This tech tip was heavily inspired by one aspect of Rick Sims’s 2021 HVACR Training Symposium session titled “Conductsation.” The entire presentation is worth a watch, but the advice in “Conductsation” and this tech tip are tailored to hot, humid climates. The contents of this tech tip may NOT work for colder climates. Dr. Allison Bailes from Energy Vanguard has a great article that addresses the unique challenges of colder climates, including the formation of ice dams in the winter.

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Rick Sims’s “Conductsation” symposium session from a few years ago is one I keep coming back to time and time again because we see so many misinformed building designs in our market (Florida). His presentation contained a ton of great information about different causes of duct sweating in humid markets like Florida, but he had one particularly interesting point: ventilation design in unconditioned attics can either take advantage of natural air movement and cause little trouble, or they can dump moist air into the attic and make it rain on the ducts and air handlers (in attic installations). 

If an attic is NOT in the thermal building envelope (i.e., if it’s not conditioned), ventilation is required per the latest International Residential Code (IRC). The IRC lays out requirements for vent area and placement in R806.2, and this code helps us design vents in ways that take advantage of heat and moisture’s natural movement to prevent water vapor from condensing on duct surfaces—and potentially causing bigger structural problems.   

The IRC’s Guidance

Section R806 of the 2021 IRC covers roof ventilation practices, with R806.1 laying out the basics for vent opening sizes and screening materials for protection from the elements and animals—not particularly interesting for our purposes. 

However, R806.2 sets forth the minimum ventilation area requirements. The general requirement is 1/150 of the vented space (practically speaking, 1 square foot of vent area per 150 square feet of attic area), but there is an exception. The minimum vent area could be as low as 1/300, so long as the following conditions are met:

• If in Climate Zones 6, 7, and 8, a Class I or II vapor retarder must be installed on the warm-in-winter side of the ceiling. Other climate zones do NOT require a vapor retarder by code.
• No less than 40% and no more than 50% of the ventilation area (i.e., where vents are installed) shall be in the rafters or upper attic space (within 3 vertical feet of the ridge or highest point) EXCEPT in cases where wall or roof framing members conflict with vent installation. In those cases, vents are permitted to be installed more than 3 feet below the highest point. The remaining ventilation must be provided in the bottom third of the attic space.

We’re based in the humid part of Climate Zone 2 (but some of South Florida is in Climate Zone 1), so the vapor retarder condition doesn’t apply to us. However, that second condition matters a lot. Placing the vents outside the locations indicated in the code—or with the improper venting area in each location—can directly contribute to duct and air handler sweating in attics.

Dew Point and Ventilation

When we talk about moisture problems in attics, we’re referring to when surfaces sweat, which can then lead to other nasty consequences like fungal and bacterial growth. Sweating only happens when wet air makes contact with a surface below the dew point. 

The dew point is when the air is at 100% relative humidity (saturation) and cannot carry any additional water vapor with it. (Before anybody comes at me, I know air doesn’t actually “carry” or “hold” water vapor; it’s just a useful way to think about saturation. In reality, water vapor molecules exist in the air thanks to vapor pressure AND water’s partial pressure. As the air temperature increases, the vapor pressure increases. When there is more water vapor in the air, the partial pressure increases. The partial pressure cannot exceed the vapor pressure in our normal everyday conditions, so we get saturation when they’re equal. The ratio of partial pressure to vapor pressure IS relative humidity; a 1/1 ratio = 100%.) 

As a result, excess water vapor must come out as liquid water. In Florida, dew points above 70°F are very common in the summer months. The supply air in the ductwork is usually around 55°F, and of course, while we have duct insulation that makes the duct surface warmer than the air contents inside the duct, the duct surfaces are still significantly cooler than the unconditioned outdoor air that’s coming in. 

Therefore, when we bring in lots of unconditioned attic with no means of dehumidifying it or a clear path out of the attic, that moisture-laden air will come into contact with cold duct surfaces (and the air handler, if it’s installed in the attic). When those surfaces are below the dew point, the water vapor in the air will be pulled out of the air and condense on those surfaces. They’ll sweat. 

Now, we can mitigate that a bit without conditioning the attic when we consider where we place those attic vents.

How Moisture and Heat Behave

Since we were young, we’ve been taught that “heat rises.” While that’s a simplistic way of describing heat movement, it’s largely true; warmer air is less dense, and less-dense air rises while denser air sinks—that’s thermal buoyancy and a key driver of heat transfer via convection. 

