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Building Science

Ventilating a Home in Cold Weather

You need fresh air, but bringing in cold outdoor air can cause problems

An energy recovery ventilator - or ERV - can solve some of the problems that come with providing outdoor air to a home in cold weather, but it's not a panacea.
Image Credit: Energy Vanguard

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?

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…

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  1. Andrew Bater | | #1

    HRV Cold Weather Defrost Problem & Solution
    I have an HRV that shuts off the input fan in order to defrost.

    This has annoyed me for a few years now, as when that happens the house is depressurized and we suck in raw cold air from elsewhere. Since our home is very tight, most of that comes in through a little gravity damper in our mechanical room. However if I happen to open the door to our masonry heater (fireplace) during that same defrost cycle, a face full of fumes or ashes.

    This last ridiculous cold spell spurred me to action.

    I had noticed that the insulation on the fresh air input duct for the HRV was often waterlogged due to condensation. The insulation wasn't tight enough to keep moisture out. I have flirted with replacing that section of ductwork with a new duct and duct heater. This way I could slightly pre-heat the fresh air to prevent the condensation. It would also allow me to cut down the number of times that the HRV went into defrost. Unfortunately I never quite found the right duct heater, as my duct work sections weren't long enough to prevent turbulence before or after; the elements in any heater were likely to have uneven airflow over them and burn out.

    The epiphany I had the other day was to remove the insulation and wrap the fresh air duct with an electric heating cable. So far this has actually worked quite well. The small amount of heat keeps moisture from condensing on the surface of the duct. Since this duct is now exposed to the warmer air of the mechanical room, the input air temp to the HRV is now slightly tempered/higher too, so the unit goes into defrost less often.

    Pictures follow of the HRV input duct with insulation, images showing ice (!) under the insulation, and the duct with the electric heater cable wrapped around it.

    If you look carefully on the upper right of the one image, you will observe that my little fresh air gravity damper is open and the manometer reading is zero. The HRV was in defrost when I took that picture.

    Pix of the post installation HRV input (OA) and output (MA) temps also attached.

  2. Calum Wilde | | #2

    Is that -40° from


    Is that -40° from north, or from the ships head? :)


    I was faced with the same situation. I ended up buying a newer energy star qualified HRV that went into a recirc mode, moving conditioned air through the core and back into the house to defrost. Now I'm off to look at my HRV's performance curves to see what the "proper" speed should be.

  3. User avater
    Reid Baldwin | | #3

    Frost formation mechanism
    My understanding of the frost formation mechanism is different than what Allison describes. I understood the problem to occur as the temperature of the outgoing air drops to less than its dew point and also less than freezing. As a result, moisture from the outgoing air freezes on the walls of the outlet channels. Allison's explanation would not explain frost in an HRV, but as the above two comments indicate, HRV's are also susceptible. Based on my understanding of the mechanism, ERVs are less likely to frost than HRVs because the outgoing air's dewpoint drops as it transfers moisture to the incoming air.

    Renewaire ERVs do not have defrost provisions. I asked a Renewaire rep why they can get away without defrost. His explanation was that the moisture transfer was sufficiently effective that the dewpoint drops fast enough that the outgoing air temperature does not drop below the dewpoint.

  4. David Baerg | | #4

    Reply to Andrew Bater
    Hi Andrew,
    The problem with waterlogged insulation on the supply duct is a common one caused by the fact that the manufacturers of insulated flex duct typically use 2 mil poly as the vapour barrier. This is way too flimsy to withstand the installer pulling the duct through joist bays, etc and we end up with a perforated vapour barrier which allows moisture to reach the cold surface of the duct and form condensation.

    I would like to hear some suggestion for how to best protect the vapour barrier during installation. I would suggest wrapping the duct in 6 mil poly. Perhaps one could use the leg of an old pair of blue jeans. A pair of old bell bottoms from the 70s would work best, I think. You couldn't use skinny jeans. They likely wouldn't fit over a 6" duct.

