Thermal Comfort and Glazing Design

This month Lab Design published our article about thermal comfort and glazing design.  We’ve shared an excerpt of the piece here, along with graphics that illustrate our findings.

Glazing design and thermal comfort: It’s more than being cool

Glazing is a ubiquitous and invaluable architectural feature that gives occupants a connection to the outdoors while lowering lighting energy demand. Unfortunately, the thermal comfort conditions of a space can be compromised by the amount of glazing, the heat loss through the glass—quantified by a variable called U-value—or a combination of both.

In the U.S., thermal comfort conditions are primarily controlled by mechanical systems, which compensate for any shortcomings in envelope performance. To combat the discomfort created from heat lost through glazing, perimeter radiant heating is often employed. However, a higher-performing glazing system with a lower U-value will decrease that heat loss and may render the additional perimeter system unnecessary for maintaining thermal comfort along the exterior. Because of the complexities of the air and heat flow, in order to understand the trade-off between adding perimeter heating or improving the performance of a glazing system, lengthy and costly computational fluid dynamics (CFD) studies are often used. However, computational methods also exist to evaluate thermal comfort. Utilizing these models, PAYETTE decided to create a simpler tool that could be used earlier in the design process to assess the impact of glazing decisions on the thermal comfort of occupants.

Glazing Properties and Thermal Comfort

Main factors affecting thermal comfort in winter conditions and the relationship to glazing configuration and performance.

Glazing Percentages / View Factor

Range of glazing configurations considered for this study.

U-value vs. View Factor @ -0.5 PMV

Combinations of glazing configuration and performance that ensure occupant comfort without the need for perimeter radiant heating. The three dashed black lines indicate the limit above which perimeter radiant heating would be needed (shown in gray) for three different outdoor temperatures (5, 15, 25°F).  As a reference, typical U-values for double glazed (0.4) and triple glazed (0.25) insulated glass units are shown as an overlay to the calculated U-value for the different glazing scenarios.

References:
Olesen BW. Vereinfachte methode zur vorausberechnung des thermischen raumklimas. HLH – Heizung Luftung Klima Haustechnik. 1995;46:219-25.

Schellen, L., Timmers, S., Loomans, M., Nelissen, E., Hensen, J. L. M., & Marken Lichtenbelt, W. v. 2012. “Downdraught assessment during design: Experimental and numerical evaluation of a rule of thumb,” Building and Environment, vol. 57, no. November, pp. 290-301.

Van Hoof, J. “Forty years of Fanger’s model of thermal comfort: comfort for all?”, Indoor Air 2008; 18: 182–201.

Center for the Built Environment Thermal Comfort Tool.