Wet air is the same. Despite how oppressive and “heavy” humid air might feel on our bodies, water vapor is actually lighter than air in terms of atomic mass. Most of the air around us is either nitrogen or oxygen, which are molecules N₂ and O₂, respectively. One nitrogen atom has an atomic mass of ~14 atomic mass units (amu); multiply that by two, and the molecular mass of one molecule of N₂ is 28 amu. Oxygen has an atomic mass of 16 amu; multiply that by two to get a molecular mass of 32 amu. Water vapor consists of two hydrogen atoms and one oxygen atom (H₂O), so since hydrogen’s atomic mass is 1 amu, 16 + 1 + 1 = 18 amu. 

Since water vapor is less dense than the other molecules that make up air, it will rise. However, convection is NOT the main way water vapor can make its way to the top of an attic.

Vapor Pressure and Permeability

On top of the density-driven movement, water vapor can pass through vapor-permeable building materials, even when air cannot. As a result, water vapor can get into the roof decking and other foundational materials based on their vapor permeability. From there, that water vapor responds to changes in vapor pressure. 

Vapor permeability is measured in units called perms. No, not the 80s hair—these perms indicate how well water vapor can pass through a material. Though if you want to get real nerdy, one perm is equal to one grain of water vapor per hour, per square foot, per inch of mercury (pressure). A building material with more perms allows more vapor to pass through (and is at least vapor-open, even if it doesn’t allow air to pass and is air-closed), and a building material or vapor retarder with fewer perms allows less to pass through.

Plywood sheathing has a permeability of 10 perms under “wet-cup” conditions (which is considered semi-permeable, bordering on permeable), whereas foil-faced polyiso has a permeability of <0.1 perms (practically vapor impermeable). Oriented strand board (OSB), another common sheathing material, Plywood and OSB sheathing are at least semi-permeable and will allow some water vapor to pass and stay in the material. OSB becomes more permeable when wet, too.

The movement of water vapor is driven by pressure, following the high-to-low rule. When the roof decking absorbs solar radiation and heats up, the vapor pressure increases. Since vapor moves from areas of high pressure to low pressure, some of that water vapor in the high-pressure roof decking will be released to the lower-pressure surrounding air and into the upper areas of the attic. Ideally, it would exit via a ridge vent at the top, but that’s not always the case.

Open-Cell Spray Foam and the Ping Pong Effect

In many cases with unvented attics, we’ll see open-cell spray foam applied directly to the roof sheathing. This spray foam is meant to serve as insulation, but its low density also means it is vapor-open—about 30 perms. Water vapor travels into and out of it easily, and that water makes its way to the wood-based roof decking. 

During the day, the sun beats on the roof and raises the vapor pressure in the building materials. Raising the vapor pressure forces moisture OUT of the roof decking, through the spray foam, and into the attic. Since warm, wet air is less dense than dry air, the water vapor rises. 

At night, it’s a different story. The temperatures drop at night, and the water vapor returns to the building materials, where it is stored. The next day, the process repeats, and the water vapor travels a little higher with each “ping” and “pong” between the roof material and attic air. 

That moisture accumulates at the attic ridge over time. When moisture can’t exit the attic ridge and just passes back and forth between the building materials and the air, it can rot building materials that don’t dry very well.

When Convection and Vapor Pressure Don’t Drive Air & Moisture Movement…

We have yet to mention a significant pressure driver: wind. Wind helps move air and creates the pressure differentials needed to drive air out of the attic.

Soffit vents take in air down low, and wind drives the motion that brings air to those. Ridge vents exhaust it up high via natural convection (with some guidance from baffles). While the wind pushes air above the roof and against its slopes, it helps create negative pressure when it blows across the ridge vent(s), forcing some attic air out.

But while wind is helpful in well-designed ventilation systems, it can also cause lots of problems when we have significant leaks or poor vent designs. When wind pushes air straight into the attic via poorly placed off-ridge exhaust vents or significant gaps in the structure, that air doesn’t have the same designed ventilation pathway as it would with properly planned soffit and ridge vents that follow the IRC’s guidance. 

Additionally, that air contains water vapor. Generally speaking, bulk water is the worst way for water to enter (as opposed to water vapor in the air or via vapor diffusion), but water vapor in the air is a definitive second-place. It also doesn’t help that wind often comes with severe (read: very wet) weather conditions. 

Vent Placement: Expectation vs. Reality

Based on the way heat and water vapor behave and the IRC’s guidance, it would make sense for a Florida home to have 60% of the attic ventilation come in via low soffit vents and 40% at ridge vents in the peaks. 

With that design, we can take advantage of solar radiation, convection, water vapor’s natural tendencies, and baffles to keep the fresh air on the edges of the attic and on a clear ventilation pathway—that is, away from centrally located supply ducts and air handlers. When the air in the center of the attic has no reason to move or be displaced by incoming air, there isn’t nearly as much condensation. 

Unfortunately, that’s not quite what we see a lot of the time.

