Common Problems with Cold Weather Ventilation

Posted by Allison Bailes on January 22, 2018

When I woke up Saturday morning, the temperature outdoors was -40 degrees. The wind chill was -100 degrees! It was just unbelievably, impossibly, inhumanly cold outside. Fortunately, that was on a mountaintop in New Hampshire and not where I was. I happened to have woken up on a mountaintop in North Carolina, where the temperature was a much warmer -3° F.

And when it's cold outside, most people prefer to be in warm, cozy home with no drafts. Of course, we all know that no drafts means we have good airtightness, and good airtightness means we need mechanical ventilation. But how do you ventilate a home when it's really, really cold outdoors without causing problems? And what are those problems with cold weather ventilation?

The 3 whole-house ventilation strategies

Exhaust-only ventilation. This one's the cheapest to install and the most common. It relies on fans that are already in the home: bathroom exhaust fans and kitchen range hood. These fans are set to run continuously or intermittently with a controller. As they exhaust air from the home, the resulting negative pressure inside causes air from outside to come in through leakage sites in the building enclosure.

Supply-only ventilation. Like exhaust-only but the fans blow outdoor air into the house, creating a positive pressure. It can be done with regular bath fans or specially designed fans or by hooking up a controller to the central air handler.

Balanced ventilation. Most people automatically think of the energy recovery ventilator (ERV) and heat recovery ventilator (HRV) here, but balanced ventilation simply means supplying and exhausting equal quantities of air. I've written previously about different ways to do balanced ventilation.

When it's really cold, bringing in outdoor air needs to be done with some forethought. There's a reason we don't just open the windows year-round. So let's take a look at what can go wrong and what we can do to make things better.

Cold drafts

The problem. With any of the three strategies, you could end up with comfort problems if the occupants feel cold drafts. This can happen in a variety of ways. With exhaust-only, you could cause a draft through a leaky window or door near an occupied zone. With supply-only, you could be blowing untempered cold air right on occupants with a bad design. You could also use the central heating system to blow tempered air through all the vents in the home, but that air will feel cold to occupants, even if it's close to room temperature. Same goes for ERVs and HRVs.

The solutions. This is mostly a design issue. Good HVAC contractors will tell you: Never blow air directly at the occupants. When ventilating with cold outdoor air, this is even more true. You want that air to be well-mixed by the time it reaches them. With exhaust-only ventilation, this is hard to do...and yet another reason to avoid that strategy. Another aspect of this is the ventilation rate. If you're ventilating more than necessary, this problem is more likely.

Dry air

The problem. Cold air is dry air. 100% relative humidity sounds like really humid air, but when it's at a temperature of 32° F, it's actually pretty dry in terms of actual moisture concentration. If you warm up that same air to 70° F, the relative humidity drops to 20%. The colder it is outdoors, the lower the moisture in the air and the dryer your home becomes when that air comes inside, either through intentional whole-house ventilation or through infiltration. When you reach for that lotion, remember that the source of the problem is cold, dry outdoor air.

The solutions. Exhaust-only and supply-only ventilation with cold air will dry out the indoor air. That's why indoor humidity runs lower in winter. In homes with a lot of air leakage, the indoor air can be bone dry. The same is true in homes with too much ventilation. Make sure you're not ventilating more than you need to. 

The question of how much ventilation you need is a big one, and one that I've addressed here previously. The standard answer is to ventilate at a rate specified by the ASHRAE residential ventilation standard, 62.2. The building code is diverging a bit from 62.2, and the rate in the 2018 International Residential Code (IRC) is:

(0.01 cfm/square foot of conditioned floor area) + (7.5 cfm/person)

For code purposes, the number of people in a home is defined as the number of bedrooms plus one.

But of course, what rate you choose depends on where you are in the process. If you're building a new house, you have to go by your local code. If you're living in a house with mechanical ventilation system, you can run it how you wish. You can use the formula above to get an idea of  whether or not you might be overventilating. If the result of your calculation is that you need 50 cubic feet per minute (cfm) of ventilation air and you're running your 200 cfm HRV continuously, I think it's fair to say you're ventilating more than you need to.

