Where Your Home Loses Heat: Windows, Doors, Walls & Attic

Every Quebec home is a leaky bucket in January: heat pours out through the roof, the walls, the glass, the foundation and a thousand small gaps, and the furnace or heat pump pours it back in at Hydro-Québec rates. The losses are not evenly spread, and neither are the returns on fixing them. Here is where a typical house in the Montreal area actually loses its heat, why windows and doors take a share out of proportion to their size, and how to decide what to upgrade first.

The Typical Heat-Loss Breakdown

Energy audits of detached and semi-detached homes in southern Quebec produce a fairly consistent picture. For a house built between 1960 and 2000 with original or lightly upgraded components, roughly 25 to 35 percent of heat loss goes through the windows and doors, 20 to 30 percent through uncontrolled air leakage, 15 to 25 percent through the walls, 10 to 15 percent through the attic or roof, and 10 to 20 percent through the basement walls and slab. Newer homes built to the 2012 or later code shift the balance toward windows and air leakage, because walls and attics have improved faster than glazing.

Two things stand out in those numbers. First, windows and doors account for only about 15 to 20 percent of the exterior envelope area of a typical house, yet they carry the largest single share of the loss. Second, air leakage — which is not a component at all, but the sum of every crack around windows, doors, rim joists, electrical penetrations and attic hatches — is nearly as large as the windows themselves.

The percentages shift with the house. A 1950s bungalow with an uninsulated attic will lose more through the roof than the ranges above suggest; a 1980s two-storey with a full-height picture window on the north side will lose more through glass. The point of the breakdown is not the exact figure but the order of magnitude: windows, doors and air leaks together typically account for half or more of the heating bill.

• Windows and doors: 25 to 35 percent — largest share, smallest area.
• Air leakage: 20 to 30 percent — mostly at joints, penetrations and old window frames.
• Walls: 15 to 25 percent — depends heavily on the era of construction.
• Attic and roof: 10 to 15 percent — the cheapest to fix per unit of savings.
• Foundation and slab: 10 to 20 percent — often uninsulated in homes built before 1980.

Why Glass Loses So Much

Insulation is measured by R-value, and the gap between glass and everything else is enormous. A wall in a 1980s Quebec home is roughly R-12 to R-20; an attic with 250 mm of blown insulation is R-30 to R-40; a code-built wall today reaches R-24 or more. A single-pane window is about R-1. A double-glazed clear unit from the 1980s is R-2. A modern double low-E argon window reaches R-3.5 to R-4, and triple glazing R-5 to R-7. Even the best window is a hole in the wall, thermally speaking, with one-fifth the resistance of the wall around it.

That is the conduction story: heat moving through the material by temperature difference. On a night at -30 °C with the house at 21 °C, the difference across the window is 51 degrees, and a square metre of R-2 glass loses heat about ten times faster than a square metre of R-20 wall. Multiply by fifteen or twenty windows and the arithmetic explains why glass dominates the loss despite its small area.

Radiation adds to it. Warm objects in the room, including people, radiate heat toward the cold glass surface, which is why you feel a chill sitting next to an old window even when there is no draft. A low-E coating reflects that infrared back into the room, which is the single largest reason modern windows outperform old ones. Frames matter too: an aluminum frame without a thermal break conducts heat like a radiator in reverse, while a multi-chamber vinyl or hybrid frame adds resistance and keeps the interior surface warmer.

Air Leakage Versus Conduction: Two Different Problems

Conduction is heat passing through a solid material. Air leakage is heated air physically leaving the house and being replaced by cold outdoor air. They are measured differently, fixed differently, and they feel different: conduction makes a surface cold; leakage makes a draft. A house can have excellent insulation and still be expensive to heat because air escapes through the top of the building and is pulled in at the bottom — the stack effect that is strongest precisely when it is coldest outside.

Old windows are a major contributor to both. The glass conducts, and the frame leaks: sash weatherstripping that has flattened, wood that has shrunk away from the stop, pulley pockets in double-hung frames that open straight into the wall cavity, and the joint between the window frame and the rough opening that was stuffed with fibreglass forty years ago and never sealed. A blower-door test on a pre-1990 home in Montreal frequently shows windows and doors as the largest measured leak paths after the attic hatch and the rim joist.

This is why replacing a window fixes two categories of loss at once. A new unit with compression seals, welded corners and a low-E sealed glass unit stops the conduction, and a proper installation with foam-sealed frame gaps, a sealed interior air barrier and exterior flashing stops the leakage around it. A window replaced without attention to that perimeter seal recovers the conduction savings and leaves the leakage on the table — which is the difference between a good installation and a fast one.

Attic, Walls and Foundation: Where the Rest Goes

The attic is the easiest and cheapest heat-loss fix in most Quebec homes, and it should usually come first or alongside windows. Topping up blown cellulose or fibreglass from R-20 to R-50 is a modest expense for a typical house, is eligible for Rénoclimat support, and pays back in three to six years. Sealing the attic hatch, the top plates and every pipe and wire penetration at the same time addresses the largest single air-leakage path in the house.

