A window is made up of several distinct parts — including the frame, sash, glazing unit, sill, head, jambs, and hardware — each doing a specific job that affects energy efficiency, security, and long-term durability. Knowing what these parts are called helps you spot problems early, communicate clearly with contractors, and choose the right replacement windows and frames for your home.
Key takeaways:
- The frame is the fixed outer structure; the sash is the moving part that holds the glass.
- The glazing unit (IGU) is where most of a window’s thermal performance comes from.
- Warm-edge spacers, Low-E coatings, and gas fills are components inside the IGU — not the glass itself.
- Hardware quality directly affects how airtight and secure a window is over time.
- Every part ages at a different rate, so knowing which one is failing saves you money on repairs.
The Frame: The Foundation of the Window
The frame is the rigid border that holds the entire window assembly inside the wall opening. It consists of three sub-parts:
- Head — the horizontal top member of the frame
- Jambs — the two vertical side members
- Sill — the sloped horizontal bottom member that sheds water away from the wall
In a vinyl (uPVC) window, the frame is a hollow multi-chamber profile. Those internal chambers trap still air, which is a poor conductor of heat. Better frames have five or more chambers, which keeps the interior surface warmer and reduces condensation on cold Ontario mornings.
The frame also includes a nailing fin or installation fin on new-construction windows, or a brick mould and casing on retrofit replacements. These are the parts you or your installer fasten to the rough opening in the wall.
When you see fog, rot, or daylight around the edges of a window, the frame or its flashing is usually the culprit — not the glass. If you’re noticing these old windows issues, it’s worth having the frame inspected at the same time as the glass unit.
The Sash: The Moving Part
The sash is the inner framework that holds the glass and, in operable windows, is the part that actually moves. It sits inside the frame and is made from the same uPVC profile material in a vinyl window.
There are two common sash configurations:
- Single sash — one fixed or moving panel
- Double sash — two panels, as found in double-hung or slider designs
In a double-hung window, both the upper and lower sashes slide vertically. In a casement or tilt-and-turn window, the sash is hinged and swings open on one or more axes. The sash is where most of the weather-sealing happens, so worn or warped sashes are the most common reason a window starts to feel draughty.
Sash width matters when you’re measuring for replacements. For an accurate job, you measure the sash opening — not the outside of the frame — so the new unit fits cleanly without shimming.
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The Glazing Unit (IGU)
The insulated glass unit, or IGU, is the sealed glass assembly that fits inside the sash. It is the thermal engine of the window. A modern IGU is not just two panes of glass — it’s a precision-built sandwich of materials:
| Component | Function |
| Outer pane | Structural and weather barrier |
| Spacer bar | Separates panes; holds desiccant to absorb moisture |
| Gas fill (argon or krypton) | Slows heat conduction between panes |
| Inner pane | Structural; often carries coatings |
| Low-E coating | Reflects infrared heat, controls solar gain |
A double-glazed IGU has two panes; a triple-glazed unit has three, with two gas-filled cavities. Triple glazing achieves U-factors in the range of 1.0–1.3 W/m²K, compared with around 1.6–1.9 W/m²K for typical double glazing — a meaningful difference for an Ontario home running heating for six months of the year.
When you see condensation between glass panes, the IGU seal has failed. At that point, the gas has escaped, and the window’s insulating value is much lower than it should be.
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Spacers and Gas Fills
The spacer bar runs around the perimeter of the IGU, separating the two or three panes and sealing the cavity. Old-style aluminium spacers conduct heat readily, creating a cold strip at the edge of the glass — the “cold edge” effect. This is the zone most likely to attract condensation in winter.
Warm-edge spacers (such as foam-and-foil hybrid designs, or structural silicone foam types) cut that edge-of-glass heat loss significantly. TotalSeal uses warm-edge hybrid spacers that lower U-values and improve condensation resistance by roughly 25% versus standard aluminium spacers.
Gas fills — typically argon, sometimes krypton for narrower cavities — replace the air between panes. Argon improves overall glazing efficiency by around 25–30% compared with air-filled units, because it is denser and conducts heat more slowly.
Low-E Coatings and Glass Coatings
Low-E (low-emissivity) coatings are microscopically thin metallic layers applied to one or more surfaces of the glass. They work by reflecting long-wave infrared radiation — the heat your furnace generates — back into the room during winter, and by reducing solar heat gain in summer when applied to the correct surface.
Common coating positions follow a numbered system based on which glass surface they occupy (surfaces 1 through 4 in a triple-glazed unit, counting from outside to inside).
| Coating type | Best use | Effect |
| LoĒ-180 (surface 2) | Heating-dominated climates | High solar gain, strong insulation |
| LoĒ-272 (surface 3) | Mixed climate | Balanced solar and thermal control |
| LoĒ-366 (surface 2 or 3) | All-season performance | Maximum solar rejection, strong U-factor |
For Toronto’s Climate Zone 2 designation under ENERGY STAR™ certification requirements, windows must meet a U-factor of ≤1.22 W/m²K and a solar heat gain coefficient (SHGC) of ≥0.20. Most quality triple-glazed windows with a LoĒ-366 coating meet or exceed these thresholds.
If you want to go further, an interior i89 coating on the room-side surface can cut additional heat loss by an estimated 10–15%, though it is more common on commercial glazing than residential.