You also may be able to reduce the amount of ventilation air further when it's really cold. Why? Because the stack effect increases with temperature difference. The colder it gets outdoors, the more infiltration you'll have because of warm air rising inside the home. Some ventilation devices have controls that automatically cut down the amount of ventilation air when it's cold.

Another way to reduce the drying effect of ventilating in cold weather is to use an energy recovery ventilator. They exchange heat and moisture, thereby allowing you to keep the humidity levels higher. Instead of exhausting the humidity you already have and replacing it with dry air, some of the water vapor transfers across the membrane and comes back into the house.

And that brings up the next problem of cold weather ventilation...

Frozen ERV cores

The problem. I don't live in a cold climate and don't have direct experience with this, but if you talk to people in the really cold places, like IECC climate zones 6 and higher, they'll say you have to use a heat recovery ventilator rather than an energy recovery ventilator. The reason behind their claim is that when that water vapor transfers through the energy recovery ventilator membrane, it encounters that incoming cold air and freezes. As the mass of ice in the ventilator grows, you lose not only the moisture recovery and heat recovery but also the ventilation air. Ice, as it turns out, is a pretty good air barrier.

The solution. I'm sure some people will tell me in the comments to this article about actual freeze-up cases. What I know is that frozen ERV cores don't have to happen in cold climates. How do I know? Because the Cold Climate Housing Research Center (CCHRC) has done some research on it. They studied 14 houses with 8 models of energy recovery ventilators in Fairbanks, Alaska in the winter of 2013-14. None froze, even though temperatures got as low as -40. (I love that temperature! Or at least the interesting factoid about it.)

The solution is simple: a defrost cycle. Different models have different methods for preventing ice, so there's not just one to do it. Controls could close and shut off the intake side every once in a while, thus turning your ERV into an exhaust-only system temporarily. Similarly, the intake side could have a damper that switches from outdoor air to room air periodically. Another option is that the incoming air can be tempered with room air to keep the core above the freezing point. Frozen ERV cores aren't a given in cold climates.

Frozen HRV condensate lines

The problem. An energy recovery ventilator can get freezing at the core, where the interior air's water vapor permeates through the capillary core membrane to the cold side. A heat recovery ventilator can freeze up, too. In an HRV core, the moisture can condense on the cold, impermeable membrane. That's why HRVs have a drain in the bottom and a condensate line to carry away the liquid water. And that's where the trouble happens. If the end of that condensate line is outdoors, you can get a chunk of ice in it.

The solutions. You can fix this in a few ways. You could wrap the condensate line with heat tape, which uses electric resistance heat to prevent freezing at the end when the temperatures are low enough. That's an inelegant solution that uses more energy, though. Better would be to run the HRV condensate line into a sewer drain line somewhere inside the house. That way the condensate should never encounter below-freezing temperatures. (But make sure you get a qualified plumber for this as there are right ways and wrong ways to do it.) If the condensate line must run to the outdoors, design it so all condensate runs out quickly.

Condensation on air intake ducts

The problem. This was a problem I ran into on one of my very first jobs, way back in 2004. My client was building an ENERGY STAR home so I worked with him to make sure the HVAC contractor installed mechanical ventilation. The intake air duct ran from a wall cap on the outside wall through the basement ceiling to the air handler. In their first winter, they discovered a problem: a line of water spots on the basement ceiling, right below where that intake air duct ran. Turns out, they put no insulation on it.

When cold surfaces run through humid air —  and in winter, the indoor air is where the humidity is — condensation is possible. That's exactly what happened here. That cold ventilation duct turned into a condenser, dropping its condensate on the basement ceiling. This was a case of accidental dehumidification.

The solutions. Condensation on the ventilation intake duct, or even frost if it's cold enough, from having uninsulated intake duct. It could be totally uninsulated, as was the case with my client, or just poorly insulated. Make sure the duct is fully insulated and, as with ducts in crawl spaces, the outer insulation jacket is sealed up.

