Soundproof Windows: What Actually Cuts Outside Noise

There is no truly soundproof window. Andersen says so directly: some windows reduce sound well, but none blocks it entirely ¹. What makes the difference is the glass build-up (how many panes, how thick, whether one is laminated and how far apart they sit) and how airtight the closed window is ^{1 3}. A typical dual-pane window has a Sound Transmission Class (STC) rating of 25 to 30, and Andersen suggests looking in the 30 to 35 range if noise is the priority ¹.

So the practical answer is: compare published STC and OITC ratings for the exact glass package being quoted, prefer laminated or mixed-thickness glass, and make sure the window seals well and is installed tightly. Treat any promise of a quiet room as a sales claim, not a measurement.

How sound gets through a window

Sound reaches a room two ways through a window: through the glass and frame, which vibrate, and through any gap where air can pass. A guide from the National Research Council Canada (NRC) puts it simply: if air can leak through the seals, sound can pass through too ³. It adds that cracks around windows or doors can let in as much sound energy as the entire wall surface of the house ³.

Windows are usually the weakest part of a wall for sound. The NRC guide gives an example of a 1 square meter window in a wall of 11 square meters: the window alone reduces aircraft noise by 30 dB and the wall by 50 dB, but together they reduce it by 40 dB ³. Halving the window area raises the combined figure to 43 dB, and doubling it lowers it to 37 dB ³. The point is not the exact numbers but the mechanism: the weakest element sets the result.

What the ratings mean

• STC (Sound Transmission Class) is the rating most window makers publish. Andersen describes it as a measure for indoor sounds such as talking, TV and music ¹.
• OITC (Outdoor-Indoor Transmission Class) is weighted toward outdoor sounds such as traffic, aircraft and construction ¹.
• For both, a higher number means more sound is blocked ¹.

Andersen's own scale gives a feel for the numbers. At STC 30, where it says most dual-pane windows sit, you could hear and follow a loud conversation outside; at 30 to 35 you might hear it but not follow it; at 36 to 40 even loud conversation would hardly be audible ¹.

Ratings are measured in a lab on a standard test size. Pella's design manual notes that its acoustic tests use the standard sizes in ASTM E1425 and that the ratings stand for all sizes of the same configuration ². A field installation with air leaks will not match a lab number, which is why the seal matters as much as the glass.

Laminated glass

Laminated glass is two layers of glass with a plastic interlayer between them. Andersen explains that the interlayer stiffens the pane and damps its vibration, so less sound passes through, and says typically only one pane in the unit is laminated ¹. It is offered across many window types ¹. The NRC guide is more cautious: it says the gains from laminated glass or special gas fills may be no larger than those from slightly heavier glass or a slightly wider air space ³.

Mixed pane thickness

Two panes of the same thickness tend to vibrate at the same frequency, which lets some sounds through more easily. Pairing panes of different thickness breaks that up; Andersen calls this an STC upgrade and says thicker glass generally blocks sound better ¹.

Pella's published table for one product line shows how much the glass alone moves the rating. For a two-panel Impervia sliding window tested at 59-1/2 by 47-1/2 inches, the STC and OITC ratings it lists are ²:

These are one manufacturer's ratings for one product, not a general rule ². Other lines, frame materials and window types will rate differently, so ask for the rating of the exact unit and glass you are being quoted.

Pane spacing and triple pane

The gap between panes matters more than many buyers expect. The NRC guide explains that a small air space creates a low-frequency resonance, and that ordinary thermal double glazing with about 13 mm (1/2 inch) between the panes is no better than a single layer of the same glass against aircraft noise ³. Its table lists aircraft noise reduction of 27 dB for both single 4 mm glass and 4/13/4 mm double glazing, 32 dB with a 100 mm (4 inch) air space, and 36 dB for 6 mm glass with the same 100 mm space ³.

Those wide air spaces are hard to find in house windows, which are designed around thermal performance ³. The same guide says lab tests found triple glazing no better than double glazing with the same total weight of glass and the same overall spacing between the outer panes ³. Andersen makes a similar point: going from dual-pane to triple-pane will not necessarily give the same jump as going from single to dual ¹. More on what a third pane does for energy is in triple-pane windows.

One less expensive route the NRC guide mentions is adding a separate exterior storm window with as large an air space as possible ³. Storm windows covers the interior and exterior types.

Seals, operation and installation

Operable windows depend on their weatherstripping. The NRC guide notes that new windows may seal very well but the seals can wear with use, and that combining high sound insulation with an operable window is hard to keep up reliably over the years ³. That has three practical consequences:

• Window type. A fixed window has no operating seal to wear. For an operable window, ask how the sash seals against the frame and how the weatherstripping is replaced; window types are compared in casement windows and sliding window cost.
• Installation. Gaps between the new frame and the wall let sound in just as drafts do. Ask how the installer will air seal the perimeter, and whether it is an insert or a full-frame job (full-frame vs. insert replacement).
• Area. On the noisy side of the house, a smaller window lets less sound in, as the NRC example above shows ³.

What it may cost

Cost surveys rarely price "soundproof windows" as a category, because the noise performance comes from the glass option. Fixr's 2026 window replacement guide, updated January 21, 2026, lists laminated glass at $300 to $1,200 per window in its glass-type table ⁴. Frame type, size and installation add to that, and the full ranges are in window replacement cost.

Questions to ask before you buy

• What are the STC and OITC ratings for this exact window and glass build-up, and at what test size?
• Is one pane laminated, or are the panes different thicknesses?
• How wide is the air space between the panes?
• How will the perimeter be air sealed, and what happens to the existing frame?

Ask each company to quote the same glass package, so the prices compare line by line. When you are ready, you can compare quotes from window installers.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Reported: Andersen's explanation of STC and OITC, its scale for STC 30 to 40, the 25 to 30 STC typical for dual-pane, laminated and mixed-thickness glass, and triple-pane, checked October 26, 2026.
• ² Reported: Pella's 2026 Architectural Design Manual STC and OITC table for two-panel Impervia sliding windows and its note on ASTM E1425 test sizes, checked October 26, 2026.
• ³ Official record: National Research Council Canada guide by J.S. Bradley (August 1998) on sound through windows: seals, air space, the window and wall example, the aircraft noise reduction table, laminated glass, triple glazing and storm windows, checked October 26, 2026.
• ⁴ Reported: Fixr's laminated glass range of $300 to $1,200 per window, updated January 21, 2026, checked October 26, 2026.