Dehumidifier. This method attacks the problem head on.
- Supply air plus vapor diffusion vent. A good one-two punch.
- Exhaust fan. This can work in a mixed-humid climate but may not be up to the task in a hot-humid climate.
- Vapor diffusion vent. It may be adequate by itself in less humid areas. Or maybe you have enough duct leakage in the encapsulated attic so you don’t have to add more supply air.
We get asked about humidity in encapsulated attics a lot. It’s a real issue, and now you know that there are solutions. The problem is that every house is different, and the number one answer to building science questions is “It depends.” If you’re having trouble sorting through this, contact us at ev****@************rd.com. We do consulting and can help you with isolating the problem and coming up with a good solution.
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 bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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This came at exactly the right time. I’ve got someone I’m helping with a humid attic problem. He’s already tried a dehumidifier and spent way too much time up there himself. Consumer grade dehumidifiers have short life spans and just make it more hot up there.
He didn’t realize he was fighting outside air. The open cell foam needed some additional touch ups, it had a few significant air leaks (something I believe you’re familiar with) Since it’s almost impossible to get a supply duct up from the basement, I think we’re gonna be going with the exhaust fan method. This just helped me narrow my thoughts down on the issue without having scour the internet. It’s like you read my mind.
Andrew: The exhaust fan should work well to control summer humidity in Michigan. But if you’ve got open-cell SPF in a cold climate, you also have to worry about what happens in winter. Code requires a vapor retarder over open-cell SPF in cold climates.
Thank you for the timely article.
What specific peak RH humidity value at the ridge vent is consdered too high, to trigger the need for additional humidity mitigation? Does the peak RH humidity value change if the encapsulated attic already has a vapor diffusion vent? Does the peak RH humidity value change based on how often it occurs, and if so, what is the allowable frequency, or frequency at which humidity mitigation is needed?
Regarding the strategy of sending some conditioned air from the AHU into the encapsulated attic, this is definitely on of the least expensive and least disruptive solutions if there is already an AHU in the encapsulated attic. However, I was advised it could not be done unless I applied either a very expensive fire retardant to the spray foam in the rafters, or encapsulated the spray foam in gyp board, as without that it would be against code. Is it now allowed?
Regarding the strategy of using a dehumidifier, in a small cf attic, would a dessicant type dehumidifier work? They are smaller, lightweight, don’t have compressors (less to break), low cost, use an energy efficient to operate and some can be routed to a drain. If the encapsulated attic has a condensate drain for the AHU, it may be able to use the same condensate drain…
cal: If the air spends a significant amount of time above 70% RH, you may be growing stuff. If you still have high humidity after putting in a vapor diffusion vent, you still have a problem. I don’t know what the “allowable frequency” of high humidity would be, but if you’re getting high readings on summer days, it’s too much.
Regarding supply air necessitating an ignition or thermal barrier, I had not heard that before. I have heard that putting a return vent in the attic isn’t allowed for fire reasons. See Joe’s Cool Hand Luke article I mentioned in my article.
Yes, a desiccant dehu can do the job. I didn’t realize you could be small ones, though. Do you have any info on brands, sizes, where to buy?
I’ve only read about the desicaant dehu’s, and know nothing about them. When researching, I found the Eva Dry website. Their EVD 4000 is a dessicant dehumid that removes 7 liters/day and weighs 13.5 lbs. They also have a model EVD 2400 which uses something they call “Peltier” technology that removes 25 oz day but only weighs 5 lbs. Neither of these models have a compressor.
Not sure if links are allowed so did not include.
Any issues you see with direct transfer from living space to attic? Basically a few ceiling return grilles with no duct attached. My new construction has a steel storm shelter in a room but with no gypsum at the bottom of the trusses so entering the area allows you to look up through a gap between the corridor wall and the face of the shelter. Elsewhere, I have a floor level mechanical room with all of my fan coils, water heater, and ERV. I will not be installing gypsum on the bottom of trusses in this room as well. Same concept as directly conditioning the attic but without actually discharging air directly. just making it more of an accessory space but not fully separated. At present, not planning on grilles like I mentioned initially but had I not had these other openings I would plan on transfer grilles.
Lee: That’s an interesting setup. As for issues, you may get more noise through those open areas and they may stay warmer in summer as an encapsulated attic can get up into the mid-80s Fahrenheit…worse if there’s not enough insulation or it’s not airtight.
Heat rises so not too concerned with the effects of the grilles in cooling season. The key is trying to naturally ventilate the attic without affecting building pressurization as an exhaust fan would. In my application, I will have a heat pump water heater in my mechanical room so a very minor amount of cooling and dehumidification is possible.
Well, actually it’s warm air that rises. But in summer, cool air falls is the controlling factor and you get what’s usually called reverse stack effect. It shouldn’t be as bad with an encapsulated attic, though, because the highest temperature should be in the 80s Fahrenheit, not 130ish.
With closed cell foam encapsulating the attic roof deck, do you recommend adding return air with the supply air if that’s the method? Would an ERV system be a better alternative than an exhaust-only fan?
Would there even be an issue if the attic is encapsulated in closed cell? Closed cell is vapor impermeable, so the moisture would have no way to get to the decking, rafters or ridge…
cal: No, closed-cell SPF is fine under the roof deck. In fact, that’s how most attics are encapsulated with spray foam in cold climates.
