Atriums and Energy: Designing for Performance
Atriums are basic building elements that can have a significant effect on energy consumption. Because atriums are so fundamental to a building's geometry, their effects should be studied early, and as thoroughly as possible. While energy modeling is usually undertaken only at the end of construction documentation, it's usually impossible to go back and make large “broad-stroke” scale revisions at that late point in the project development. For this reason — and others as well — it is much more effective, and beneficial to the project quality, to quantify the passive effects of an atrium early in the design process.
Roles That Atriums Play
Energy impacts aside, a sky-lit central court in a multistory campus building delivers a number of benefits. To begin with, a space that is open to the sky gives occupants access to natural light, and also often provides outside views into the central space. This can make the difference between a space that is merely functional and one that is also beautiful and special. We know that an atrium can reduce the need for artificial lighting. But it also can function as a building's social center, providing circulation and casual meeting space. Further, it can become a destination, or act formally as an event space. The large volume of such a space is visually impressive, and can be effective in reinforcing the occupants’ understanding of the building organization and assist with wayfinding. However, atriums might also be planned for reasons that are purely formal or aesthetic, and their effects on building performance left unconsidered. In the long run, such an approach can be a mistake. This article will look at one way to avoid this and provide a project example that illustrates some advantageous approaches to atrium design, as well as ways to understand its calculable benefits and condition designers in thinking more broadly about how best to incorporate an atrium into the plan and program for a building.
Quantifying the Effects of Design Options
The good news is that designers now have tools available to help discover early in the design process which basic building geometries will work best. Also, working together early in an integrated design process, engineers and architects can start quantifying the effects of various options at a point when those decisions can have a more significant effect on energy performance.
Until very recently, designers relied on an intuitive sense of what would work and what would not. For example, in most temperate climates it is generally recognized that orienting a building as a bar or rectangle along an east-west axis will minimize early morning and late afternoon sun exposure (avoiding glare and excessive heat gain). In addition, most designers recognize that placing windows strategically along a south-facing facade and providing shading over the windows will admit direct sunlight only during the heating season, and admit only reflected ambient light the rest of the time. In climates where cooling loads dominate, north-facing windows are preferred. In climates where more energy is used in heating, it makes sense to admit more south light, more often. These strategies are usually understood and referenced by designers, although they are often considered guidelines, and don't always get put into rigorous practice.
New Tools Enable Up-Front Analysis of Design Decisions
In the last few years, tools such as Autodesk's Ecotect, and a plug-in from the Glasgow-based company Integrated Environmental Solutions, (IES) that runs with Google Sketchup, are beginning to make it possible for design teams to adjust a building's form and orientation more precisely to local conditions. As a result, in the early planning stages, designers can perform very basic comparative energy modeling to begin to understand the effects an atrium can have on the building envelope and the subsequent effects on heating, ventilation, and cooling.
Using simple computer models, designers can compare glazing orientations and building proportions, plugging in regional climate data such as seasonal average temperatures, wind speed, wind direction, and sun angles. This process can provide quantifiable data comparing various configurations of the programmed building volume to study the effects of changes in building proportion (ratios of height, width, and depth) and, if an atrium is included in the design, what the optimal position and exposure for that atrium should be. In the past, only engineers could perform this type of modeling, and even now, the more complex models should still be left to the experts. However, architects can now perform basic modeling in-house, and gain insight into some of the basic decisions early on, before an expensive engineers' model is commissioned. This also enables designers to adjust the building form in response to the local environment. Climate conditions in the Southeastern U.S. will require a different approach to fundamental geometry than those in the desert Southwest, New England, or the Pacific Northwest.
Oregon State University
If there is one easy rule, it's that an atrium is much more likely to have a net positive effect on energy usage if it is passively conditioned. For example, at Oregon State University's Kelley Engineering Center in Corvallis, engineers were able to show that the addition of an atrium to the building design would have a net positive effect on building energy consumption. This is because, while the atrium area accounts for additional program square footage, the only "conditioning" it receives is through exhausted conditioned air from the classrooms and offices. Otherwise the Kelley Engineering Center atrium is passively ventilated, and takes advantage of natural light, eliminating the need for artificial lighting during daylight hours.
One benefit of this strategy is that the atrium provides an air volume maintained at a temperature somewhere between the exterior and the interior conditioned space. Since the unconditioned space is passively tempered, and held against part of the conditioned envelope, loads on the HVAC system are lower than if those spaces were left exposed to the exterior. This allows a higher percentage of glass to be included in spaces facing the atrium, as well. Since the tempered air would otherwise have been exhausted to the exterior, this strategy uses no additional energy, and ends up providing a net savings.
In addition to this, materials with a high thermal mass were chosen for the interior of the atrium to help buffer temperature swings from day to night. A variety of other mechanical strategies were also employed that have been shown to lower energy usage in the Kelley Engineering Center to 40 percent of the state energy requirements. Again, the key rule here is that these are generally passive strategies.
Atriums Affect Other Building Systems
The success or failure of any design element can depend on how every member of a design team views it. If designers and engineers are strictly concerned with meeting the minimum program and code requirements of their own pieces of the work, then it is likely that they are not seeing the potential of interrelated systems. By focusing on function, it is possible to move beyond the basics and get each system to contribute in more than one way. An atrium is just such a system, and one that can affect numerous other systems positively, if the stakeholders are open to the possibilities and the design team recognizes the potential.
Mick Richmond, AIA, NCARB, LEED-AP BD+C, is a project architect with Yost Grube Hall Architecture (www.ygh.com) in Portland, OR. Since 2001, he has pursued a focus on sustainable design and building technologies, and is a coordinating member of the Portland AIA Committee on the Environment.