Costs and Benefits of Low Solar Gain Windows

The shading capacity of low solar gain windows makes them useful in southern climates that have long cooling seasons. While solar gain is just one of many factors that determine a window’s energy performance, understanding the principles, costs and proper applications of low solar gain windows can help you save energy and money.

What is solar gain?

All buildings absorb heat from the sun through walls, roofs and windows. When we talk about solar gain at windows it’s about the amount of the sun’s heat that is absorbed by and radiates through the glazing. Heat transfer via radiation depends on the material properties, the temperature gradient (outside vs. inside) and the area or size of the surface.

In applicable terms, a window’s solar heat gain is expressed as the solar heat gain coefficient (SHGC), a number between 0 and 1. This number represents the window’s ability to shade. The lower the number, the less solar heat is transmitted through the window. If “1” is equal to 100% of the sun’s solar heat, a SHGC of 0.39 is 39%. A window with an SHGC rating of 0.39 blocks 61% of the sun’s heat. A low solar gain window is a window with a SHGC of 0.4 or less.

This factor is part of the National Fenestration Rating Council (NFRC) label that is seen on most new windows. This label also informs consumers of the window’s U-Factor or insulating capacity and other performance considerations like visible transmittance (VT), air leakage and condensation resistance.

Window Technology & Solar Gain

There are several technologies that control solar heat gain in windows. The SHGC through glazing can be altered by tinted glazing, reflective glazing or by applying a spectrally selective low-emittance (low-E) coating.

Low-E coatings have become popular because this technology helps decrease solar gain, allows a higher transmittance of visible light and helps reduce radiant heat loss in colder months. The solar heat gain through low-E windows is rated as high, moderate or low depending on the coating and the SHGC.

Low solar gain low-E windows are also called low-E2 or solar control low-E. It’s easy to confuse low-E with low solar gain but they are not the same. Low-E is a way to control SHGC and insulation. A true low solar gain window is a low SHGC window (0.4 or less).

For comparison:

double-glazed clear glass: U-factor 0.50 SHGC 0.76 VT 0.81

double-glazed low-E glass: U-factor 0.27 SHGC 0.58 VT 0.78

double-glazed low solar gain low-E glass: U-factor 0.25 SHGC 0.39 VT 0.71

Compared to a clear glass window, the low solar gain window provides twice the insulation, blocks nearly twice the amount of solar heat and is comparable in visible transmittance. The significant difference in energy-efficiency between the two windows is the result of the low solar gain low-E coating that filters short and long-wave radiation.

The simple technology is producing energy saving results. In Texas, building energy codes now require windows that have a SHGC of 0.40 or less. The state’s Public Utility Commission estimates that low solar gain windows result in an annual energy savings between 2.68 and 9.5 kWh per square foot of window glass per household per year.[1]

The Bottom Line - what do low solar windows cost?

Over the past thirty years, window performance has become a priority for builders and homeowners increasing from 3-5% of total construction costs in the 1970s to near 10% today. The jump reveals that the advancing technology has increased the cost of windows.

Much of the difference can be attributed to double-glazed windows replacing single-glazed as the standard for new construction. The extra layer of glazing boosts the windows thermal resistance or insulating capacity. Double-glazing has also allowed implementation of coatings, low conductance spacers and insulating gas fills.

With low solar gain low-E windows, double-glazing, gas fills, aluminum spacers and low-emittance coatings are all contributors to the window’s final cost. Another variable is the material from which the window frame is made.

Window frames today come in a variety of materials including wood, vinyl, fiberglass, aluminum and composites. Windows made with vinyl frames cost about half the price of those with wood frames. Vinyl is a popular choice because it’s low-maintenance and has high insulating properties.

On average, windows with low-E coatings will be about 10-15% more expensive than a comparable window (same glazing, frame materials) with clear glass. Let’s say you’re building a house in Florida, you need twenty windows and you’re looking at a clear glass model that costs $250. The total cost for your project would be $5,000. Adding a low solar gain low-E coating would add about $625 to your overall window costs.

Is the extra cost worth it? According to the National Fenestration Rating Council (NFRC), the increased energy-efficiency of low-E coatings is estimated to reduce energy loss by 30-50%. In Florida, a home with double-glazed clear glass vinyl windows can expect average annual cooling costs around $550. Doing the math with the NFRC’s estimates means upgrading to a double glazed vinyl window with a low solar gain low-E coating can shave about $165 - $275 off your average annual energy costs. At this rate, the payback would be between 2 to 3 ½ years. Even a conservative estimate at half of NFRC’s lowest estimate equals an annual energy cost savings of about $85 and payback time of around 7 years.

Plus, there are other relative cost considerations. If you’re building a new home, installing region appropriate high performance windows means you’re lowering the building’s energy demand. Lower demand means that the size of air conditioning equipment can be reduced which equals a first cost savings. Decreased demand on HVAC systems also means less maintenance and longer lifespan of the units.

Another benefit to low solar gain low-E coated glass is that it can reduce harmful UV rays and protect the interior finishes of your home, decreasing maintenance costs. High performance windows also improve the thermal comfort inside your home, increase natural daylighting capabilities and reduce the need for artificial light. All of these factors equal increased energy-efficiency and increased energy-efficiency equals an increase in the resale value of your home.

It doesn’t matter if you’re building or renovating, windows are a significant factor in energy-efficiency. For warmer climates, low solar gain windows make sense and since windows are an investment you’ll live with for 2-3 decades, you want to get the most efficient systems available. The initial costs of high performance windows are relative to the significant lifelong energy savings they provide.

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