Condensation on the intake duct also could happen because of an intake duct that isn't properly sealed. That duct is under negative pressure. There's a fan somewhere pulling air in from outdoors. If the duct isn't sealed, it can pull humid interior air in, resulting in condensation. So when you install ventilation intake ducts, you've got to seal them completely, insulate them completely, and seal the insulation jacket completely.

Condensation in walls

The problem. Water vapor likes cold surfaces. It will condense or adsorb on them with glee. (Or is it with gusto? I can never remember.) A supply-only ventilation system in a cold climate can result in positive pressures indoors, which can push humid, indoor air into wall cavities. When that humid air finds cold sheathing on the other side of the insulation, it starts accumulating. If enough accumulates over a long enough period of time, you can start a biology experiment in the wall. That's bad for durability and for indoor air quality. I don't recommend it.

The solutions. In warmer climates (generally zones 4 and lower), cold weather rarely lasts long enough for this to be a problem. In cold climates, this issue has garnered a lot attention because it's real. It's one reason some people recommend using exhaust-only ventilation rather than supply-only. That's far from a guarantee against moisture accumulating in your sheathing, though. Those exhaust fans could be pulling air in down low while the stack effect is pushing it out up high. The photo below (by Joe Lstiburek) shows moisture damage from positive pressure on the second floor of the building. The first floor is fine. (This is from his article, How Do Buildings Stack Up?)

Peeling paint on the second floor from interior moisture caused by positive pressure (photo by Building Science Corp., used with permission)

The real solution here is fix the building enclosure. You can do that by installing exterior insulation that keeps the sheathing warmer or by keeping humid indoor air from leaking into the wall cavity.

Don't forget the V in HVAC

Homes are getting a lot more airtight these days. New homes have to get blower door tests showing they hit certain thresholds. Older homes are getting air-sealed by home performance companies. That means we have a lot more whole-house ventilation systems in use. And no matter how it's done, there's technology involved. There's design. There's variation in conditions, both interior and exterior.

Buildings are put to the test when it's really cold outdoors, as it has been recently with the ridiculously named Bomb Cyclone. (It sounds like a dessert at a chain restaurant.) Building enclosures fail. Heating systems fail. And ventilation systems fail. Things happen at really low temperatures that don't happen when it's a more normal cold.

If you're experiencing any of the cold-weather ventilation problems I described above, the first step is figuring out where things went wrong. Was it the technology? The design? The installation? Knowing some building science can help a lot as you figure it out.

My hope is that eventually whole-house ventilation will be accorded the status it deserves. The initials HVAC seem to accord it equal weight to heating and air conditioning, yet that's not currently the reality. But for now, I'm anxious to hear your cold weather ventilation stories and problems. What problems did I miss? What solutions have you found?


Related Articles

Why Do Airtight Homes Need Mechanical Ventilation?

4 Ways to Do Balanced Ventilation

Cold Air Is Dry Air

Accidental Dehumidification - A Preventable Mess



† Now really, I don't have to give units for that temperature. Not because you already know it but because in the two temperature scales in common use, -40° F is equal to -40° C. Even the science nuts who insist on Kelvin know it's impossible ever to reach -40 K because of the third law of thermodynamics. And the same is true of the Rankine scale, which is to Fahrenheit what Kelvin is to Celsius and for which you should not bother creating any neural connections. Therefore, it is never necessary to give units for the temperature -40.


Add a new blog comment!

Enter your comments in the box below:

(Please note that all blog entries and comments are subject to review prior to posting.)


While we will do our best to monitor all comments and blog posts for accuracy and relevancy, Home Energy is not responsible for content posted by our readers or third parties. Home Energy reserves the right to edit or remove comments or blog posts that do not meet our community guidelines.

