Thermal comfort: why windows matter

According to the annual surveys conducted by the International Facilities Management Association (IFMA), the top two complaints of occupants in office buildings are always “it’s too hot” or “it’s too cold” [1]. Thermal discomfort can also have a significant impact on productivity – which anyone who has tried to type with cold hands, or think clearly when they are too hot, knows all too well. Quantitatively, studies show that there is an approximate 1% drop in productivity for every 1ºF deviation from the optimum temperature of 71-72ºF (either higher or lower) [2]. Interestingly, when occupants are thermally uncomfortable, their dissatisfaction with other aspects of indoor environmental quality also increases [3]. Thermal comfort therefore seems to be one of the most important indoor environmental quality factors to get right.

Why should we care? Because poor performing fenestration can be a significant source of thermal discomfort in buildings, which can provide additional justification for those who are trying to reduce window area. If we can encourage the use of higher performing fenestration that delivers both higher energy performance and thermal comfort, designers can use more glazed area without taking a thermal comfort penalty.

Also, while we know that it is hard to achieve a short-term monetary payback on energy savings for window retrofits, achieving a payback through improving occupant productivity by enhancing thermal comfort is more feasible. Since the cost of human capital in commercial buildings is nearly 100 times more than the cost of energy, a 1% improvement in productivity can be much more significant than, say, a 5% decrease in energy performance. In addition, by making the perimeter space adjacent to the façade more comfortable to occupy, useable space can be increased. A significant monetary value can be placed on this additional usable and rentable square footage.

Using high-performance windows can make a dramatic difference to thermal comfort. An online calculator has been developed by Payette to assess the thermal comfort, including the downdraft phenomenon, specifically next to glazing [4]. The graph above shows the thermal discomfort of occupants due to downdrafts as a function of distance from a façade with four different window thermal performances when the exterior temperature is 0ºF.  The model assumes the occupants are centered on windows that are 11 feet wide, have a sill height of 2.5 feet and are 7 feet tall. Using the limit of “20% dissatisfied” as the standard for comfort, occupants would have to move over 7 feet away from the poorest performing window to be comfortable, yet for the highest performing window, occupants could be comfortable at only 2 feet away. That’s 5 feet more of usable space around the perimeter of the building. That’s huge!

Owners and occupants desire large expanses of windows for views and daylight. By promoting high-performance windows, these positive benefits can be delivered without compromising thermal comfort and productivity, while increasing space utilization.