If your home backs onto Autoroute 15, sits under the YUL flight path or faces a bus corridor in Villeray, you have probably searched for "soundproof windows" and landed in a fog of acronyms. STC — Sound Transmission Class — is the number that cuts through the marketing, but only if you know what the digits actually mean. Here is how to read STC ratings, which glass upgrades move the number, and how much quieter each point really makes your bedroom.
What STC Actually Measures
Sound Transmission Class is a single-number rating derived from laboratory testing under ASTM E90: a window is mounted between two chambers, noise is played on one side across frequencies from 125 to 4000 Hz, and the transmission loss is measured on the other. The resulting curve is fitted to a standard contour and summarized as one integer. Higher is quieter — each additional point represents roughly one more decibel of average sound blocked.
The critical caveat is that STC was designed around speech frequencies, not traffic. Truck rumble, bus idle and aircraft approach all carry heavy low-frequency energy below the range where STC is most sensitive, which is why two windows with identical STC can perform differently against Autoroute Métropolitaine rumble. Some manufacturers publish OITC (Outdoor-Indoor Transmission Class), a companion rating weighted toward low frequencies — when you can get it, it is the more honest number for highway and flight-path homes.
Still, STC remains the rating you will actually find on Quebec spec sheets, and it ranks products reliably: a window rated STC 34 will beat a window rated STC 28 in any real house, against any real noise source.
Typical Numbers: Where Windows Actually Land
A standard double-glazed residential window — two panes of 3 mm glass with a 12 mm airspace — lands around STC 26 to 28. That is the baseline most Quebec homes have today, and it is also roughly what a decent wall assembly loses through its weakest point. Ordinary triple glazing, despite its thermal advantages, adds little acoustically: three identical light panes with narrow gaps often test at STC 28 to 30.
Purpose-built acoustic configurations move the number meaningfully. Asymmetric double glazing reaches STC 31 to 33, adding a laminated pane pushes into the 34 to 38 range, and specialized assemblies with wide cavities can exceed 40 — though those usually require deeper frames than standard residential lines allow.
For orientation, here is what the common configurations deliver in practice.
- STC 26–28: standard 3+3 mm double glazing — the baseline
- STC 29–31: asymmetric glass (3 mm + 5 mm) or wider airspace
- STC 32–34: asymmetric plus one laminated pane
- STC 35–38: thick laminated glass with acoustic PVB interlayer
- STC 40+: deep-cavity or secondary-glazing assemblies
Asymmetric Glass: The Cheapest Real Upgrade
Every pane of glass has a resonant weakness — a frequency band where it vibrates sympathetically and lets sound pour through, called the coincidence dip. When both panes in a sealed unit are the same thickness, they share the same dip and fail together at the same frequencies. Making one pane thicker than the other — 3 mm paired with 5 mm is the classic recipe — staggers the dips so each pane covers the other's weakness.
This single change typically buys 2 to 4 STC points and costs remarkably little, often $15 to $40 per window at order time, because the manufacturer is simply substituting one glass thickness on a line they already run. It is the highest-value acoustic upgrade in the catalogue and the first thing to specify for any bedroom facing a noisy street.
Asymmetry works in triple glazing too: a 3-5-3 or 4-3-3 stack outperforms the standard equal-pane triple with no thermal penalty. If you are already ordering triple glazing for the Quebec winter, asking for mixed thicknesses is close to free noise control.
Laminated Panes and Airspace: The Serious Tools
Laminated glass bonds two sheets of glass around a plastic (PVB) interlayer, and that soft layer does something solid glass cannot: it damps vibration, converting acoustic energy to trivial heat instead of re-radiating it. Substituting one laminated pane into a sealed unit typically adds 3 to 5 STC points, and acoustic-grade PVB interlayers add another point or two. Laminated glass also brings security and UV-filtering benefits, which is why it is standard on high-end urban projects.
Airspace is the other lever. The air gap between panes acts as a spring, and a wider gap is a softer spring that transmits less energy — but sealed insulating units cap out around 16 to 20 mm before convection undermines thermal performance. The dramatic acoustic gains come from very wide cavities of 50 to 100 mm, which is the domain of secondary glazing: an interior pane added inside the existing window, common on heritage buildings in Old Montreal where exterior alterations are restricted.
Budget-wise, a laminated acoustic upgrade adds roughly $80 to $200 per window depending on size. For a three-bedroom facade upgrade, expect $400 to $900 in glass premiums over standard units — usually the best-spent money in the whole project for anyone living with chronic traffic noise.
