More on Daylighting with Windows

More on Daylighting with Windows

I read with great interest your excellent article on daylighting in buildings (

EBN

Vol. 8, No. 9, September 1999). I would offer two clarifications. First, you state that south orientation is the best for daylighting, and north second best. It is true that the south offers the most daylighting potential; however, control of direct beam, glare, and distribution of south light is problematic, requiring a significant investment in light shelves and shading devices. North daylight has a very high luminous efficacy and is already diffuse, thus eliminating the need for expensive daylight control elements.

Second, it is always good to point out that the amount of glazing area needed for daylighting is quite small relative to what is typically found in commercial buildings for “view” glazing. If windows are properly sized for daylighting, they will have a beneficial effect on the overall energy use and load profile of the building. If glazing is oversized in the name of daylighting, a sin seen fairly frequently, the energy benefits will not be achieved.

You are quite right to emphasize the design complexity of daylighting. The impact of all possible daylighting design parameters on the heating, cooling, and lighting loads is daunting. Parameters include glass area, window geometry, orientation, optical properties, thermal properties, and shading strategies. Computer simulation helps in the optimization of daylighting and energy use, but issues such as glare, daylight distribution, control, and light quality are not well modeled in the current generation of whole-building simulation programs. Few projects will have the budget required to do specialized ray-trace modeling, or physical daylight models, so a lot is still left to the “art” of the architect or daylighting specialist.

Nevertheless, a whole-building energy design approach that includes daylighting as a fundamental part of the strategy can yield significant energy and greenhouse emission savings. In two of our buildings at the National Renewable Energy Laboratory we have measured energy cost savings compared to equivalent code-compliant buildings of about 45% and 65% respectively. These buildings were designed and constructed within a strict fixed-cost budget, so they did not cost extra. They did require a willingness on the part of the design team to tradeoff non-functional aesthetic cost items for functional aesthetic features that defined both the daylighting systems and the aesthetic impact of the buildings.

Ron Judkoff, Director

Buildings & Thermal Systems Center

National Renewable Energy Lab

Golden, Colorado