Sustainable Technology: Exteriors

Durable, Sustainable Exteriors

There seem to be as many wall types as there are definitions of "sustainable." Clearly, to be sustainable, the building wall must be durable - able to stand the test of time. Sustainable wall systems resist the effects of gravity, wind, and seismic and climatic forces (rain, air pressure, and thermal fluctuations). However, the massive, yet highly durable, stone walls found in the finest examples of historic architecture are no longer economically viable or aesthetically desirable for modern, sustainable walls.

Many recent building enclosure technologies build on basic building physics to deliver thermal comfort, ventilation, and protection from undesirable natural elements. Among them:

Cutting-edge Glass
Constant improvements in glass and coatings have resulted in spectrally selective glazing that significantly reduces the amount of solar heat gain and loss. Based on the climate, tinted glass and low-e coatings can maximize the amount of visible light while limiting the amount of long-wave radiation. Studies have shown natural light to improve occupant productivity, and limiting long-wave radiation reduces cooling loads in the building. In winter, spectrally selective glazing can also help reduce heat loss, saving on energy to heat interior air.

Light Shelves and Solar Shading Devices
These old technologies are making a comeback, in part due to the ability of fabricators to attach cantilevered components without compromising the structural integrity of the curtain wall. Light shelves and solar shading devices alternately work to bounce natural light indirectly into the building and minimize excess direct light entering the windows. As light shelves and solar shading device attachments can sometimes form a thermal bridge, insulation strategies need to be employed to significantly reduce or eliminate the bridge. One way to reduce the thermal bridge is to keep the thermal barrier continuous through the attachment connection. The solution needs to be carefully evaluated by structural and thermal analysis - a thermal analysis will show if any surface temperature is cold enough to condense at interior temperature/humidity conditions.

Building enclosure commissioning is equivalent to "test driving" the exterior enclosure. Once the performance levels are established, and documents and construction independently reviewed by the building enclosure commissioning agent, tests are conducted to "try out" the building enclosure before occupancy. By applying calibrated volumes of water with corresponding depressurization in the building, the effect of severe storms can be simulated to determine if the construction meets performance criteria. When combined with HVAC commissioning, the whole building can be fine-tuned to maximize energy savings potential and enclosure durability.

Two basic warnings: Beware of new products or systems that have not been through rigorous testing (your sustainable building should not be an experiment for short service-life products), and do not rely solely on sealants as they are hole fillers, not waterproofing. All exterior finishes should have backup drainage planes and flashings for the greatest durability.

High-performance walls, when properly designed, constructed, and maintained, lead to more efficient control of the interior environment and provide the promise of truly sustainable building walls.

Chris Decareau ( is an associate principal at Boston-based Simpson Gumpertz & Heger Inc., a structural and building enclosure engineering firm.