click to enlarge
Figure 1: Glazing properties and thermal comfort. Images: Payette

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.

Thermal comfort is defined as the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation. The most prevalent model to predict it, proposed by P.O. Fanger, indicates the variables that influence a person’s thermal comfort sensation consist of four external factors—air temperature, radiant temperature, air speed, humidity—and two personal factors: the metabolic rate of the occupants’ activity (met) and clothing insulation value (clo). This model is the basis for ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, and it considers the previously mentioned variables needed to generate a Predicted Mean Vote (PMV), a quantitative method that predicts the average comfort vote on a seven-point scale that predicts occupants feeling hot (+3.0) to cold (‑3.0), with 0.0 as neutral. Another metric to express thermal comfort is Percent People Dissatisfied (PPD), which is derived from the PMV and predicts the percentage of thermally uncomfortable people in a space. Both are commonly used metrics to define thermally acceptable environments. ASHRAE 55 recommends that PMV levels fall within the range of ‑0.5 to 0.5, or less than 10% PPD. The Indoor Environmental Quality Credit 7.1 under LEED requires the PPD to fall below 20%.

Radiant temperature is the factor most impacted by changes in glazing performance and design. Radiation is the heat transfer between objects that aren’t in contact but have different surface temperatures—like a person and the surrounding walls—and is very important in determining an occupant’s thermal comfort. The mean radiant temperature for an occupant is a function of two variables: the radiant temperature of the surrounding surfaces and a view factor associated to each surface. The view factor is a geometric relationship that an occupant has with various surfaces within a space, and shows how much influence a surface temperature will have on the radiant heat exchange of an occupant and the surrounding surface. For instance, sitting next to a 6-ft-wide window on a winter’s day versus a 2-ft-wide window will make an occupant feel colder because there’s a larger influence from the surface temperature of the glass.

Another factor of concern for thermal comfort is the presence of downdraft along tall, vertical pieces of glass when the outside air temperature is low. Downdraft occurs as warm interior air hits the cold surface of the glass and falls, creating cold convective currents with temperatures and air speeds that can cause discomfort. Disturbances from convection are a function of the glazing performance (U-value) and the height of the glass; a rule of thumb proposed by Olesen correlates glazing U-value and height to discomfort due to downdraft. Recent studies have empirically tested this equation and found it adequate but conservative (Schellen); and for this study, we have used it to determine those conditions when downdraft may be problematic.


click to enlarge
Figure 2: Glazing percentages/view factor.

Design scenarios
To evaluate the impact of glazing amount, performance and height on thermal comfort, we analyzed 12 glazing scenarios with varying view factors and glazing percentages from the three most common glazing configurations (punched windows, ribbon windows and fully glazed walls). Figure 2 illustrates the view factors, glazing percentage and the window layout of the options investigated.

Using typical indoor conditions, we used the Center for the Built Environment’s Thermal Comfort online tool to find a mean radiant temperature that would yield to a comfort level of -0.5 PMV (the lower threshold from ASHRAE 55-2010’s acceptable comfort range). We then evaluated which combination of glazing configurations (view factors) and thermal performance (U-value) would help us both achieve this mean radiant temperature goal and ensure that downdraft wouldn’t be an issue.

Figure 3 shows the final outcome of our calculations, and allows for easily identifiable combinations of glazing U-value and percentage that would negate the need for perimeter radiant heating. Three dashed black lines indicate the limit above which perimeter radiant heating would be needed to ensure an adequate mean radiant temperature for three different outdoor temperatures (5, 15 and 25 F). Solid black lines indicate the comfort threshold related to downdraft, regardless of outdoor temperature. To ensure occupant comfort without the need for perimeter radiant heating, one must find a combination of U-value and percent glazing that falls below both curves.

Not surprisingly, these results indicate that the required U-value to ensure occupant comfort decreases as percent glazing increases, but also highlight the fact that, even when very little glazing is used, there’s a limit to the allowable U-value to avoid downdraft. It’s important to note the large difference in threshold between full-height glazing and strip windows: Given a certain glazing performance and adding a window sill to the design could spare the need to install perimeter radiant heating, with little sacrifice on daylighting or views.

