Homeowners rarely expect the side effect of new windows: within weeks the house feels stuffier, the bathroom mirror stays fogged longer and the bedrooms are warm and heavy by morning. Nothing went wrong — the leaks that used to ventilate the house by accident are gone. This guide explains what changes in your indoor air once the envelope is tight, and how Quebec homes keep it fresh without throwing the savings back out the window.
What changes in your air when the leaks are gone
An older Quebec house with original windows breathes through its cracks. Blower-door tests on pre-1980 homes in the Montreal area commonly show 5 to 10 air changes per hour at 50 Pa, and in winter the stack effect and wind push a steady, uncontrolled trickle of outdoor air through the sash joints, the sill gaps and the trim. It is wasteful — you are heating air you did not ask for — but it does dilute whatever the household produces.
Replace the windows with A3-rated units, foam the rough openings properly and seal the trim, and a large share of that accidental ventilation disappears. Homeowners who go on to insulate the attic and air-seal the basement can bring a 1960s bungalow down to 3 or 4 air changes at 50 Pa. The same amount of moisture, carbon dioxide and chemical off-gassing is still being produced inside, but far less of it leaves on its own.
That is the paradox of every good envelope upgrade: the tighter the house, the more deliberate its ventilation has to be. New-construction programs solved this years ago with the rule 'build tight, ventilate right'. Renovators, who tighten the house one project at a time, are the ones who tend to forget the second half.
CO2, humidity and VOCs: what the numbers mean
Carbon dioxide is the easiest indicator to track because people produce it continuously. Outdoor air sits at roughly 420 to 430 ppm. A well-ventilated room stays under 800 to 1,000 ppm; a closed bedroom with two adults in a tight house can pass 2,000 ppm by dawn, a level at which most people wake up dull and slightly headachy. An NDIR sensor is a modest purchase and is the single most useful tool for deciding whether your ventilation is adequate.
Humidity rises for the same reason. Cooking, showers, laundry, plants and breathing add several litres of water vapour a day to a household, and once the leaks are sealed that water stays until a fan removes it. The visible symptom is condensation on the coldest glass; the invisible one is moisture migrating into wall cavities. Keeping winter relative humidity in the 30 to 40 percent range, lower during a deep cold snap, is a ventilation problem first and a window problem second.
Volatile organic compounds are the least visible of the three. New flooring, paint, cabinetry, furniture and even the foam and sealants used around your new windows off-gas for months. Health Canada publishes residential guideline values for formaldehyde and other compounds, and the only practical way to stay under them in a tight house is a steady exchange of air. If a renovation coincides with the window replacement, ventilate more, not less, for the first season.
- CO2: under 1,000 ppm is comfortable; sustained readings above 1,500 ppm mean the room is under-ventilated
- Relative humidity: 30 to 40 percent in winter, dropping toward 25 percent when it is -25 °C outside
- VOCs: highest in the months after new materials go in; ventilation is the remedy, not time alone
The HRV (VRC): Quebec's standard answer
A heat recovery ventilator — VRC or échangeur d'air in Quebec French — is a balanced system with two fans. One exhausts stale air from the bathrooms, kitchen area and laundry; the other supplies an equal amount of outdoor air to the bedrooms and living spaces. The two streams cross inside a core that transfers 60 to 80 percent of the heat from the outgoing air to the incoming air without mixing them. In practical terms, you get a full change of air every few hours while losing only a fraction of the heat you paid for.
Quebec's building code has required a designed mechanical ventilation system in new houses for more than a decade, and in our climate that almost always means an HRV rather than an energy recovery ventilator. The ERV's core also transfers moisture, which is helpful in a humid summer and in very dry, very tight new homes, but in a typical winter the HRV's tendency to remove excess humidity is exactly what a house with condensation problems needs. A good contractor sizes the unit to the house, balances the two airflows and commissions it with a flow meter; an unbalanced HRV either pressurizes the house or sucks in cold air through every remaining gap.
The cost of retrofitting an HRV into an existing house in the Montreal area depends mainly on whether it ties into forced-air ductwork or needs its own dedicated ducts to bathrooms and bedrooms. Rénoclimat, Quebec's home-renovation efficiency program, has offered financial assistance for HRV installation as part of an audited renovation, so it is worth checking the current terms before you sign. Running costs are modest: a modern unit on continuous low speed draws about as much as a light bulb, and filters need a rinse every three months.
- HRV (VRC): recovers heat only; lowers indoor humidity in winter — the default choice for Quebec
- ERV (VRE): recovers heat and moisture; suits very dry tight homes or humid-summer comfort
- Both need balancing at commissioning, a defrost cycle for -20 °C operation and quarterly filter cleaning
Trickle vents and passive options: what they can and cannot do
Trickle vents — small closable slots in the head of a window frame — are standard in the United Kingdom and parts of Europe, and homeowners who have lived there sometimes ask for them. In Quebec they are rare for a reason: at -20 °C the air coming through a slot in the frame is a cold draught right at the glass, it carries no heat recovery, and it promotes frost on the very sash it is supposed to protect. A few Canadian manufacturers offer them, and they can make sense in a shoulder-season sunroom or a rental unit without an HRV, but they are not a substitute for balanced mechanical ventilation.
