Windows and doors: what your windows' age tells you

How heat escapes through windows and doors

Glass conducts heat, cold panes draw radiant heat from your body, and air leaks around sashes and frames (NRCan ↗; NRCan ↗). Radiation is less than 10% of heat loss in most houses, and most of it goes through windows (NRCan ↗). Single-glazed windows lose about 10 to 20 times as much heat as the same area of insulated wall (NRCan ↗). Some “drafts” near windows are cold air sliding down the glass, a convection current rather than a leak (NRCan ↗).

A window's U-factor measures how fast heat passes through the whole unit: glass, edges and frame. Lower is better; it is the inverse of an R-value (NRCan ↗).

How window age matters

PNNL figures are whole-window values for wood windows, converted from imperial units (1 Btu/h·ft²·°F = 5.678 W/m²·K) (NRCan ↗). For new windows, the 2020 National Building Code sets a maximum of 1.84 in zones 4 and 5, 1.61 in zones 6 and 7A, and 1.44 in zones 7B and 8 (National Building Code ↗).

Doors

Doors usually matter less than windows simply because there are fewer of them, unless you have patio or garden doors (NRCan ↗). Insulated-core doors resist heat flow better than solid doors or doors with glass (NRCan ↗). Worn weatherstripping, misplaced strike plates, poorly fitting frames and warped doors are the main causes of door air leaks (NRCan ↗).

Signs your windows and doors are costing you

• Fog or condensation between the panes of a sealed unit: the seal has failed, and NRCan says replacing the glazing unit is the lasting fix (NRCan ↗).
• Condensation between a storm window and the inner window: house air is leaking past the inner sash, which is common upstairs where the stack effect pushes air out (NRCan ↗).
• Heavy condensation or frost on the room side, usually a sign that humidity is too high for the glass. With the house at 21°C and −20°C outside, NRCan's table shows condensation starting above about 34% indoor humidity on double glazing, 50% on double low-e and 67% on triple low-e (NRCan ↗).
• Rot, mould or staining around frames, cracked putty or paint, worn weatherstripping, or hardware that no longer closes tightly (NRCan ↗).
• Doors out of square with their frames, letting drafts in at the edges or threshold (NRCan ↗).

What an energy advisor checks

An EnerGuide evaluation records the number, type and location of all windows and exterior doors and measures airtightness with a blower door (NRCan ↗). With the house under pressure, smoke or a thermal camera can show where air leaks in at frames and sashes (NRCan ↗). If condensation is your complaint, ask about humidity and ventilation too; NRCan's suggested solutions start with lowering humidity and improving ventilation (NRCan ↗).

Before working on old windows, doors or trim, consider a lead paint test: homes built before 1950 were often painted with lead-based paint (NRCan ↗).

When replacement pays, and when it doesn't

The U-factors tell most of the story. By our arithmetic from those figures, swapping a single-glazed window (about 5.0) for an ENERGY STAR unit (1.22 or lower) cuts heat flow through that opening by about three-quarters, and replacing a clear double-glazed unit (about 2.9) cuts it by a little over half (PNNL, US DOE ↗; NRCan ↗). Adding a storm panel to a single-glazed window lowers its U-factor by 47 to 53% with clear glass and 59 to 64% with low-e glass (PNNL, US DOE ↗).

Replacement makes the most sense when glazing is single or a sealed unit has failed, or when frames are rotten. NRCan suggests a new sealed glazing insert where the frame is sound and full replacement where it is not; a full tear-out also lets the installer make an airtight, insulated seal to the wall opening (NRCan ↗). It pays least when windows are already double-glazed and sound: NRCan notes some windows need only minor air sealing, and ranks air leakage control first in any upgrade strategy (NRCan ↗; NRCan ↗). Test the numbers for your fuel and climate in the upgrade payback calculator.

The ENERGY STAR Canada criteria

Since January 1, 2020, ENERGY STAR windows and doors sold in Canada must have a U-factor of 1.22 W/m²·K or lower, or an Energy Rating (ER) of 34 or higher, plus air leakage of 1.5 L/s/m² or less; skylights must be 2.29 or lower (NRCan ↗). Products designated Most Efficient for 2025 reach 1.05 or lower, or an ER of 40 or more (NRCan ↗).

Glass also lets in free solar heat, measured as the solar heat gain coefficient (SHGC). For new or substantially renovated homes NRCan suggests an SHGC of 0.3 to 0.4; above 0.4 suits homes built before 1980 or not well insulated and air sealed (NRCan ↗).

Fixes, from simplest to biggest

Mistakes to avoid

• Expecting new windows to cure condensation. Tighter windows can raise humidity and make it worse; reduce moisture and improve ventilation as well (NRCan ↗).
• Caulking outside first. It can trap warm, moist air in the wall; seal inside, then outside (NRCan ↗).
• Sealing the storm tighter than the window. With exterior storms the inner window must be the tighter one, and the storm's weep holes must stay open (NRCan ↗).
• Comparing centre-of-glass ratings. They make a product look more efficient than it is; compare whole-window U-factor or ER (NRCan ↗).
• Sealing shut or shrinking a bedroom window. Each bedroom needs a window or door that opens from inside, with an opening of at least 0.35 m² and no dimension under 380 mm (National Building Code ↗).
• Losing the labels. Keep ENERGY STAR labels until any rebate claim is processed (NRCan ↗).
• Solar-control film on every window. It adds little insulation and cuts useful winter sun (NRCan ↗).

Rebates and next steps

Programs open and close often, so check our rebates by province for what is available now. In Ontario, start with Home Renovation Savings assessments. Some programs include a post-upgrade EnerGuide evaluation that needs proof of the work, so keep paid invoices and receipts (NRCan ↗).

To compare this work with other upgrades at your local fuel prices, try the upgrade payback calculator, then put the order of work into your home plan. You can also find a registered energy advisor near you.