    Regarding the defrost cycle, houses with wood burning appliances need to be equipped with an HRV (or ERV) that uses a recirculating style of defrost.

  5. Andrew Bater | | #5

    Duct Type & Insulation / Recirculating Defrost / ERV versus HRV
    David, the "before" supply duct picture that I attached was probably deceiving; we didn't use the normal flex duct.

    All our ductwork in and out of the HRV is actually rigid. I know that is atypical due to concerns about transmitted noise. You may be able to discern that we isolated the HRV by placing it on a vibration pad on a shelf.

    The point about supply ductwork moisture ingress is still valid though. That insulation, while pretty carefully taped and sealed, apparently wasn't perfect and let in moisture. Before I went to the heat tape method, I did look briefly whether I could find dual wall insulated rigid duct in the 6" size range. It's out there, but not common.

    Agreed also on the need for a recirculating style of defrost for any home where wood burning appliances are in use. That wasn't something I had any clue on when that HRV was purchased; defrost methods don't seem to make spec sheet bullet points.

    Our ventilation discussions back at construction centered around whether we should do an ERV or an HRV. At that point the guidance seemed to be: ERVs hot climate, HRVs cold climate. I think we are all a little smarter about that now. Frankly our selection of an HRV was probably OK the first year or two, as we slowly got rid of all the construction materials moisture. Now though it would be nice to have an ERV, as I perceive it would prevent the house from drying out as much as it does.

  6. Jon Kunesh | | #6

    Question for Andrew
    You state: Agreed also on the need for a recirculating style of defrost for any home where wood burning appliances are in use. Just curious as to why? Air filtration?

  7. Andrew Bater | | #7

    Recirculating Defrost versus "Simple" Defrost
    Jon, the HRV in my home is very similar (if not the exact model) to the one shown in the image at the top of Allison's article. At temperatures below freezing frost will develop on the core as the extremely cold outdoor air interacts with the warm humid indoor that is being exhausted. Here is the exact description of what happens to rectify that:

    "The ERV and HRV units are equipped with an automatic defrost feature to eliminate any ice build up on the core.
    • Automatic defrost is initiated once every hour when the fresh air supply temperature drops to 23°F (-5°C) or colder.
    • The defrost cycle operates by turning off the supply fan while continuing to operate the exhaust fan.
    • The exhaust fan speed is adjusted proportionally based on the outdoor temperature, initially operating at low speed.
    • As the outdoor temperature continues to drop, the exhaust fan speed will increase, and will operate at maximum speed when the outdoor temperature is -4°F (-20°C) or less.
    • The VNT5150 Series HRV/ERV defrost cycle runs for 4 minutes with the supply fan off, followed by 40 minutes of continuous normal operation.
    • Defrost cycles will continue to repeat as long as the temperature is 23°F (-5°C) or less."

    Our home measured less than .9 ACH50 when last measured for airtightness. It was the tightest home the rater had measured up to that point. It's probably even tighter than that now, as we had the Marvin rep here to adjust window seals, found a few more places to spray foam, etc. What happens when the HRV goes into defrost mode is that our home basically becomes depressurized. There are no holes or openings for the air that the HRV is exhausting to come from. It's almost like running a mini-blower door test.

    During the aforementioned depressurization the "make up" air must come from somewhere. My first winter in the home this really surprised me; if I opened the door on my masonry heater to poke at the fire I would get a blast of fumes. Subsequent to that I installed a little relief damper in our mechanical room to minimize that problem. I also got better at building fires, no need to poke at them these days!

    A recirculating defrost HRV or ERV would not cause this depressurization problem, as although it still uses warm inside air to defrost the coil, through some mechanism of dampers it keeps that air loop entirely within the building envelope.

    By the way, you are right that there is an issue of filtering with the "simple" style defrost unit I have. The outside air that is sucked into the home during defrost depressurization either comes in through cracks in the building envelope or in my case through my little relief damper. Either way that air is not filtered. The good news is that typically outside dirt levels seem to be lower in the winter particularly if the ground is snow covered.

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