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
LaboratoryDesign|JUL|AUG 2014 15
continued on page 16
Figure 2: Glazing percentages/view factor.
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