What +5 STC Feels Like
Decibels are logarithmic, and human perception follows suit: a reduction of about 5 points is the threshold where nearly everyone describes the room as "noticeably quieter," and a jump of that scale — say from a worn STC 26 window to a well-sealed STC 31 or 32 unit — is commonly perceived as cutting the intrusive noise roughly in half. Going from 28 to 38 is transformative: conversation-level traffic noise drops to a murmur you stop registering.
The perceptual math explains why chasing one or two points is pointless but chasing five is not. Moving from STC 30 to 31 is not detectable in a real bedroom; moving from 28 to 33 changes how you sleep. Structure your upgrade decisions around 5-point jumps and you will spend where it counts.
It also explains why the difference between brands quoting 29 and 30 should never decide a purchase. Installation quality, seal integrity and frame choice will swamp a one-point laboratory difference every time.
Frames, Seals and Installation Matter as Much as Glass
Sound is like water: it finds the gap. A premium acoustic glazing unit mounted in a leaky frame performs like a cheap one, because air paths transmit sound almost unimpeded. Compression-seal windows — casements and awnings that pull tight against their gaskets — consistently outperform sliding types by 2 to 3 effective points because sliders must leave a friction gap to operate.
The installation joint is the other silent killer. The gap between frame and rough opening must be filled with low-expansion acoustic foam or mineral wool and sealed on both faces; a hollow, poorly foamed perimeter can cost 5 points or more, erasing everything you paid for in glass. Insist that your contractor documents the perimeter sealing method, and remember that in Quebec any contractor performing this work must hold an RBQ licence.
When comparing quotes across our window lineup, ask for the tested STC of the whole window assembly — glass, frame and seals together — rather than the glass-only number, which is always flattering and never what your bedroom experiences.
Choosing by Location: Montreal Noise Realities
Within earshot of the Décarie, the Métropolitaine or Autoroutes 13, 15, 20, 25 and 40, low-frequency rumble dominates: prioritize laminated glass and maximum cavity width, and check OITC if available. Under the YUL approach corridors over Saint-Laurent, Dorval, Lachine and parts of Ahuntsic, aircraft events are intermittent but intense — laminated asymmetric triple glazing in casement frames is the proven recipe there.
On urban arterials like Papineau, Saint-Denis or Notre-Dame, the mix of bus pulses, sirens and voices spreads across the spectrum, and a solid STC 32 to 34 window handles it well. Near rail lines in Pointe-Saint-Charles or Saint-Henri, treat the problem like highway rumble and weight your budget toward the facades actually facing the tracks.
A final planning note: you only need acoustic packages on exposed facades. Ordering premium glass for the quiet rear elevation wastes budget better concentrated where the noise lives — a mixed order, standard glazing in back and acoustic in front, is completely normal and any good manufacturer will quote it that way.
Frequently Asked Questions
What STC rating do I need near a highway in Montreal?
Target STC 32 to 34 as a whole-window rating for facades directly exposed to autoroute noise, achieved with asymmetric laminated glazing in a compression-seal frame. Because highway rumble is low-frequency, ask for the OITC rating too when available — it better predicts performance against truck and bus noise.
Does triple glazing block more noise than double glazing?
Not automatically. Three identical panes with narrow gaps test only 1 to 2 points better than standard double glazing, because the panes share the same resonant weaknesses. Triple glazing becomes acoustically strong when the panes have different thicknesses and one is laminated — then it combines winter insulation with STC in the mid-30s.
How much does upgrading to acoustic glass cost in Quebec?
Asymmetric glass thickness is the bargain at roughly $15 to $40 extra per window, and a laminated acoustic pane adds about $80 to $200 depending on size. A typical noisy-facade upgrade of four to six windows carries $400 to $900 in glass premiums — modest against the cost of the windows themselves.
Is a 2-point STC difference between two windows meaningful?
Not perceptibly. Differences under 3 points are barely detectable in a furnished room, and installation quality can easily swing results by more than that. Compare windows in 5-point steps — the threshold where rooms sound clearly quieter — and put the marginal dollars into perimeter sealing instead.
Why does my new high-STC window still let in truck rumble?
Two usual causes: STC underweights the low frequencies where diesel rumble lives, so a high-STC unit can still pass bass energy, and installation gaps — an unsealed frame perimeter, worn weatherstripping or a nearby air leak — let sound flank the glass entirely. Check the OITC spec and have the installation joint inspected before blaming the glazing.
Can I improve the STC of my existing windows without replacing them?
Meaningfully, yes: fresh weatherstripping and a properly sealed perimeter recover several effective points on a leaky unit, and interior secondary glazing with a wide air gap can add 8 to 12 points — often outperforming replacement. Full replacement wins when the frames themselves are degraded or you want thermal gains at the same time.