Balancing Glazing, Building Envelope, and Thermal Comfort
In the developed world, mechanical systems such as perimeter heating, compensate for shortcomings in envelope performance to provide a thermally comfortable environment. However, with an increased interest in maximizing energy efficiency and façade transparency, as well as providing healthy spaces for occupants, this model is due for reconsideration.
When it comes to glazing in the winter, perimeter heat is the most common solution provided to avoid potential occupant thermal discomfort. It is often used because it is thought to be cheaper than upgrading the thermal performance of the glazing. However, in the northeast U.S., this system has a first cost ranging between $250–$400/linear foot, which in some instances may outweigh the cost of an upgraded envelope (between 5% and 20% price increase per square foot to upgrade from double-pane glass to triple-pane).
Currently, it is challenging for architects to quantify early in the design process how glazing performance and geometry affect the need for supplemental perimeter heating. What if the design team could understand, as early as schematics, which façade properties negatively or positively impact occupant comfort? What if there was a way to avoid the use of perimeter heat by selecting the right glazing geometry and performance?
To achieve this goal, a team of building scientists and designers at Payette, a Boston-based architectural firm, developed the Glazing and Winter Comfort Tool. It is a free web tool based on existing scientific research that aims to improve the design community’s understanding of thermal discomfort triggers in the winter.
Glazing affects thermal comfort in two ways: occupants can feel cold due to radiant losses to the glass or due to cold downdraft. When occupants sit close to a cold window, they may experience radiant discomfort. Radiant thermal discomfort is influenced by window height and width, the location of the occupant from the window and the temperature of the inner windowpane. It is the U-value of the window and the temperature of the outside air that determine how cold a glass pane gets.
Downdraft occurs when warm interior air hits the cold interior glass surface and falls due to negative buoyancy, creating cold convective currents. This downdraft can cause occupants’ hands or feet to feel cold, particularly when bare. Downdraft discomfort is primarily influenced by the height of the window and the temperature of the inner windowpane.
For a given glazing geometry, performance, interior, and exterior conditions, thermal comfort is quantified through the use of predicted percentage dissatisfied (PPD). PPD represents the percentage of occupants that may feel thermally dissatisfied under a given set of conditions. ASHRAE Standard 55 considers that an occupant will be thermally comfortable when the PPD in the space is 10% or lower, while LEED allows PPD values of up to 20% in a space.
The Glazing and Winter Comfort Tool was designed to evaluate the impact of glazing geometry and thermal performance on occupant comfort in the winter. Inputs include window and room dimensions, window performance properties, indoor and outdoor conditions and occupancy characteristics. The impact of these variables on thermal comfort is represented through a chart that quantifies PPD against occupant distance to the façade. Markers along the resulting curve indicate whether thermal discomfort is dominated by downdraft or low radiant temperatures. The comparison of up to three glazing scenarios allows the design team to quickly understand and convey the differences between each option.
In a world where most of our time is spent indoors and glazing is increasingly becoming a prominent design feature, it is critical to view occupant thermal comfort with the same significance as transparency, daylight and energy use. The Glazing and Winter Comfort Tool empowers the architect and engineer to make smart decisions early in the design process, and it enables the owner to know what to expect once their building is built and fully occupied.
About the Authors
Lynn Petermann, AIA, is an associate and Alejandra Menchaca, Ph.D., LEED AP, Member ASHRAE, is an associate/building scientist at Payette in Boston.