Yes. If you seal a house and do nothing else, the air gets bad, and faster than most people expect.
That is the honest answer, and it is why the question deserves a real response instead of reassurance. Air sealing is the highest-return energy upgrade available to Canadian homeowners. It is also the one upgrade that creates a new problem if you stop halfway.
The building science answer is four words long: build tight, ventilate right. The two halves are not optional extras of each other. Sealing without ventilating is a mistake. Ventilating through random cracks in your wall is a different mistake. Here is how the whole thing actually works.
Leaky Houses Are Not Well-Ventilated Houses. They Are Randomly Ventilated Houses.
The instinct behind the question is that a drafty house at least breathes. It does, in a sense. It breathes badly.
Air leakage through a building is driven by two forces. Stack effect happens because warm air rises, so a heated house pushes air out near the ceiling and pulls it in near the floor. Wind pressure pushes air in on the windward side and out on the leeward side.
Both depend on weather. On a still, mild October day, a leaky house might exchange less air than a tight house with a running ventilator. On a windy -25°C night, that same house might exchange far more than anyone wants, which is expensive and uncomfortable.
There is no control, no filtration, and no choice about where the air comes from. Makeup air in a leaky house is pulled from wherever the path of least resistance leads: the crawlspace, the attached garage, the gap around the sewer stack, the soil beside the foundation.
That is the part people miss. A drafty house does not pull fresh air from the front yard. It pulls it through the dirtiest cavities in the building.
What Actually Builds Up When Air Stops Moving
This section is the meat of the question, so it gets the most space. Five things accumulate in a sealed house without ventilation, and they arrive in a predictable order.
Moisture arrives first and does the most damage
A household of four generates a surprising amount of water vapour every day. Breathing and perspiration alone contribute several litres. Showers, cooking, dishwashing, laundry, houseplants, and aquariums add more.
| Moisture source | Approximate litres per day, family of four |
| Breathing and perspiration | 4 to 8 |
| Showers and baths | 1 to 3 |
| Cooking and dishwashing | 1 to 2 |
| Laundry, if line-dried indoors | 2 to 5 |
| Houseplants and pets | 0.5 to 2 |
| Unsealed crawlspace or damp basement | 3 to 20 |
In a leaky house, most of that leaves through the cracks. Seal the cracks and it stays put.
Indoor relative humidity climbs. Above roughly 50% in winter, condensation forms on the coldest surfaces in the house, which are windows first, then wall corners, then closets on exterior walls. Sustained moisture on those surfaces grows mould within days.
Worse, warm humid indoor air pushed into wall and attic cavities condenses inside the assembly where you cannot see it. That is how sealed houses rot from the inside. It is the single most serious failure mode of a retrofit done halfway.
The target range in a Canadian winter is 30% to 45% relative humidity, and lower during extreme cold. If your windows are wet, your house is telling you the ventilation is not keeping up.
Carbon dioxide climbs, and you feel it before you notice it
Outdoor air sits around 420 ppm of CO2. Indoor air in a well-ventilated home stays under 800 ppm. Above about 1,000 ppm, measurable declines in concentration and decision-making show up in controlled studies. Above 1,500 ppm, most people report headaches, drowsiness, and stuffiness.
A closed bedroom with two adults and no ventilation routinely reaches 2,000 to 3,000 ppm by morning. That happens in leaky houses too, which is why so many people wake up groggy and blame their mattress.
CO2 is also the best available proxy for general ventilation adequacy. A cheap monitor in the master bedroom tells you more about your air quality than any other single instrument.
The building itself off-gasses
Cabinets, laminate flooring, paint, carpet, foam insulation, furniture, and adhesives release volatile organic compounds. So do cleaning products, candles, and cooking. Formaldehyde from pressed-wood products is the classic example, and it off-gasses for years after installation.
In a leaky house these are diluted continuously by accident. In a sealed house they concentrate until something removes them, and no filter on a furnace removes gases. Only outdoor air dilution does.
Renovations make this worse temporarily, because new materials off-gas hardest in their first months, which is exactly when a freshly retrofitted house has just been sealed.
Radon deserves its own paragraph in Canada
Radon is a naturally occurring radioactive gas from uranium decay in soil. It enters through foundation cracks, sump pits, floor drains, and the gap where the slab meets the wall. It is the second leading cause of lung cancer in Canada after smoking.