Older Quebec houses often have a passive fresh-air intake ducted into the furnace return, sometimes with a motorized damper, plus bathroom fans on timers. This 'exhaust-only' approach works up to a point in a leaky house, since the fans pull replacement air through the cracks. In a house that has just received tight windows, the same fans may struggle to draw air, backdraft a wood stove or water heater, and pull soil gas up through the basement slab — one more reason the balanced HRV became the norm.
If an HRV is genuinely out of reach for now, the minimum is to make sure every bathroom fan actually vents outdoors, put the main one on a 24-hour timer for 20 minutes an hour, and pair it with a fresh-air intake so the house is not running under negative pressure.
Strategic window venting in winter
Even a house with a well-balanced HRV benefits from occasional burst ventilation, and a house without one depends on it. The technique that works in a Quebec winter is short and total rather than long and partial: open two windows on opposite sides of the house wide for five to ten minutes, let the air change completely, then close them. Walls, floors and furniture hold their heat through such a short exchange, so the house recovers in minutes; a single window left cracked all day, by contrast, chills one room continuously and costs far more.
New windows make this easier. A casement with multi-point hardware can be latched in a secure ventilation position, and an awning window sheds rain and snow while open, which is why it is the best choice for a kitchen or bathroom that needs a quick purge. Do the burst after cooking, after a shower, after painting or bringing in new furniture, and before bed in a room that will be closed all night. Aim for outdoor air that is dry and cold rather than mild and damp — in January that is nearly always the case.
Keep interior doors open or undercut, especially bedroom doors at night, so that whatever ventilation you have reaches the rooms where CO2 accumulates. The best HRV in Quebec cannot supply a bedroom whose door seals tight against a carpet.
- Open two windows on opposite sides, wide, for five to ten minutes
- Do it after cooking, showers, painting and before bed in closed rooms
- Prefer awning windows in wet rooms; latch casements in a secure vent position
- Leave bedroom doors ajar or undercut so supply air actually arrives
Novoclimat homes and the 'build tight, ventilate right' rule
Novoclimat, Quebec's certification program for new energy-efficient homes, is built on exactly this principle. A certified house has to meet a demanding blower-door target, is required to have a certified heat recovery ventilator with a minimum recovery efficiency, and must have that HRV balanced and verified by an approved inspector before the certificate is issued. The program's designers understood that airtightness without ventilation is not a healthy home; it is a sealed box.
Homeowners renovating an older house are, in effect, walking the same path without the checklist. Each envelope project — windows, attic insulation, basement air sealing, a new front door — brings the house closer to Novoclimat tightness. The mistake is to arrive there without the second half of the recipe. If you are planning a sequence of upgrades, budget the HRV as part of the second or third project rather than as an afterthought once the symptoms appear.
For the broader picture of how air leakage, insulation and windows interact in a Quebec house, our guide to understanding energy efficiency covers the terms and the trade-offs in plain language.
A note on radon
Radon is a naturally occurring radioactive gas that seeps from the soil into basements, and it deserves a mention in any discussion of airtight homes. Health Canada's guideline is 200 becquerels per cubic metre; several parts of Quebec, including areas of the Laurentides, the Outaouais, the Estrie and the Montérégie around Mont-Saint-Hilaire and Oka, have been mapped with a higher potential. Radon has no smell, and the only way to know your level is an inexpensive three-month test kit, ideally run in winter.
Tightening the envelope does not create radon, but it reduces the dilution that used to keep concentrations down, and exhaust-only ventilation can actively draw more of it through the slab by depressurizing the basement. A balanced HRV helps modestly on both counts. Where a test shows an elevated level, the proper fix is sub-slab depressurization — a sealed pipe and fan under the basement floor — which no window or ventilation choice replaces.
Frequently Asked Questions
Do I need an HRV after replacing my windows?
The code does not require one for a window replacement, but if the house has no designed ventilation and you notice condensation, stuffiness or CO2 readings above 1,500 ppm in bedrooms, the answer is yes. It is the most effective way to keep air fresh without losing the heat your new windows are saving.
Why is my house more humid since the new windows went in?
The old windows leaked, and that leakage carried moisture out all winter. The new ones do not, so the same moisture from cooking, showers and breathing now stays in the house. Run bathroom and kitchen fans longer, do short burst ventilation and consider an HRV; the windows are doing their job.
HRV or ERV for a Quebec home?
An HRV in most cases. It recovers heat and lets excess humidity leave, which is what a Quebec house needs in winter. An ERV keeps more moisture inside and suits very dry, very tight new homes or households that prioritize summer humidity control.
Can I just crack a window instead of buying an HRV?
A window cracked all day chills one room, wastes heat and does little for the bedrooms. Short, total burst ventilation several times a day is far more effective and much cheaper, and it is a reasonable stopgap — but it depends on someone being home to do it.
What CO2 level is too high in a bedroom?
Readings consistently above 1,500 ppm at night indicate the room is under-ventilated; above 2,000 ppm most people notice poorer sleep and morning headaches. Under 1,000 ppm is the comfortable target, achievable with a balanced HRV supplying the bedroom or a door left ajar.
Does an HRV waste the heat my new windows are saving?
No. A modern HRV recovers 60 to 80 percent of the heat in the outgoing air, so a full air change costs a fraction of what the same ventilation through leaks or an open window would. It is the combination — tight windows plus recovered ventilation — that produces the lowest bill.