Understanding U-Factors: Why U-Factors Matter in Hot Climates

There is a misconception that U-factor doesn’t need to be a key factor in window design in hot climates. Helen Sanders of Technoform North America, based in Twinsburg, Ohio, says U-factor does matter in these areas, not just in cold climates.

Understanding the Misconception

“The misconception exists because the driving force for conductive heat flow across any object—in this case a window—is the difference in temperature between the hot and cold sides. The greater the difference in temperature, the greater the rate of heat flow,” she explains. “In the case of fenestration, we think the temperatures in question are those of the inside and outside air, which leads us to conclude that temperature difference in cold climates is much higher than that in hot climates.”

For example, in Minneapolis in winter, the outside temperature could be -20 degrees Fahrenheit while the inside temperature is 70 degrees Fahrenheit. That means there’s a temperature difference of 90 degrees. Compared to Phoenix in the summer, the outside temperature could be 110 degrees Fahrenheit while the inside temperate is 70 degrees Fahrenheit for a temperature difference of 40 degrees.

“This leads us to believe that conductive heat flow is a bigger problem in cold climates than hot climates, which is reflected in the fact that the U-factor requirements set for fenestration are less stringent in hotter climate zones than colder climate zones,” says Sanders.

However, she points out that assuming the exterior air temperature is the one that matters for the evaluation of hot climate heat transfer is flawed. This is because solar absorption by framing members increases the temperature of the exterior framing significantly above the ambient air temperature.

“Exterior frame temperatures can exceed 150 degrees Fahrenheit. So, when you evaluate the temperature difference now, it is more like 80 degrees Fahrenheit (150-70), similar to the cold climate,” adds Sanders. “The SHGC for a window is the weighted average of the glass and the frame. The SHGC calculation for the frame actually captures the impact of absorption and conduction through the frame and shows the frame’s solar heat gain to be directly proportional to the frame’s U-factor. The higher (worse) the U-factor of the frame, the worse its SHGC is, and the worse the overall fenestration’s SHGC¹.”

Free flow of heat from the outside of the frame to the inside and through the edge of glass also can cause thermal comfort and safety issues, according to Sanders. In a study done in Singapore, the interior surface temperature of a non-thermally broken frame reached over 120 degrees Fahrenheit.

“Not only can that be uncomfortable to sit next to, but it is also hot enough to cause first-degree burns,” she says.

Reducing Heat Flow

The key to reducing the rate of heat flow from the hot exterior framing elements to the room side surfaces is to break its path. Sanders says this can be done by implementing polyamide thermal breaks, which separate the outer and inner framing members with a low thermal conducting material.

“The wider the thermal break, the better it will be at slowing down the heat flow. Once you constrain the heat flow through the frame, you then have to constrain the heat flow through the edge of glass, since heat will find the path of least resistance to get from the hotter side to the colder side,” she says. “Warm-edge spacers, which are specifically designed to reduce the rate of heat flow across the edge of glass, are great solutions to augment the use of thermal breaks in the adjacent frame. These warm-edge spacers are much less conductive to heat than the typical aluminum box spacer, and so, they improve the U-factor and reduce heat transfer from hot to cold sides.”

Edge of Glass Considerations

When specifying edge-of-glass products, Sanders says it’s important to choose insulating glass edge seal components with durability and service life in mind. This is because “thermal performance is only as good as the insulating glass durability,” since a unit failure will negate the center-of-glass performance, a key factor in overall window performance.

“Service life is key to leveraging the embodied carbon used in manufacturing the glazing product, too. One of the most important aspects of reducing the carbon emissions from constructing buildings is to make them, and their components, last longer. Increasing thermal performance, but reducing lifetime, is not a good trade off in the building lifecycle,” explains Sanders, adding that the edge seal should be considered a system rather than a collection of components that can be swapped out and used interchangeably.

Silicone is the typical choice for secondary sealants in commercial insulating glass because of its structural strength, UV resistance and resistance to liquid water, she adds. However, when using a silicone sealant, it’s critical that the primary seal, comprised of the spacer and primary sealant (PIB), has a low moisture vapor and inert gas transmission rate. Unlike other sealants, silicone has higher gas and water vapor transmission rates and cannot act as a secondary back-up barrier.

“Adhesion of silicone or other sealants to the back of the spacer also is extremely important since loss of adhesion can cause premature seal failure, movement of the spacer itself and higher edge seal extension when under climatic loads,” says Sanders.

She recommends that architects specify spacer-sealant systems that have been qualified repeatedly according to ASTM E2190 and EN1279-2,3.

Solar Heat Gain

To achieve the optimal balance of daylight admission and solar control, it’s important to manage solar heat gain performance.

“Solar gain is a significant issue in hot climates, and also should be managed through a combination of strategies, including solar heat gain control through center of glass, frame and edge. Lowering the SHGC of the fenestration should not be used alone,” says Sanders.

Design strategies to lower the SHGC include careful orientation of the building and appropriate placement of fenestration, natural shading and permanent shading.

“Since half of the sun’s energy hitting fenestration is in the visible light spectrum, once the near infrared energy is blocked, further reduction in SHGC of the glass typically results in a reduction in visible light transmission, leading to darker tinted glazing in the extreme,” says Sanders. “Typically, the best high-performance solar control coatings can reach a center of glass solar heat gain of around 0.27 with a visible light transmission of around 62%. Improving the frame thermal performance can help reduce the overall fenestration SHGC without having to trade off visible light transmission further, depending on the target.”

1. Helen Sanders, PhD, Technoform North America; “U-Factor Matters in Hot Climates,” Façade Tectonics Institute 2020 World Congress

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