Paul: I actually put an ERV extract vent in my encapsulated attic. I just got it going at the beginning of this year, so I can’t say how well it works yet. When you add air from an encapsulated attic to your ERV extract, though, you lose a little of the recovery efficiency. On a summer day, the attic will be warmer than the living space. That raises the temperature of the exhaust air, putting it closer to the outdoor temperature. With a smaller ΔT, the heat transfer isn’t as efficient.
I had the underside of my roof deck sprayed with open cell this spring and it has been interesting dealing with high humidity in the upstairs. I just got a dehumidifier integrated and it has helped but at this point I’m not sure I will ever see any power savings. Since the open cell was only done to R-20 would it make sense next time the roof is being replaced to add rigid foam to the top of the roof deck?
Blake: Yeah, R-20 with spray foam at the roof deck is common and even allowed by some building codes, including here in Georgia. I’m not a fan of that:
Does Spray Foam in the Attic Make It a High-Performance Home?
https://www.energyvanguard.com/blog/does-spray-foam-in-the-attic-make-it-a-high-performance-home/
I actually had less than that when we bought this house, but then I had a bunch more open-cell SPF sprayed over the original foam. Adding rigid foam during a roof replacement is a great idea in my opinion. It may be tricky if you have a complex roof, and you might need to do some new detailing at the eaves, though.
I start with 2 inches of closed cell foam and then 8 to 10 inches of open on top of that, and I prefer the humidity sensing bath fans from Lowe’s, they can come on at 60% (mode with a blue light)and apparently after they run a while they turn off 🙂
Punchy, Succinct, and valuable. Thank you, Dr. Bailes. This is how it should be done.
Great post Allison! I have taught on these except I didn’t think to add a vapor diffusion port in allowable climates. The only point I would add is that we find opening the mechanical to often be the most expensive solution because it opens a bypass to the thermal barrier requirement. If we have a damper that will close during a fire, they let us do it, but when we don’t we have to paint the foam with an intumescent coating, that can raise the price of the job by over $1.25/foot of roof deck.
My attic has open cell spray foam under the roof deck without a ducted supply or return. When I asked my HVAC installer about needing them he stated he installs in foam houses all the time and never seen a problem. After moving in 5 years ago I immediately noticed high temps and humidity then after researching my options, I installed a 100 cfm fan in my closet ceiling that comes on based on temperature and humidity setpoints and a passive return on the other end of the house in the laundry. While it did help somewhat, it created another issue, hot or cold laundry depending on time of year. I am considering installing an active HVAC supply and return to the attic, and since the attic would become a plenum, I plan to install a smoke detector that will shutdown the HVAC if smoke is detected. I know this isn’t building code compliant but my Tennessee county opted out of TN building codes.
Hi Barry,
The statement from your HVAC installer may be truthful from his point of view, because they don’t know building science and haven’t been called to account.
My best and free advice is to track down the source of the humidity. Vapor always moves from wet to dry—through vapor diffusion, capillary action, infiltration at joints and ridges, or even through gaps in the control layers like the WRB. Think in terms of source control. If you attack the challenge with root-cause analysis, your solutions will be longer lasting.
My training and experience tell me that a durable building is primary and fundamental. Tools like data loggers, thermal imaging cameras, blower door tests, theatrical smoke or smoke pens are invaluable for building forensics. They reveal how air and moisture behave inside assemblies, whether the culprit is leakage, insulation gaps, or even unintended chemical interactions like spray foam Part A/Part B isocyanates.
Try not to chase a quick fix like the result with your laundry room. A systems view will pay off. Buildingscience.com is a wealth of research on this subject.
I look forward to hearing what Dr. Bailes adds to the discussion.
Cheers,
John
Thanks John, I appreciate the advice. I have data loggers in the attic, living and crawlspace/basement. I am scheduling a blower door test this fall and plan to go from there. My builder did not do one because it isn’t required here and I wasn’t familiar with them at the time so didn’t request one. I had a standing seam metal roof installed last year and I could see some small areas of daylight in my attic so I think that is my primary source of moisture infiltration. I also have a partial basement and they didn’t spray foam the two sides of the inner masonry walls so that could be a moisture source also. Once I get the results of the test I will go from there but most likely will end up getting some foam added to the home. Again thanks for your response. Barry
So very very timely! Thank you. Are there any specs for properly sizing an attic dehumidifier? Would there be as much humidity in an encapsulated attic as there is in an encapsulated crawlspace? If so, I could just follow the same guidelines from Aprilaire. But I get the feeling its not exactly apples to apples. We’re also planning on adding an inline whole house dehumidifier tied into the heatpump ductwork up in the encapsulated attic. Is there a way to just add an intake valve (forgive me, I do not know the right terminology) into the dehumidifier and not have to buy a separate unit? Thanks in advance!
Great article. I posted this on Instagram just to share it, since Grein building advisor just reposted it.
Got a question from someone who said what is that large galvanized pipe? That can be a red flag in a spray foam attic. Is that for the range hood?
Travis: That galvanized pipe in the third photo is a furnace flue. Yes, it’s a red flag because you generally don’t want atmospheric combustion in spray foam encapsulated attics. See this article:
The #1 Question to Ask before Putting Spray Foam in Your Attic
https://www.energyvanguard.com/blog/The-1-Question-to-Ask-before-Putting-Spray-Foam-in-Your-Attic
Code allows you to do it if there’s enough volume of air for the combustion. In this case, though, I replaced the furnace with heat pumps before doing the spray foam.