“Low-E coating position is one of the most misunderstood specs I see on quotes. For a Toronto home that heats eight months of the year, you want that coating on surface 3 or 4, not surface 2 — putting it on surface 2 in a cold climate chokes off the free solar gain you’d otherwise get on a clear January day.” — Marcus Holt, P.Eng., building envelope consultant, Mississauga, Ontario
The Sill, Head, and Jamb Extensions
Once the window unit is in the wall, it gets surrounded by interior trim and jamb extensions — flat boards that bridge the gap between the window frame and the interior wall surface (drywall). These are not structural, but they matter for air sealing and aesthetics.
The interior stool (commonly called the sill) is the flat shelf at the bottom of the window opening on the inside. Above the window, the apron is the trim piece below the stool. Getting these details right matters especially on deep walls — like those found on older Toronto brick homes with 2×6 or masonry construction.
On the outside, the brick mould or casing frames the window against the exterior wall and is the first line of defence against wind-driven rain. If the exterior caulk between the casing and siding cracks, water gets behind the frame — one of the most common sources of wall rot around windows.
Hardware: Locks, Cranks, and Hinges
Window hardware covers every moving mechanical part:
- Casement operators (cranks) — the gear mechanism that pushes the sash open; a worn casement crank is one of the most common operable window repairs
- Multi-point locks — engage at two or more points along the sash, pulling it tightly against the weatherstrip for an airtight seal
- Hinges and friction stays — on casement, awning, and tilt-and-turn windows; hold the sash at the correct angle
- Tilt-and-turn handle — a single lever that controls two distinct opening modes; see how a tilt-and-turn window works for a breakdown of the mechanism
- Sash lifts and cam locks — on hung and slider windows
- Weep holes — small channels at the sill that drain any water that gets past the exterior seals
Multi-point locking hardware, like the Roto systems used in TotalSeal windows, distributes clamping pressure across two or three lock points rather than one central latch. This keeps the sash pulled evenly against the frame gasket, which both improves security and reduces air infiltration year-round.
“I inspect a lot of windows where the hardware is original to the 1990s installation. The glass is often still fine, but the lock cam has worn enough that the sash sits 2–3 mm proud of the frame on the latch side. You can feel the cold coming in. Upgrading to multi-point locks is sometimes all a window needs before spending on a full replacement.” — Diane Kowalski, RHI (Registered Home Inspector), OAHI member, Toronto
Weatherstripping and Gaskets
Weatherstripping sits in a channel around the perimeter of the sash and compresses when the sash closes, blocking air and water. In vinyl windows, this is usually a co-extruded thermoplastic elastomer (TPE) or EPDM rubber gasket — the same material is used in automotive door seals.
Good weatherstripping keeps the window performing well between the glass and the frame, and it should last 15–20 years under normal conditions. When it gets brittle or flattened, draughts return even if the glazing unit is still intact. Replacement weatherstripping is available by the metre for most standard profiles.
Screens
Operable windows typically include a removable insect screen fitted to the exterior of the sash opening. Screens are a fibreglass or aluminium mesh stretched over an aluminium frame with a spring-loaded corner system for easy removal.
Screens are not part of the window’s thermal performance — they actually reduce visible transmittance (VT) by roughly 10–15% — so removing them in winter makes sense for homes trying to maximise passive solar gain.
Parts Summary Table
| Part | Location | Primary function |
| Frame (head, jambs, sill) | Outer border in wall | Structural housing, air/water barrier |
| Sash | Inside frame | Holds glass; moves in operable windows |
| IGU | Inside sash | Thermal insulation, noise reduction |
| Spacer | Inside IGU perimeter | Pane separation, moisture control |
| Gas fill | Inside IGU cavity | Thermal resistance |
| Low-E coating | On glass surface(s) | Infrared reflection, solar control |
| Weatherstripping | Sash perimeter | Air and water sealing |
| Hardware | Sash and frame | Operation, security, compression |
| Jamb extensions | Interior wall | Finish trim, air sealing |
| Screen | Exterior of sash | Insect barrier |
When Parts Fail: What to Watch For
Not every window problem means a full replacement. Knowing which part is failing helps you decide whether you need a repair or a new unit. That said, if your windows are over 20 years old, how often windows should be replaced is worth reviewing — multiple worn components often mean it’s more economical to replace the whole unit than patch individual parts.
For homeowners comparing options, our cost estimator gives a realistic budget range for full replacement across different window styles.
If you’re also exploring whether energy-efficient window replacement could qualify for the Canada Greener Homes Grant or provincial programmes, make sure the IGU specs — U-factor, SHGC, and ENERGY STAR™ certification — are listed on the quote.
FAQ
What is the difference between a sash and a frame?
The frame is the fixed outer structure installed in the wall. The sash sits inside the frame and holds the glass. In operable windows, the sash moves; in fixed windows, it is stationary but still a separate component from the outer frame.
Can I replace just the glass unit without replacing the whole window?
Yes, if the frame and sash are structurally sound. An IGU replacement (glass swap) is cheaper than a full window replacement. However, if the frame is warped, rotted, or poorly insulating, replacing only the glass leaves the bigger problem in place.
What does a warm-edge spacer actually do?
It replaces a standard aluminium spacer bar at the IGU perimeter. Because it conducts far less heat than metal, it keeps the glass edge warmer in winter, which reduces condensation and lowers the overall U-factor of the window.
How many lock points does a good window have?
A quality operable window should have at least two lock points; three is better for larger sashes. Multi-point locking pulls the sash evenly against the weatherstrip, improving both security and air sealing at the same time.
What does ENERGY STAR™ certified mean for a window in Ontario?
It means the window meets Natural Resources Canada’s performance thresholds for Climate Zone 2, including a maximum U-factor of 1.22 W/m²K and a minimum SHGC of 0.20. ENERGY STAR™ certified windows may qualify for government rebate programmes.
