U-Factor Matters in Hot Climates

The case for thermal breaks and warm-edge spacer



Photo of Helen Sanders, PhD

Helen Sanders, PhD

General Manager

Technoform North America




It is commonly thought that fenestration U-factor is not a key determinant in the performance of façades in hot climates, and generally the focus of code bodies and designers is on solar heat gain, especially reducing the gain through the transparent areas. The assumption is that the difference in air temperature between inside and outside is much lower in hot climates than in cold climates, so the driving force for heat transfer is less. However, the main mechanism for heat transfer through a window frame in hot climates is solar absorption and then conduction of that absorbed heat through to the room side surfaces of the frame. Exterior frame temperatures can exceed ambient air temperatures by a significant amount, especially dark colored frames, because of solar absorption. Temperature differences between inside and outside can be generated through this mechanism which are much more comparable to those seen in cold climates in winter. The use of thermal breaks and warm-edge spacer can significantly reduce this solar heat gain mechanism through the edges of fenestration.

Data from a research study by the Solar Energy Research Institute of Singapore (SERIS) reviewed herein demonstrates how the thermal performance of the window frame significantly impacts the heat transfer through, and thermal comfort performance of, fenestration systems in a hot climate. A whole building modeling study is used to illustrate the resultant impact of fenestration U-factor on energy usage compared to solar heat gain coefficient (SHGC). Thermally broken frames are shown to be a must have for buildings in hot climates, just like they are in colder climates. In addition, the use of warm-edge spacer in the Space Needle renovation demonstrates how reducing thermal transfer at the edge of glass is essential for reducing the cooling load in summer, heating load in winter, and improving thermal comfort year-round.

Introduction and Background

Conventionally, designing using fenestration with low thermal transmittance (low U-factor) is thought to be important only in cold climate zones. This is demonstrated by the relatively high U-factors allowed in

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Field Test Study

Experimental Details

To demonstrate the impact of thermal breaks in aluminum fenestration systems in a hot climate, a research project conducted by SERIS will be reviewed here (SERIS, 2016). Four different

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Building Energy Modeling Study

Input details

A study carried out by BSD used IES building energy modeling software to compare the energy performance of a large 24 story (8,290 m2) prototypical office building (fig. 5)

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Heat gain and U-factor

The data from the field test and the building energy modeling study support the fact that the solar heat gain of fenestration framing is dependent on the

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Conclusion and Future Work

The results of the field study, building energy modeling analysis, and experience from a real case study support the conclusion that reducing the thermal conductance of fenestration frames and edge

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The author would like to acknowledge the Solar Energy Research Institute of Singapore (SERIS) for their work on the field evaluation of thermal breaks, and the support for that study of Technoform, the Building and Construction Authority of Singapore, Meinhardt, Yongnam and the Singapore Green Building Council. Thank you to Building System and Diagnostics (BSD) for their contract building energy modeling services.

Rights and Permissions

Building Construction Authority of Singapore, “Guidelines on Envelope Thermal Transfer Value for Buildings”, version 1.01 (2004). https://www.bca.gov.sg/PerformanceBased/others/ETTV.pdf

Building Systems and Diagnostics (BSD), “A Report on Thermal Performance of Fenestration For Green Mark Version 5.0”, (2016).

National Fenestration Rating Council (NFRC), ANSI/NFRC 200-2017, Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence, 2017.

Solar Energy Research Institute of Singapore (SERIS), “Pilot Study on Energy Savings Potential of Thermally Broken Aluminium Frames in the Tropical Climate”, (2016), commissioned by Technoform.

Wright, John L, Alex McGowan, “Calculating the Solar Heat Gain of Window Frames”, ASHRAE Transactions, 106 part 2 (1999)

Zemanski, Mark W, Richard H. Dittman, Heat and Thermodynamics, sixth edition, McGraw-Hill, 1981, p86-87.