Low-E Glass Explained: What Utah Homeowners Need to Know

What Low-E Actually Means

Low-E is short for low emissivity. Emissivity is a measure of how readily a surface radiates heat energy. Clear glass has an emissivity of about 0.84, which means it radiates 84 percent of the heat energy that strikes it. A Low-E coating drops that emissivity to 0.04 or lower, meaning the coated surface radiates only 4 percent of the heat energy and reflects the rest back toward its source.

In practical terms, a Low-E coating turns your window glass into a radiant heat mirror. It is a microscopically thin layer of metallic oxide, usually silver or tin oxide, deposited onto the glass surface. The coating is so thin (a few hundred nanometers) that it is invisible under normal viewing conditions. You might catch a faint bluish or grayish tint if you look at the glass edge-on in direct sunlight, but face-on the glass looks clear.

What makes Low-E remarkable is its selectivity. It blocks infrared radiation (heat) and ultraviolet radiation (the wavelengths that fade furniture and carpets) while allowing the vast majority of visible light to pass through. Your rooms stay bright and naturally lit. Your furnishings stay protected. And the heat stays where you want it, inside during winter, outside during summer.

How Low-E Coatings Control Heat

Heat moves through windows in three ways: conduction (direct transfer through solid material), convection (air currents carrying heat), and radiation (infrared energy traveling through space). Traditional strategies like double-pane glass and gas fills address conduction and convection. Low-E coatings tackle the third mechanism, radiation, which accounts for roughly 60 to 65 percent of heat transfer through a standard insulated glass unit.

During a Utah winter, your furnace heats the air inside your home. That warm air heats interior surfaces, including the inner pane of your windows. Without Low-E, that glass radiates infrared energy through the window assembly and out into the cold. With a Low-E coating on the appropriate surface, that infrared energy bounces back into the room. Your furnace runs less because less heat escapes.

During summer, the process reverses. Solar radiation heats the outer pane of your window. Without Low-E, that heat radiates inward, warming your rooms and forcing your air conditioner to work harder. With Low-E on the right surface, a significant portion of that radiant heat is reflected back outside before it enters your living space.

The net result is a window that works for you in both seasons. In Utah, where January and July feel like different planets, this dual-season performance is particularly valuable. For a full picture of how these coatings fit into overall window energy performance, see our complete guide to energy-efficient windows for Utah winters.

Hard-Coat vs Soft-Coat Low-E

Not all Low-E coatings are created equal. The two manufacturing methods produce coatings with different performance characteristics and durability profiles.

Hard-Coat (Pyrolytic) Low-E

Hard-coat Low-E is applied during the float glass manufacturing process while the glass is still molten hot. The coating bonds directly to the glass surface and becomes extremely durable. You can handle hard-coat Low-E glass, clean it, and even use it in single-pane applications because the coating withstands direct exposure to air and contact.

The trade-off is performance. Hard-coat Low-E has a higher emissivity (typically 0.15 to 0.20) compared to soft-coat, which means it reflects less radiant heat. It also tends to have a slightly more visible tint and lower visible light transmission.

Hard-coat Low-E was the original technology and is still used in some budget window lines and specialty applications. For most Utah homes seeking maximum energy performance, it has been surpassed by soft-coat.

Soft-Coat (Sputtered) Low-E

Soft-coat Low-E is applied in a vacuum chamber after the glass is manufactured. Multiple layers of silver and metal oxide are deposited onto the glass surface in a precisely controlled process called magnetron sputtering. The result is a coating with emissivity as low as 0.02 to 0.04, dramatically better than hard-coat.

The downside is that soft-coat Low-E is delicate. It cannot be exposed to air, moisture, or physical contact. It must be sealed within an insulated glass unit with the coated surface facing the interior gas cavity. This is perfectly fine for modern double-pane and triple-pane windows, where the coating lives protected between the panes for the life of the window.

Soft-coat Low-E delivers 30 to 50 percent better performance than hard-coat for blocking radiant heat transfer. It allows higher visible light transmission (typically 70 to 80 percent versus 60 to 70 percent for hard-coat), so your rooms are actually brighter with the better-performing coating. For Utah homes, soft-coat Low-E is the clear choice.

Low-E Surface Positions Explained

This is where window glass technology gets specific, and where understanding the details helps you make a smarter purchase. In a double-pane window, there are four glass surfaces, numbered from outside to inside:

• Surface 1: Exterior face of the outer pane (faces the weather)
• Surface 2: Interior face of the outer pane (faces the gas cavity)
• Surface 3: Exterior face of the inner pane (faces the gas cavity)
• Surface 4: Interior face of the inner pane (faces the room)

In a triple-pane window, there are six surfaces (1 through 6), with surfaces 3 and 4 belonging to the middle pane.