Recent Blogs
Common Problems with Cold Weather Ventilation

Common Problems with Cold Weather Ventilation

Allison Bailes

When I woke up Saturday morning, the temperature outdoors was -40 degrees†. The wind chill was -100 degrees! It was just unbelievably, impossibly, inhumanly cold outside. Fortunately, that was on a mountaintop in New Hampshire and not where I was. I happened to have woken up on a mountaintop in North Carolina, where the temperature was a much warmer -3° F. [continue reading]

Using God Talk—or Not—to Help the Planet?

Using God Talk—or Not—to Help the Planet?

Jim Gunshinan

I am always impressed with the work of Susan Shelton and the Shelton Group. So an email message with the subject line from her office, “Age, religion & environmental messaging: what works?” immediately caught my attention. [continue reading]

Rebuilding After a Hurricane, and the Next, and the Next...

Rebuilding After a Hurricane, and the Next, and the Next...

Kelly Vaughn

This year’s hurricane season is the most expensive on record, with $202.6 billion in damages according to Bloomberg. These storms across the Atlantic had devastating impacts on people’s lives and homes, on communities, and on infrastructure in the hardest-hit areas. [continue reading]

Degree Days Shuffle

Degree Days Shuffle

Jim Gunshinan

A 2015 study, “Impacts of global warming on residential heating and cooling degree-days in the United States,” comparing heating degree-days (HDD), cooling degree-days (CDD), and HDD + CDD across the United States during the historical period 1981–2010 and the future period 2080–2099 predicts that New York City weather will become more like today’s Oklahoma City weather and Seattle’s will become more like present day San Jose’s weather. In general, ... [continue reading]

Cool—and Warm—Clothing

Cool—and Warm—Clothing

Jim Gunshinan

“Why do you need to cool and heat the whole building? Why don’t you cool and heat individual people?” asked Yi Cui, professor of materials science and engineering at Stanford University. [continue reading]

Smarter Homebuilding: How to Best Incorporate Smart Tech Features

Smarter Homebuilding: How to Best Incorporate Smart Tech Features

Eric Murrell

The old way of integrating technology into a home could be a major headache. If a client doesn’t know exactly what kind of tech they want early in the building process, they could later face the agony of tearing into drywall or missing out on certain features altogether. Speakers and security products required hard wiring, advanced lighting systems required an electrician, and something as simple as installing outdoor security cameras presented its own ... [continue reading]

Building Science Can be Fun

Building Science Can be Fun

Jim Gunshinan

We've come a long way in understanding and providing homeowners with the means to create healthy indoor air. But comfort is a separate matter from good IAQ. Ian Walker and Robert Bean, building scientists from the United States and Candid respectively, and both experts on ventilation, discuss this and much more in an entertaining yet very interesting and informative video produced by Debra Little.  [continue reading]

People Who Save Energy

People Who Save Energy

Jim Gunshinan

According to the American Council for an Energy Efficient Economy (ACEEE), there are millions of workers in the country creating homes that are more energy efficient, and safer and more comfortable to live in. In fact, the Department of Energy puts that number at more than 2-million people working for energy efficiency in the United States. Compare that to the 1.9 million workers producing electricity, coal, natural gas, and gasoline. And the number working for energy ... [continue reading]

Coping with Dust and Smoke From Fires

Coping with Dust and Smoke From Fires

Alex Stadtner

October will long be remembered in Northern California, where wildfires destroyed over 8,000 structures, killed scores, and forever changed Sonoma and Napa counties. Survivors face new threats and hazards seldom addressed in design-build magazines and chat rooms. [continue reading]

Integrating Building Performance with Design

Integrating Building Performance with Design

Elizabeth Grant

Now, more than ever, it is important for design professionals to be advocates of environmentally responsible design. Architects should aim to create places that occupants care about and care for, and maintain and enjoy for a long time. Unfortunately, architects may not have adequate training or experience to handle performance mandates relating to building integrity and comfort. This can result in situations where these parameters are either inadequately addressed, or are relegated to consulting engineers ... [continue reading]

Earn BPI CEU credits
Home Performance with EnergyStar

Harness the power of

Get the Home Energy