When July heat and humidity settle over Montreal, your air conditioner works overtime — and old, leaky windows quietly undo much of that effort. This guide explains how solar heat gain, low-E coatings and airtight seals decide your summer comfort, and how modern windows can cut cooling costs while paying themselves back over time.
How old windows waste your air conditioning
Air conditioning is expensive because it fights two battles at once: it removes the heat pouring in through your building envelope, and it strips humidity out of the incoming air. Windows are the weakest link on both fronts. A tired single-pane or early double-pane unit lets solar radiation flood in through the glass and lets conditioned air escape through worn seals and cracked caulking, so your compressor cycles far more often than it should.
The heat that enters through a sunlit window is not subtle. On a clear late-June afternoon in Montreal, a south- or west-facing pane with no low-E coating can transmit hundreds of watts of solar energy per square metre straight into your living room. Multiply that across several large windows and you have the equivalent of a space heater running all afternoon — while your AC tries to cancel it out.
Air leakage is the second, quieter drain. Failed weatherstripping, shrunken caulk lines and warped sashes create gaps that let cool, dehumidified interior air slip out and hot, moist outdoor air seep in. Because that infiltration also carries moisture, your system spends extra energy just wringing water back out of the air, which is why a leaky home feels clammy even when the thermostat reads a comfortable number.
- Solar gain: uncoated glass lets radiant heat pour in on sunny afternoons
- Air leakage: worn seals let cooled, dehumidified air escape
- Humidity load: infiltrating outdoor air forces the AC to dehumidify harder
- Short-cycling: constant heat gain makes the compressor run more often
Low-E coatings and SHGC: your summer defence
Two glass properties decide how much solar heat reaches your rooms: the low-emissivity (low-E) coating and the Solar Heat Gain Coefficient (SHGC). A low-E coating is a microscopically thin metallic layer applied to the glass that reflects infrared energy while letting visible light through. In summer it bounces the sun’s radiant heat back outside; in winter it reflects your interior heat back in — the same coating works both seasons.
SHGC is the single number that tells you how much of the sun’s heat a whole window lets through, on a scale from 0 to 1. A window with an SHGC of 0.25 admits only a quarter of the available solar heat, while one at 0.55 admits more than half. For cooling-dominated exposures — big west- and south-facing windows that bake all afternoon — a lower SHGC keeps rooms noticeably cooler and takes real load off your AC.
The nuance in Quebec is that we need heat in winter and want to reject it in summer, so glazing is a balancing act rather than a race to the lowest number. A common strategy is a moderate SHGC on north and east elevations to keep some passive solar warmth, paired with lower-SHGC, high-performance low-E glass on the sun-blasted south and west faces. A good manufacturer will spec the glass package per orientation rather than sell one coating for the whole house.
- Low-E coating: reflects infrared heat out in summer, in during winter
- SHGC 0.25–0.35: strong solar rejection for hot west/south exposures
- SHGC 0.40–0.55: keeps useful winter warmth on north/east exposures
- Argon fill & triple glazing: slow heat transfer through the whole unit
Airtight seals and why they matter in humidity
Glass gets most of the attention, but on a humid Quebec afternoon the seal around the sash is just as important as the coating on the pane. A modern casement window closes with a crank that pulls the sash tight against continuous compression weatherstripping and locks it there with multiple points around the frame. That mechanical squeeze is why casements consistently test as the most airtight operable style — there is no gap for humid air to sneak through.
Compare that to an old slider or single-hung, which relies on brushes and fins that wear out and never fully seal at the meeting rail. Every one of those small leaks adds to your latent cooling load, meaning your air conditioner burns energy pulling moisture out of air that never should have entered. Sealing the envelope is often the difference between a home that feels crisp and one that feels sticky at the same thermostat setting.
Air-tightness also protects the building itself. When warm, moist outdoor air leaks into a cool wall cavity, it can hit the dew point and condense inside the assembly, feeding mould and rot over the years. A tightly sealed, properly installed window keeps that moisture on the outside where it belongs, which is why installation quality — low-expansion foam, a continuous air barrier tie-in, correct flashing — matters as much as the window you buy.
How new windows cut your cooling bill
Replacing failing windows with a modern ENERGY STAR unit attacks your cooling bill from three directions at once. The low-E glass rejects a large share of incoming solar heat, the airtight seals stop conditioned air from leaking away, and the insulating gas fill and multiple panes slow the temperature difference from crossing the glass. Together they shrink the heat load your AC has to fight, so it runs less and cycles more gently.
