When I woke up Saturday morning, the temperature outdoors was -40°F*. 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?
Three 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.
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 is 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.
The problem. Cold air is dry air. Air at 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, some of them will say that 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 way 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.
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. Joe Lstiburek has a great photo of a building showing moisture damage at the top of the building with no damage lower down.
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?
*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°.
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