Health Canada’s guideline is 200 becquerels per cubic metre. A meaningful share of Canadian homes exceed it, with higher rates in parts of the Prairies, Ontario, Quebec, and the Atlantic provinces.
Air sealing the above-grade envelope does not directly increase radon entry, but it can change pressure relationships in ways that pull more soil gas indoors. Ventilation dilutes radon, and balanced ventilation avoids the depressurization that draws it in.
Test for it. A long-term test kit costs under $60, runs for three months, and is the only way to know. Sealing the foundation and adding sub-slab depressurization is the fix if levels are high, and it is not expensive.
Combustion appliances can start pulling the wrong direction
This is the one that can kill quickly rather than slowly.
Any naturally vented combustion appliance, an older gas water heater, a mid-efficiency furnace, a wood stove, or a fireplace, relies on a chimney draft to carry exhaust out. That draft is weak. If the house is depressurized, exhaust can reverse direction and spill combustion gases, including carbon monoxide, into the living space.
Depressurization comes from exhaust equipment: range hoods, dryers, bathroom fans, and central vacuums. In a leaky house, replacement air arrives through the cracks and pressure stays roughly balanced. In a tight house, a 600 cfm range hood can pull hard enough to reverse a water heater flue.
Two rules follow. Any house being air sealed should have its combustion appliances checked for spillage under worst-case depressurization. And sealed houses should move toward sealed-combustion or electric appliances, which draw their combustion air directly from outside through a dedicated pipe.
The summary table
| Contaminant | Main source | Warning sign | What removes it |
| Water vapour | Occupants, showers, cooking, wet basement | Window condensation, musty smell | Ventilation, source control |
| Carbon dioxide | Breathing | Morning headaches, stuffiness | Ventilation only |
| VOCs and formaldehyde | Building materials, furnishings, cleaners | Chemical odour, irritation | Ventilation, low-emission materials |
| Radon | Soil gas through foundation | None, it is undetectable by senses | Testing, sub-slab depressurization, ventilation |
| Carbon monoxide | Backdrafting combustion appliances | CO alarm, flu-like symptoms | Sealed combustion, combustion safety testing |
| Particulates | Cooking, outdoor smoke, pets | Visible haze, allergy symptoms | Filtration plus ventilation |
An HRV Is a Heat Exchanger With a Fan Attached
Here is the piece of equipment that resolves the whole problem.
A heat recovery ventilator runs two air streams past each other through a core. Stale indoor air heads out. Fresh outdoor air comes in. The two streams never mix, but heat passes between them through the core walls.
In January, outdoor air at -15°C enters the core, meets 21°C exhaust air on the other side of a thin barrier, and arrives in your house at 10°C to 15°C. Your heating system tops up the last few degrees instead of heating from -15°C.
Good units recover 60% to 85% of the heat, a figure reported as sensible recovery efficiency. So the ventilation you must have costs a fraction of what opening a window would.
Incoming air also passes through a filter, which means outdoor pollen, wildfire smoke particulate, and road dust get caught before entering. A leaky house has no such option.
HRV or ERV depends on your humidity problem
An energy recovery ventilator does everything an HRV does, plus it transfers moisture between the two air streams.
That distinction sounds minor and is not.
| Consideration | HRV | ERV |
| Transfers heat | Yes | Yes |
| Transfers moisture | No | Yes, partially |
| Winter effect | Removes indoor humidity with exhaust air | Retains some indoor humidity |
| Summer effect | Brings in outdoor humidity | Reduces incoming humidity |
| Best suited to | Homes with excess winter humidity | Homes that get too dry in winter |
| Regional fit in Canada | Prairies with damp basements, homes with high occupancy | Humid summer regions, very cold dry winters, low occupancy |
| Core freezing risk | Higher, needs defrost strategy | Somewhat lower |
| Typical cost difference | Baseline | 10% to 25% more |
The practical rule: if your windows sweat in winter, you want an HRV. If your winter indoor humidity drops below 25% and your skin cracks, an ERV keeps some of that moisture in the house. Occupancy matters as much as climate. Six people in a small house make plenty of moisture regardless of province.
How Much Ventilation Is Actually Required
Canadian requirements come from CSA F326 and Part 9 of the National Building Code. The approach assigns a rate to each room type and sums them.
| Room | Typical required continuous rate |
| Master bedroom | 10 L/s (about 21 cfm) |
| Each additional bedroom | 5 L/s (about 10.5 cfm) |
| Living room | 5 L/s |
| Dining room | 5 L/s |
| Kitchen | 5 L/s |
| Family or rec room | 5 L/s |
| Basement, unfinished | 10 L/s |
| Each bathroom | 5 L/s |
| Utility room | 5 L/s |
A three-bedroom house with two bathrooms, a finished basement, and normal living space typically lands between 45 and 65 L/s, roughly 95 to 140 cfm. That is well within the range of a standard residential HRV.