The position of the Low-E coating determines its primary function:

Low-E on Surface 2 (most common in cold climates): This position primarily reduces winter heat loss. The coating reflects interior radiant heat back into the room before it can escape through the window. For Utah's cold winters, this is the most important surface to coat.

Low-E on Surface 3: This position primarily reduces summer heat gain by reflecting solar radiation back outside before it penetrates fully through the glass. For Utah's hot summers and intense high-altitude sun, this is the second most valuable surface.

Dual Low-E on Surfaces 2 and 3: This is the optimal configuration for Utah because it addresses both winter heat loss and summer heat gain. Both Cardinal LoE-180 and AGC Comfort Select offer dual-surface Low-E products designed specifically for climates like ours.

For triple-pane windows, a common high-performance configuration places Low-E on surfaces 2 and 5, with the middle pane acting as an additional insulating barrier. Some premium configurations coat surfaces 2, 3, and 5 for maximum thermal control.

Best Low-E Configuration for Utah

Utah's Climate Zone 5 designation and our dramatic seasonal swings mean the ideal Low-E configuration balances winter insulation against summer solar control. Here is what to specify for different window exposures in a typical Wasatch Front home:

South-Facing Windows

Use Low-E on surface 2 with a moderately high SHGC (0.30 to 0.35). You want to retain some passive solar heat gain from the low winter sun while still blocking UV and managing summer heat. Avoid aggressive solar-blocking coatings on south windows; they sacrifice free winter heating.

West-Facing Windows

Use dual Low-E on surfaces 2 and 3 with the lowest SHGC available (0.22 to 0.25). West exposures catch the intense afternoon sun in summer. The dual coating aggressively reflects solar heat without sacrificing winter insulation.

East-Facing and North-Facing Windows

Use Low-E on surface 2 as a minimum. Dual Low-E on 2 and 3 is ideal but not as critical as on west-facing windows. Focus on achieving the lowest U-factor possible, especially on north windows that receive no beneficial solar gain.

The Simple Approach

If specifying different glass configurations for different exposures sounds complicated, there is a simpler path. Choose a dual Low-E product with a U-factor around 0.18 to 0.22 and an SHGC around 0.25 for the entire house. You will sacrifice a small amount of passive solar gain on south windows, but the overall performance will be excellent and the installation process simpler.

Common Myths About Low-E Glass

Several misconceptions about Low-E glass persist, and they sometimes cause homeowners to make suboptimal choices.

Myth: Low-E glass makes your house dark. Fact: Modern soft-coat Low-E allows 70 to 80 percent of visible light through. The 15 to 20 percent reduction from clear glass is barely perceptible. Your rooms will be well-lit.

Myth: Low-E coating wears off over time. Fact: Soft-coat Low-E is sealed inside the insulated glass unit and never contacts air, moisture, or cleaning products. It lasts the full life of the window. Hard-coat Low-E is physically bonded to the glass and is comparably durable.

Myth: Low-E is only useful in cold climates. Fact: Low-E coatings reduce heat transfer in both directions. In hot climates, they keep solar heat out. In cold climates, they keep furnace heat in. In Utah, where we have significant heating and cooling loads, they work overtime in both seasons.

Myth: Low-E kills houseplants. Fact: Low-E glass blocks UV radiation, which is not the primary wavelength plants use for photosynthesis. Plants primarily use visible light in the red and blue spectrums, which passes through Low-E glass freely. Your houseplants will thrive. If anything, they may appreciate the UV protection.

Myth: You do not need Low-E if you have double-pane windows. Fact: Double-pane without Low-E addresses conduction and convection but leaves radiation unchecked. Adding Low-E to a double-pane unit reduces overall heat transfer by 30 to 50 percent beyond what the double pane alone provides. For the small added cost, there is no reason to order insulated glass without Low-E in any climate.

Making the Right Choice

Low-E glass is not optional for Utah homes. It is a fundamental component of any window that claims to be energy efficient. The technology is proven, the cost premium is minimal ($15 to $30 per window at manufacturing scale), and the energy savings are well-documented. When shopping for replacement windows, confirm that the units include soft-coat Low-E and ask which surfaces are coated.

The combination of Low-E coatings with argon or krypton gas fills and quality frames creates a window system that dramatically outperforms what was available even a decade ago. When you layer in federal tax credits and Utah utility rebates, which often require the kind of performance numbers that Low-E makes possible, the financial case is compelling.

If your home still has windows without Low-E coatings, whether single-pane or early double-pane from the 1980s and 1990s, the upgrade to modern Low-E glass will be one of the most noticeable comfort improvements you have experienced. Drafts diminish, cold spots near windows disappear, and your energy bills drop measurably. That is not marketing; that is physics.