Types of Low-E Glass: How Different Coatings Affect Window Performance

The main types of Low-E glass differ in two ways: how the coating is made and what performance it is designed to deliver. Hard-coat and soft-coat describe manufacturing methods; passive solar-gain and solar-control describe how a product balances indoor warmth, incoming sun and daylight. Those categories overlap, so the coating name alone won’t tell you whether a window suits your home. Compare the complete window’s U-value, solar heat gain and visible light transmission, then consider the room’s orientation, shading and comfort needs. A well-matched specification can improve comfort; the wrong one may reduce useful winter sun or leave a sunny room prone to overheating.

What does Low-E glass do?

Low-E means low emissivity. The glass has a thin coating that reduces the transfer of heat by long-wave infrared radiation. In colder conditions, a suitable coating can reflect more indoor heat back into the room; depending on its design, it can also limit some solar heat entering from outside.

The coating is designed to transmit useful visible light, but the balance between light and solar heat varies by product. Low-E glass is not automatically dark, nor does every Low-E product block the same amount of sun. It also works as part of a glazing system: the glass panes, gap between them, frame, seals and installation all affect the finished window’s performance. A coating cannot compensate for poor sealing or an unsuitable frame.

What are the main types of Low-E glass?

It helps to separate two kinds of description. “Hard coat” and “soft coat” refer to how the coating is applied and protected. “Passive” and “solar control” describe the balance of heat and light the glass is intended to achieve. They are not competing labels on a single scale: a particular coating method can be used in products with different performance aims.

Hard-coat Low-E glass

Hard-coat Low-E, also called pyrolytic coating, is applied while the glass is being made. The coating bonds to the hot glass, making the finished surface relatively robust compared with many soft-coat products. Depending on the product, it may be suitable for uses where the coated surface is not enclosed within a sealed insulating glass unit.

Hard-coat products are often associated with passive solar gain, but the label alone does not establish the product’s heat or light performance. Check the technical data for the exact glass configuration. Its durability can be useful, while a particular product may offer a different balance of insulation, solar control and appearance from a soft-coat alternative.

Soft-coat Low-E glass

Soft-coat, or sputtered, Low-E is deposited onto finished glass in a vacuum process. These coatings can use multiple thin layers to tune how the glass handles heat and light. Many soft-coat products are designed to sit on a protected surface inside a sealed double- or triple-glazed unit, rather than being left exposed. Follow the manufacturer’s specification for the correct position and handling.

Soft-coat products include both higher-solar-gain and solar-control options. They are often selected for their range of performance choices, but “soft coat” is not a performance guarantee. Compare the published values for the actual window, not just the coating description.

This table compares manufacturing and handling, not which glass is “better.” The right choice depends on the product’s measured performance and where it will be installed.

Passive, solar-control and spectrally selective options

These descriptions focus on what the glazing is meant to do. Product terminology can vary, so use it as a starting point and check the figures rather than assuming a label has a universal meaning.

These are broad performance aims, not guaranteed results for every product carrying the label. A spectrally selective coating, for example, still needs to be assessed for its visible light transmission and solar heat gain.

Which performance figures should you compare?

Ask for figures for the complete window where possible, and check that competing products are being compared on the same basis. Centre-of-glass figures describe the glazing area; whole-window figures also account for the frame and other parts of the unit. Mixing the two can make an otherwise fair comparison misleading.

• U-value: Indicates heat transfer through the window. A lower U-value generally means less heat passes through, but confirm whether the figure applies to the glass alone or the whole window.
• Solar heat gain coefficient (SHGC) or g-value: Indicates how much solar energy passes through the glazing. A lower value means less solar heat enters; a higher value allows more of it through.
• Visible light transmission (VLT): Shows how much visible daylight passes through. A higher value generally means more light, though the view, tint and reflections also affect how the glass looks.
• Appearance and reflectance: Ask how the glass will look from inside and outside. Coatings and other glass treatments can affect colour, reflectivity and the view.

These measures describe different things. A low U-value does not tell you whether a room will gain too much summer sun, and a low SHGC does not tell you how bright the room will be. Look at the values together with the exact frame and glass build-up.

How do orientation and room use affect the choice?

Start with the room, not a general claim that one coating is best. Consider when direct sun reaches the window, whether nearby buildings or trees provide shade, how much winter warmth is desirable and whether the room already overheats. In New Zealand, north-facing windows often receive more sun than south-facing ones, but roof overhangs, surrounding buildings and local conditions can change the result.

• Choose a higher-solar-gain option for consideration where winter sun is useful and the room is unlikely to overheat. Check how it will behave during warmer months as well.
• Consider solar control for a room with strong sun exposure, particularly if keeping the room cooler is a priority. Confirm that the chosen product still provides the daylight and appearance you want.
• Look at a selective or moderate-balance option when you want daylight but need to limit solar heat. The product data should show whether it achieves that balance for your needs.

Shading can matter as much as the coating choice. Curtains, blinds, external shading and landscaping can affect comfort, but their suitability depends on the window and the room. If overheating is already a problem, discuss both the glazing specification and practical shading options rather than expecting Low-E glass alone to solve it.

Does the coating’s position in the glass unit matter?

Yes. In a standard double-glazed unit, the exterior pane has an outdoor-facing surface and a cavity-facing surface; the interior pane has a cavity-facing surface and an indoor-facing surface. These are commonly numbered from outside to inside, making the cavity-facing sides surfaces 2 and 3. The coating’s position can influence how the unit manages heat and solar energy.

There is no universal rule that homeowners should use a particular surface. The suitable position depends on the coating, glass unit and design goal. Ask the supplier to state which surface is coated and to provide the performance figures for that exact configuration. Do not assume a unit can be reversed without affecting its intended performance or appearance.

A practical checklist before choosing Low-E glass

1. Describe the room’s problem or goal. Is the priority reducing heat loss, limiting strong sun, retaining daylight, or balancing these needs?
2. Note the exposure. Consider orientation, shading and the times of year when the room is uncomfortable.
3. Request the full glass specification. Ask for the pane make-up, coating type and coating surface, as well as any other glass treatments.
4. Compare the relevant performance figures. Review U-value, SHGC or g-value, and VLT. Check whether values describe the glazing or the whole window.
5. Check the finished appearance and installation details. Confirm how the glass looks from both sides and that the selected coating is suitable for the proposed unit and frame.

Common mistakes when comparing Low-E glass

• Assuming all Low-E glass blocks summer heat. Some products are designed to admit more solar heat, so check SHGC or g-value.
• Treating hard coat and soft coat as performance ratings. They describe coating methods; the figures for the specific product show its intended balance.
• Choosing only by U-value. Insulation matters, but it does not indicate the amount of solar heat or daylight entering the room.
• Equating a darker tint with better Low-E performance. Tint and Low-E coatings are different features. Assess the stated figures and appearance rather than judging by colour alone.
• Ignoring the frame and installation. The glazing is only one part of the window. A poor fit or inadequate sealing can undermine the performance of a well-specified glass unit.

When comparing types of Low-E glass, first decide whether the room needs more winter solar warmth, less incoming solar heat or a balance of daylight and comfort. Then compare the complete window’s U-value, SHGC or g-value and visible light transmission, along with the specified coating position. That gives you a sounder basis for choosing than the coating label alone.