Note that this is a continuous rate, not a burst. The unit runs quietly all the time, usually at low speed, with a boost setting tied to bathroom switches or a humidity sensor.
The Airtightness Numbers, and What Each One Means for You
Airtightness is measured with a blower door test and reported as air changes per hour at 50 pascals, or ACH50.
| ACH50 | House type | Ventilation status |
| 12 and above | Pre-1960 with no air barrier | Wildly variable, expensive to heat |
| 6 to 10 | Typical 1960s to 1980s Canadian home | Accidental, weather-dependent |
| 3 to 5 | Well-built 1990s to 2010s home | Marginal, mechanical ventilation recommended |
| 1.5 to 3 | Good retrofit or current code-plus build | Mechanical ventilation required |
| 0.6 to 1.5 | Net Zero Ready or Passive House range | Mechanical ventilation mandatory |
The threshold where people start worrying is around 3 ACH50. In practice, any Canadian house should have proper mechanical ventilation regardless of its number, because “accidental” ventilation is unreliable at every leakage level.
Installation Mistakes That Turn an HRV Into an Expensive Fan
Plenty of Canadian homes have an HRV sitting in the basement doing almost nothing. The unit is rarely the problem.
- Never commissioned. Supply and exhaust flows must be measured and balanced after install. An unbalanced unit pressurizes or depressurizes the house, which is exactly what you were trying to avoid.
- Switched off by the homeowner. Someone heard it and flipped the breaker. It has been off for four years.
- Filters never changed. Clogged filters cut airflow drastically. They need checking every three to six months.
- Simplified duct connections. Interlocking the HRV with the furnace ducting saves money and can short-circuit the airflow so fresh air is immediately exhausted.
- Intake placed badly. An outdoor intake next to a dryer vent, a driveway, or a gas meter pulls in exactly what you did not want.
- No defrost strategy. In deep cold, the core can frost up. Units need a recirculation or preheat defrost cycle appropriate for the climate.
- Stale-air pickup in the wrong rooms. Exhaust should be drawn from bathrooms, kitchen area, and laundry. Supply should go to bedrooms and living spaces.
Ask for the commissioning report showing measured supply and exhaust flows in L/s. If nobody measured, nobody balanced.
The Thing To Do Right Now
Put a $150 CO2 monitor in your bedroom and read it in the morning, because that one number will tell you more about whether your house is ventilating properly than any amount of speculation.
What It Costs to Run
An HRV moving 50 L/s continuously draws roughly 40 to 90 watts depending on motor type. Units with electronically commutated motors sit at the low end.
At 70 watts running constantly, that is about 613 kWh a year, or roughly $80 at 13 cents per kilowatt-hour. The heating penalty for the air it brings in, after recovery, typically adds another $60 to $150 a year in a Canadian climate.
Compare that to what the sealing saved. A whole-house air sealing job that cuts leakage by 40% typically saves $300 to $700 a year in heating. The ventilation gives back a fraction of it and buys you air you can actually breathe.
| Item | Typical range |
| HRV or ERV unit | $1,200 to $3,000 |
| Installation and ducting | $1,300 to $3,500 |
| Commissioning and balancing | $200 to $500 |
| Annual electricity | $60 to $120 |
| Annual conditioning penalty after recovery | $60 to $150 |
| Filter replacements | $40 to $100 per year |
The Answer, Put Simply
Sealing your house does not poison the air. Sealing it and skipping ventilation does.
The correct sequence is to seal aggressively, test the result with a blower door, check your combustion appliances for spillage, then install and properly balance an HRV or ERV sized to your house. Do those in order and you get a home that is cheaper to heat, quieter, more even in temperature, and has objectively better air than the drafty version ever did.
The drafty house was never delivering fresh air. It was pulling attic dust and crawlspace air through the walls whenever the wind blew, and charging you for the privilege in heating costs. Controlled ventilation replaces that with filtered outdoor air at a rate you choose, and recovers most of the heat on the way through.
Start with a blower door test and a CO2 reading. Those two numbers tell you where you stand and what to do next.