Let’s assume that a design team is considering two different glazing configurations (labeled as Examples 1 and 2 in Figure 3): a 20% glazed façade, utilizing punched windows, and a façade that’s at least 60% glazed, with either ribbon windows or glazing that extends to the finished floor. They want to find the highest U-value that each glazing assembly can have without the use of perimeter radiant heating, given an exterior design temperature of 15 F. For the punched window system, the team traces the 20% glazing line to find the threshold with a U-value of 0.49 [Btu/hr*ft2*F]. In this case, the occupant’s comfort sensation is a combination of the interior surface temperature of the window and the potential of downdraft due to the window height. For the second configuration, the chart indicates that the threshold U-values are 0.29 and 0.22 for the ribbon and full-height glass at 60% glazing, respectively. This means that for two glazing configurations, having the same amount of glass and access to daylight and views, the presence of a well-insulated sill will provide comfortable conditions with a lesser-performing glass. Note that beyond 80% glazing, perimeter radiant heating is needed since glazing assemblies with U-values lower than 0.2 aren’t commercially available in triple-glazed assemblies.

There are some easy guidelines generated by this study in setting thresholds for when perimeter radiant heating would be needed to maintain a PMV higher than 0.5.

  • At lower glazing percentages (punched windows), thermal comfort is dominated by the potential of downdraft, which is a function of the window height and U-value. As the window becomes taller and narrower, the view factor for that window decreases, but the impact due to downdraft increases.
  • When considering 40 to 60% glazing, an easy design choice is to create a sill at 2-ft-6-in or 3-ft above finished floor. For a design temperature of 15 F, the threshold U-value for a fully glazed wall with 60% glazing is 0.22; whereas for a ribbon window design with 63.3% glazing, the critical U-value is only 0.32, a more affordable option.
  • Beyond 80% glazing in 15 F design conditions, perimeter radiant heating will be required without resorting to glass with performance characteristics beyond triple glazing such as quadruple glazing.

click to enlarge
Figure 3: U-value vs. view factor @ -0.5PMV.

While the above conclusions focused on the glazing percentages and performance, the performance of the solid wall in which the glazing is placed needs to be addressed. It’s easy to think that an increased R-value of the solid wall would mitigate some of the performance criteria required for the glazing; however even in a condition with 22% glazing and 15 F exterior design temperature, increasing the wall performance from R 18 to R 30 has a minimal effect on the overall façade U-value (0.47 to 0.43). This indicates that when evaluating thermal comfort and the effect on radiant temperature, the properties of the glazing will always govern over adding additional insulation within the solid portion of the wall.

Beyond the technical elements of the exterior wall configurations and performance criteria, there needs to be discussion and consensus around the thermal comfort design parameters for a given project. For instance, in evaluating the exterior design temperature, ASHRAE establishes exterior design temperatures for locations. While for Boston, the heating design temperature is 13 F (99% annual cumulative frequency of occurrence), it’s unlikely that the low temperature will happen during working hours. In this case, a more representative temperature to evaluate the glazing design during the daytime would be closer to the range of 20 or 25 F. In a college research environment, where hours can be more inconsistent, it’s important to design to the stricter exterior design temperature criteria.

There will never be a “one-size-fits-all” approach to thermal comfort, since it involves a personal and psychological evaluation of a given situation, and the exact make-up in establishing a comfortable environment will never be universally established due to so many variables. However, by following these simple guidelines, architects can make informed decisions, early in the design process, to design spaces that don’t require perimeter radiant heating.

Lynn Petermann, AIA, LEED AP, joined Payette in 2013 and brings five years of experience as a thoughtful practitioner in all aspects of building design from initial programming studies through construction. Andrea Love, AIA, LEED AP, is Payette’s building scientist, leading the firm’s research projects and integrating performance analysis into the design process. Alejandra Menchaca, LEED AP, PhD, joined Payette in 2013, bringing in-depth experience in thermal comfort, daylighting and natural ventilation modeling. She serves as a principle facilitator for LEED compliance.