How to Tell If You Have Single-Pane Windows (And Why It Matters)

You know your home is drafty. You know your energy bills seem too high. But before you can make a smart decision about window upgrades, you need to answer a basic question: what type of glass do your windows actually have?

Many Utah homeowners assume they have double-pane windows because "the house is not that old." But single-pane windows were installed in homes well into the 1980s, and even some early 1990s budget construction used them in certain rooms. The only way to know for sure is to test. Here are three methods that take less than two minutes total.

Three Quick Tests

You do not need any special equipment to determine whether your windows are single-pane or double-pane. Any one of these tests will give you a definitive answer.

Test 1: The Lighter or Flame Test

This is the fastest and most reliable method. It works day or night, on any window.

How to Do It

1. Turn off the lights in the room so you can see reflections clearly.
2. Hold a lighter, match, or even the flashlight on your phone about one inch from the interior surface of the glass.
3. Look at the reflections in the glass.

What to Look For

• One reflection: Single-pane. There is only one glass surface to reflect the light.
• Two reflections, close together: Double-pane. You are seeing a reflection from the interior surface and a second, slightly offset reflection from the exterior surface of the inner pane. The gap between reflections is small, usually less than a quarter inch visually.
• Two reflections, farther apart: Double-pane with a wider gas space, or possibly triple-pane. The wider gap indicates a thicker insulating glass unit.

The key is that double-pane glass always produces two distinct reflections because the light bounces off both the inner and outer surfaces of the glass assembly. Single-pane produces only one.

Checking Multiple Windows

Do this test on several windows throughout your home, including different floors and room types. It is not uncommon for a home to have a mix of window types, especially if previous owners replaced some windows but not others.

Test 2: The Edge Inspection

If you can see the edge of the glass where it meets the frame, you can visually identify the construction.

How to Do It

1. Open the window (if possible) or look at the glass edge from a steep angle where it meets the sash.
2. Examine the visible edge of the glass.

What to Look For

• Single-pane: You see one thin edge of glass, typically 3-4 millimeters thick. The glass sits directly in the sash with glazing putty or a rubber gasket.
• Double-pane: You see two layers of glass separated by a metallic spacer bar. The spacer is usually silver or black and approximately 6-12 millimeters wide. The entire assembly (glass-spacer-glass) is typically 18-25 millimeters thick.
• Triple-pane: Two spacer bars and three layers of glass. The total assembly is noticeably thicker.

If your windows are painted shut or you cannot open them, try looking at a basement window or a window in a utility area where the glazing method may be more visible.

Test 3: The Age and Construction Check

While not as definitive as the first two tests, the age and style of your home provides strong clues.

Timeline

• Homes built before 1970: Almost certainly single-pane throughout unless windows have been replaced.
• Homes built 1970-1985: Transitional period. Higher-end homes received double-pane. Budget and mid-range construction often used single-pane, especially in secondary rooms.
• Homes built 1985-1995: Most new construction used double-pane in living areas. Single-pane may appear in garages, basements, and utility rooms.
• Homes built after 1995: Virtually all new construction in Utah used double-pane or better. Single-pane at this point would be unusual and likely a code compliance issue.

Style Clues

Certain window styles are associated with single-pane construction:

• Wood sash with rope-and-pulley counterweights: Classic single-pane double-hung windows found in pre-1960 homes.
• Aluminum sliding windows: Commonly single-pane in homes from the 1960s-1980s.
• Steel casement windows: Found in mid-century homes, almost always single-pane.
• Jalousie (louver) windows: Glass slat windows, always single-pane, sometimes found in enclosed porches or basements.

If your windows have any of these characteristics, the lighter test is worth doing to confirm.

Why Single-Pane Windows Are a Problem

Understanding why single-pane glass underperforms helps you evaluate how urgently to address the situation.

Thermal Performance

Glass is a poor insulator. A single layer of glass approximately 3 millimeters thick transfers heat almost freely. The key metric is U-factor, which measures how easily heat passes through a material. Lower numbers are better.