Off-Ridge Vents and Crossflow

If you’ve ever seen Florida McMansions, you may have seen sections of roofing about midway up the roof that jut out—very clear off-ridge vents. Rick Sims called them “big, goofy roof caps,” and they are heavy contributors to the moisture problems we see. 

These vents are intended to be exhaust vents, but they’re lower than the ridge and allow large volumes of humid air to enter the unconditioned attic via wind. Even worse, their location enables crossflow; instead of staying along the edges and exiting the structure at or near the peak, humid air gets dumped into the middle of the attic and is more likely to flow across the attic, bringing its moisture with it and causing ducts to sweat between the added moisture and movement. 

When we have off-ridge vents instead of ridge vents for exhaust, it’s also worth mentioning that the moisture that does reach the top just cycles. Again, when the roof decking loses heat at night, that moisture will condense onto the material. That can lead to its own set of problems, like roof rot. 

The Real Effects of Poorly Designed Ventilation

In very humid climates, like Florida, it’s not unrealistic to introduce a gallon of moisture every hour for every 110 CFM that comes in through those vents and is allowed to move across the attic. 

You can imagine how that moisture math quickly gets out of hand over time with inadequate exhaust ventilation. One gallon per 110 CFM per hour is 24 gallons per day per CFM, etc. The gallon per 110 CFM moisture rate is a Florida-based rule of thumb and isn’t as extreme in other climates, but it’s a reality for many homes in Florida and other coastal areas of the Southeast.

Of course, there are many more reasons why ducts and air handlers sweat in the attic, but improper vent location is a big reason.

A Word of Caution for Knee Walls

The idea that air comes in through soffit vents, rises along the outer area of the attic, and exits through the ridge vents only works if convection is allowed to work uninhibited. Now, what might hinder natural convection? 

Enter knee walls.

Knee walls separate a conditioned room from an unconditioned attic. In a hot climate, there will be a clear temperature difference between a conditioned room and an unconditioned attic; the room will be much cooler. Heat naturally wants to move from hot to cold (high to low all over again), so there will be heat losses across the wall via conduction as heat in the attic moves toward the cooler space. 

What will happen to the warm air at the top when it gets close to the cold wall? It’ll sink. This process will keep happening as more warm air moves toward the cool wall to replace the air that just sank, forming a convective loop.

By extension, ventilation strategies that rely on natural convection won’t work the same way to prevent sweating when you have a cool knee wall on one side. In those cases, ducts and air handlers should be as far away from the knee wall as possible for the best chance at reducing sweating. 

Moisture Management in Sealed Attics

Everything in this tech tip so far has been about vented, unconditioned attics. So, what about sealed attics? 

It turns out that heat and moisture behave the same way in sealed, unvented, conditioned attics. The “ping pong” effect still happens, and moisture still accumulates at the ridge. Ridge vents defeat the purpose of a sealed attic by giving air a path, so we need to rely on something else to allow moisture to escape. Ideally, we’d need a membrane that is air-closed (to prevent air from entering and bringing water with it) but still vapor-open (to let water vapor diffuse through it and get out), like a passive pressure-relief valve. That way, we keep our sealed attic and still give moisture an escape path.

Well, we’re in luck. Those membranes are a real thing, and they’re called vapor diffusion ports.

Vapor diffusion ports are air-closed, vapor-open building materials, meaning they allow water vapor to pass through (and exit the structure) but block air movement (and the negative effects of air entering the structure). In other words, they are air barriers but NOT vapor barriers. They usually come in the form of a highly permeable membrane, greater than 20 perms (required by code, but 50 perms or greater is often better). 

The sheathing terminates just shy of the ridge, the membrane fills that gap between the sheathing on each side, the membrane is adhered with proper air-sealing tape and flashing tape, and then a ridge cap conceals the membrane and allows air to pass over it. 

Keep in mind that vapor diffusion ports are approved by code for use in climate zones 1, 2, and 3. They can cause more problems than they solve in cooler climates, including microbial growth on the roof decking (which is what we’re trying to avoid in the first place). 

Again, is poorly designed venting the only reason why ducts and air handlers sweat in the attic? No, but they’re something we shouldn’t ignore. The reality is that air and moisture movement are important and will happen in an attic. Vent (or vapor diffusion port) design can either facilitate that movement or disrupt it and cause moisture to remain in the attic… and roof rot and needless duct sweating. 

Yes, these design factors are usually up to the builder, and the average HVAC tech or installer has no say in it. But we can still be mindful of that potential problem source, and in the case of knee walls, installers can make a choice to install the air handler as far away from those as possible to reduce the likelihood of sweating there.  

P.S. — This tech tip is about passive ventilation. We have an older tech tip with a video by Neil Comparetto showing a bit about the pitfalls of powered attic ventilation, if you want to learn more about that.