Thermal Comfort and Glazing Design
The February 2016 issue of Journal of the National Institute of Building Sciences includes an article by Alejandra Menchaca and Lynn Petermann on their research into making smart glazing decisions that optimize occupant thermal comfort. We’ve shared the article’s introduction here and you can read the full piece in the February issue in the print edition of the Journal.
The all-glass building is now a standard sight within cities. Façades with floor-to-ceiling glass facilitate unobstructed views to the exterior and lower lighting energy demands through the use of daylighting; however, even highly insulated windows are about 5-8 times less resistant to heat transfer than solid wall construction. Consequently, in the winter time the interior surface of windows is often too cold to provide comfortable conditions, which requires the installation of perimeter heating involving an upfront investment, as well as increased operations and maintenance costs.
How can a design team make an informed decision regarding the thermal performance of glass and the need for supplemental perimeter heating? Two aspects of thermal comfort must be considered: (1) radiant discomfort, due to the exposure of an occupant to a cold surface, and (2) discomfort due to downdraft, caused by cold convective currents cascading down the glass surface and spreading along the floor or any surfaces adjacent to the glass. Quantifying the potential level of discomfort expected in a space can typically only be done through computational fluid dynamic studies, which require both time and money, and are used as a verification tool, late in the process, rather than to inform the façade design early on.
Based on existing research regarding radiant discomfort and risk of downdraft, we developed a methodology to generate simple charts that correlate occupant thermal comfort to the thermal performance of the window assessed by its U-value as well as its proportions. These charts allow the designer and engineer to understand which variables of glazing design – thermal performance, dimensions, climate, tolerance to radiant and downdraft discomfort – have the most impact on the potential need for perimeter heating in a space. These tools become particularly useful to study the potential impact on thermal comfort of glazing units that have a low-emissivity coating on the room-side surface.