• Single-pane clear glass: U-factor approximately 1.0
• Double-pane clear glass (no coatings): U-factor approximately 0.47
• Double-pane Low-E with argon gas: U-factor 0.25-0.30
• Triple-pane Low-E with argon/krypton: U-factor 0.15-0.20

The difference between single-pane and modern Low-E double-pane is roughly a 70-75% reduction in heat transfer. That is not a small improvement. It is a fundamental change in how your home interacts with the outdoor temperature.

Sound Transmission

Single-pane glass has a Sound Transmission Class (STC) rating of approximately 26-27. Modern dual-pane windows rate 28-32, and laminated options reach 34-40. If outside noise is a problem, single-pane glass is offering you almost no barrier.

UV Transmission

Standard single-pane glass allows 70-80% of UV radiation to pass through. In Utah, where altitude increases UV intensity by 15-25% above sea level, this accelerates fading of floors, furniture, and artwork. Modern Low-E coatings block 95-99% of UV while still transmitting visible light.

Safety and Security

Single-pane glass is thinner and more fragile. It breaks more easily from impacts (a stray baseball, a fallen tree branch, hail) and provides less resistance to forced entry than multi-pane assemblies, especially those with tempered or laminated glass.

The Energy Cost of Single-Pane in Utah

Utah's climate makes single-pane windows especially expensive to live with. The Wasatch Front experiences winter temperatures that regularly drop into the teens and single digits, and summer temperatures that exceed 100 degrees Fahrenheit. Your HVAC system has to work against every BTU that passes through those single layers of glass.

Energy Star estimates that replacing single-pane windows with certified double-pane units saves $101-$583 per year nationally. For Utah, with our long heating season (roughly October through April) and extreme temperature differentials, savings typically fall in the upper half of that range.

For a home with 12-15 single-pane windows, that translates to roughly $200-500 per year in reduced heating and cooling costs. Over the 20-25 year lifespan of new windows, that adds up to $4,000-12,500 in cumulative energy savings, often exceeding the initial investment.

Your Options

Once you have confirmed that you have single-pane windows, here is how to think about next steps.

Option 1: Storm Windows (Budget Approach)

Interior or exterior storm windows add a second layer of glass or acrylic over your existing window. They create a dead air space that improves insulation.

• Cost: $100-250 per window
• Performance improvement: U-factor drops to approximately 0.50, roughly a 50% improvement over single-pane alone
• Drawbacks: Adds maintenance (cleaning two layers instead of one), can reduce operability, and does not provide the Low-E or gas fill benefits of modern integrated units
• Best for: Homeowners on a tight budget, historic homes where original windows must be preserved, or as a temporary measure while planning full replacement

Option 2: Full Window Replacement (Recommended)

Remove the existing single-pane windows and install modern double-pane or triple-pane units with Low-E coatings and argon gas fill.

• Cost: $400-800 per window installed (vinyl), $600-1,200 (fiberglass or wood-clad)
• Performance improvement: U-factor of 0.25-0.30, a 70-75% improvement over single-pane
• Additional benefits: UV protection, noise reduction, improved security, warranty coverage, zero frame maintenance (vinyl)
• Best for: Most homeowners, especially those planning to stay in the home for 5+ years

Prioritizing If You Cannot Replace All at Once

If replacing every window at once is not in the budget, prioritize by exposure:

1. North-facing windows first: These lose the most heat in winter and receive no solar gain to offset it.
2. West-facing windows second: These receive intense afternoon sun in summer, driving up cooling costs.
3. Largest windows next: Bigger glass area means more heat transfer.
4. Bedrooms: For comfort and safety (egress requirements).

For more guidance on evaluating your complete window situation, including whether other signs of failure are present alongside single-pane glass, see our comprehensive guide on 10 warning signs you need new windows. And for a detailed look at what replacement costs in your area, check our window replacement cost guide.

The bottom line is simple: if you confirmed single-pane windows using the tests above, you are paying significantly more to heat and cool your home than you need to. Every winter you wait is another season of energy waste.