In a typical older Montreal home, upgrading tired windows can trim total annual heating and cooling energy use meaningfully — often in the range of 10 to 20 percent depending on how bad the old units were and how much glass faces the sun. The summer benefit is easy to feel: rooms that used to be unusable by mid-afternoon stay comfortable, and the AC stops labouring to hold a setpoint it could never quite reach.
The comfort dividend is more than a number on a bill. Even temperatures from room to room, no hot spots by the sunny windows, less noise from the street, and no more clammy humidity all come from the same upgrade. For a deeper look at how these ratings fit together, see our guide to understanding energy efficiency.
Payback and Quebec rebates
Energy-efficient windows are an investment, and the honest answer on payback is that it depends on your starting point. If you are replacing genuinely failed single-pane or foggy sealed units, the combined heating-and-cooling savings, comfort gains and avoided repair costs shorten the math considerably. Homeowners upgrading from already-decent double-pane windows should expect a longer horizon and treat the comfort and durability as the main return.
Quebec rebates take real dollars off the top. Through the Rénoclimat program, homeowners can receive up to about $150 per rough opening for eligible ENERGY STAR window and door replacements, provided the work follows a before-and-after energy evaluation. The federal Canada Greener Homes initiative has offered support of up to $5,000 for a package of qualifying retrofits, and windows are a common line item in that package.
To qualify, you generally need ENERGY STAR certified products rated for Climate Zone D, which covers most of southern Quebec including Montreal, and an installation by a licensed contractor holding a valid RBQ number. Keep every invoice and the certification labels, book the required energy evaluations before and after, and confirm current program terms before you sign — rebate amounts and rules are updated periodically.
- Rénoclimat: up to ~$150 per rough opening for eligible windows and doors
- Canada Greener Homes: up to $5,000 for a package of qualifying retrofits
- ENERGY STAR Zone D: the certification tier for southern Quebec
- RBQ contractor & invoices: required to keep your rebate eligibility
Practical summer tips for cooler rooms
You do not have to wait for new windows to take some load off your AC this summer. The cheapest win is managing solar gain by hour: close blinds, cellular shades or exterior awnings on the south and west windows during the hottest part of the afternoon, then open up in the cooler evening. Exterior shading is dramatically more effective than interior blinds because it stops the heat before it ever reaches the glass.
Sealing is the next lever, and it is nearly free. Walk each window on a breezy day and feel for drafts around the sash and frame; refresh cracked exterior caulking and replace crumbling weatherstripping before the peak heat. Also confirm your intact screens so you can flush the house with cool air overnight when Montreal finally drops below the daytime highs.
If your windows are genuinely at end of life — fogged sealed units, sashes that no longer latch tight, frames that sweat — those band-aids only go so far. That is the point where replacement stops being a luxury and starts paying for itself in every hot, humid month. A quick assessment of orientation and glass package will tell you where new windows will do the most good.
- Shade south/west glass with exterior awnings or shades during peak afternoon heat
- Refresh caulking and weatherstripping to stop conditioned air leaking out
- Flush the house with cool overnight air through intact, well-fitted screens
- Prioritize replacing fogged, drafty or unlatchable units first
Frequently Asked Questions
Do new windows really lower my air conditioning bill?
Yes. Low-E glass rejects incoming solar heat and airtight seals stop conditioned air from leaking away, so your AC has a smaller heat and humidity load to fight. In an older home the combined heating-and-cooling reduction often lands in the 10 to 20 percent range.
What is SHGC and what number should I look for?
SHGC (Solar Heat Gain Coefficient) measures how much solar heat a window lets through, from 0 to 1. For hot, sunny south and west exposures a lower SHGC around 0.25 to 0.35 keeps rooms cooler, while north and east windows can use a higher value to keep useful winter warmth.
Are casement windows better for keeping cool air in?
Generally yes. A casement cranks tight against continuous weatherstripping and locks at multiple points, so it tests as one of the most airtight operable styles. That tight seal reduces the humid-air infiltration that forces your AC to work harder.
Can I get a rebate on energy-efficient windows in Quebec?
Yes. Rénoclimat offers up to roughly $150 per rough opening for eligible ENERGY STAR windows and doors, and the federal Canada Greener Homes program has offered up to $5,000 for a package of retrofits. You need ENERGY STAR Zone D products and a licensed RBQ contractor.
Should I choose different glass for different sides of the house?
Ideally, yes. In Quebec you want to reject summer heat while keeping some winter sun, so a good manufacturer specs lower-SHGC glass on sun-blasted south and west faces and a higher-SHGC package on north and east elevations.
What can I do this summer without replacing windows yet?
Shade south and west glass with exterior awnings or shades during peak afternoon, refresh any cracked caulking and weatherstripping, and flush the house with cool overnight air through good screens. These steps cut the load until replacement makes sense.