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 Peterman, 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.

Glazing design and thermal
comfort: It’s more than
being cool
continued from page 15

• 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.

While the above conclusions focused on the
glazing percentages and performance, the per-
formance 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 exte-
rior 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 insula-
tion within the solid portion of the wall.
Beyond the technical elements of the exterior
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 con-
figuration, 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-insu-
lated sill will provide comfortable conditions
with a lesser-performing glass. Note that beyond
80% glazing, perimeter radiant heating is need-
ed since glazing assemblies with U-values lower
than 0.2 aren’t commercially available in tri-
ple-glazed assemblies.

CONCLUSION

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.

Figure 3: U-value vs. view factor -0.5PMV.

COMINGEVENTS

; SEP TEMBER 22-24. I2SL Annual Conference. Orlando, Fla. Sponsored by the International Institute for Sustainable Laboratories (I2SL). www.i2sl.org

; NOVEMBER 5-7. SEFA U.S. Annual Trade Show and Meeting. Miami, Fla. Sponsored by the Scientific Equipment and Furniture Association (SEFA). www.sefalabs.com