Walls are a different matter. Adding insulation to an existing wall means either opening it from inside, dense-packing cellulose through holes drilled in the sheathing, or adding rigid insulation under new siding. All three are expensive and disruptive, which is why wall upgrades usually wait for a planned re-siding or interior renovation. The exception is a wall with no insulation at all, common in homes built before 1950, where dense-pack blowing is worth doing on its own.

Foundations are the sleeper. An uninsulated concrete basement wall is roughly R-1.5, barely better than a window, and in a home built before 1980 the basement often has no insulation above grade. Insulating the interior of the foundation walls with rigid foam or a framed wall with batt insulation, and sealing the rim joist where the floor structure meets the foundation, typically reduces total heat loss by 10 to 15 percent. If the basement is being finished anyway, doing it properly is close to free.

Prioritizing: What to Fix First

The rational order is set by dollars saved per dollar spent, adjusted for what else the upgrade delivers. Air sealing and attic insulation come first almost everywhere: low cost, high return, and they can be done in a weekend by a contractor. Windows and doors come next when the existing units are single-pane, aluminum-framed, leaky, or past twenty-five years old, because they combine a large share of the loss with benefits that insulation cannot give — comfort next to the glass, no condensation, quieter rooms, security and appearance.

Foundation insulation follows when the basement is being finished or is noticeably cold. Wall insulation comes last unless the walls are empty or the siding is being replaced. Heating equipment — a heat pump replacing electric baseboards or an old furnace — sits alongside the envelope work rather than after it: a smaller, better-sealed heat load lets you size a heat pump correctly and avoids paying for capacity you no longer need.

A Rénoclimat evaluation, which includes a blower-door test and a modelled breakdown for your specific house, is the best money you can spend before committing to any of this. It replaces the typical percentages above with your own numbers, identifies the largest leak paths, and is the gateway to provincial grants for insulation, air sealing and, in certain programs, windows and doors that meet the required energy ratings. Our guide to understanding energy efficiency explains the window ratings those programs look for.

• 1. Air sealing and attic top-up: lowest cost, fastest return, biggest leak paths.
• 2. Windows and doors: if single-pane, aluminum, leaky or over 25 years old; comfort and condensation gains on top of savings.
• 3. Foundation walls and rim joist: especially when finishing the basement.
• 4. Heat pump sizing: after the envelope is tightened, not before.
• 5. Wall insulation: with re-siding or interior renovation, or if walls are empty.

What It Means on a Hydro-Québec Bill

Most Quebec homes heat with electricity, and heating is 50 to 60 percent of a typical residential bill. For a 1,800-square-foot detached house in the Montreal area heated electrically, annual consumption runs 20,000 to 30,000 kWh, of which 12,000 to 18,000 kWh is heating. At current residential rates, heat alone is by far the largest item on the annual bill, more so in colder regions or larger homes.

Replacing old double-glazed clear windows with modern low-E argon units, with a properly sealed installation, delivers a measurable cut in heating consumption on the house above, more if the windows were single-pane or badly leaking. Attic insulation and air sealing together deliver a similar or larger reduction for a fraction of the cost. Doing both, plus the foundation, takes a pre-1990 home a long way down its heating bill, into the range Hydro-Québec's own efficiency programs target.

Two cautions keep those numbers honest. Savings scale with how bad the starting point is: a house that already has 2005-era low-E windows will not see a dramatic bill change from replacing them, though it may gain in comfort and condensation. And energy savings are only part of the case for windows and doors — the rest is a warm room in January, a quiet street, a front door that locks properly and a house that looks the way you want it to. Fix the cheap things first, then choose the windows for all of those reasons together.

Finding Your Own Losses

You do not need instruments to find the worst offenders. On a cold, windy evening, walk the house with the back of your hand: cold air moving at window edges, under the front door, around the garage-house door, at electrical outlets on exterior walls and at the attic hatch is air leakage. A cold pane of glass with no draft is conduction. Condensation or frost on the inside of the glass in the morning tells you the surface temperature is dropping below the dew point — a sign of old glazing or a conductive frame.

A cheap infrared thermometer from any hardware store turns this into numbers. Scan the interior surface of walls, windows and ceilings on a cold night: a wall at 18 °C next to a window at 8 °C shows you exactly where the R-value collapses. Cold stripes on a ceiling reveal missing attic insulation; a cold band at the top of a basement wall is the uninsulated rim joist.

For a proper diagnosis, a Rénoclimat advisor's blower-door test and thermographic camera will map the whole house in a few hours and give you a report with the breakdown, the leak locations and the upgrade priorities. Bring that report to your window and door consultation. It tells us which openings are costing you the most and lets us recommend glazing and installation details for your house rather than the average one.

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