It’s a well-known fact that labs consume four times more energy per square foot than a typical office building. And while ventilation and plug loads account for much of this energy use, proper design and detailing of building envelopes can have a significant impact on the energy demands of lab buildings.
Even relatively modest changes to typical façade detailing can significantly improve lab building structures so they perform as anticipated.
Thermal bridging is a fundamental of heat transfer where a penetration of the insulation layer by a highly conductive material takes place in the separation between the interior and exterior environments of a building assembly (building/thermal envelope). It’s created when materials that are poor thermal insulators come into contact, allowing heat to flow through the path of least thermal resistance created.
And, since the effects of thermal bridging can reduce the performance of a façade by as much as 70%, according to Miep Keller, AIA, LEED AP, Architect, Payette, it’s especially effective to reduce thermal bridging when designing a lab building. It’s also important to have a consistent conditioned space for labs that are sensitive to temperature and humidity changes, as well as occupant thermal comfort.
How thermal bridging is calculated
The understanding of thermal bridging of a lab building is the same as any other building type. Typically, system R-Values are determined by hand calculation. “The published thermal conductance of each material is added together to get an overall R-Value,” says Jeffrey Abramson, AIA, LEED AP, Associate, Payette.
However, the problem with this approach is it doesn’t account for thermal bridging. Programs such as THERM and WINDOW can be used to develop 2-D heat flow simulations looking at specific cases of thermal bridging. “With this additional level an analysis, a weighted system R-Value can be calculated, taking into account thermal bridging,” says Abramson.
For most lab buildings, Payette uses Lawrence Berkeley National Laboratory’s THERM program. THERM is a Microsoft Windows-based computer program for use by those interested in heat transfer. Using THERM, firms can model 2-D heat-transfer effects in building components, such as windows, walls, foundations, roofs and doors; appliances; and other products where thermal bridges are of concern. THERM’s heat-transfer analysis allows users to evaluate a product’s energy efficiency and local temperature patterns, which may relate directly to problems with condensation, moisture damage and structural integrity.
Researcher comfort, indoor air quality
Thermal bridging is of utmost importance for both researcher comfort and indoor air quality conditions. Most typically in labs, firms see 70 to 75 F with a 30 to 60% relative humidity as the most common targeted comfort zone, according to Andrea Love, AIA, LEED AP, Director of Building Science, Payette. More and more, firms are also observing lab buildings choosing not to humidify in the winter, “so the lower level of the humidity range drops with that,” says Love.
Thermal bridging in building construction occurs when thermally conductive materials penetrate through the insulation creating areas of significantly reduced resistance to heat transfer. “When there’s a large delta between the conditions outside versus inside, thermal bridging will ‘short circuit’ the exterior envelope,” says Keller. “During the winter in the northeast, there will be cold spots along the exterior walls where thermal bridging is occurring, for example; this not only increases energy usage, but also can cause thermal comfort and condensation concerns.” Any water accumulation within stud cavities has the potential to spawn mold growth affecting indoor air quality.
Thermal bridging and energy use
When thermal bridging reduces the building’s exterior enclosure performance, mechanical systems compensate in order to achieve the desired conditions. This causes the energy consumption of the building to increase significantly.
According to a study, the U.S. Dept. of Energy has shown that a typical large office building in Chicago can see a 7% increase in energy use resulting from a 50% reduction in envelope system R-Value.
However, the use of THERM can help in the sustainability of thermal bridging and the planning of a building’s façade.
According to Keller, the use of THERM and careful detailing of a façade will result in a reduction of thermal bridging. The reduction of thermal bridging will allow the building to perform as anticipated. “This means the mechanical systems aren’t compensating for the lack of a consistent thermal envelope and will reduce the energy consumption of the building as a result,” says Keller.
Payette recently used THERM to improve the sustainability of the 75-125 Binney Street Lab building through thermal bridging. And, according to Christian Blomquist, AIA, Associate, Payette, and lead designer on the building, there were two primary ways THERM was used to inform the exterior detailing of the building.
The exterior skin of the building is a rainscreen made up primarily of ultra-high-performance concrete panels by TAKTL. “This is a new product to our firm and we used THERM to identify ways to effectively limit thermal bridging between the aluminum support framing and the light gage metal framing inside the building,” says Blomquist.
Adjustments were made to the size and spacing of supports as a result of the THERM analysis and thermal isolators were introduced between the aluminum framing and the face of the exterior sheathing.
THERM was also used, according to Blomquist, for the development of details at balconies and areas of the façade where the exterior rainscreen peels away from the building and becomes free floating—a series of details incorporating 1-in thermal isolation pads between the primary building structure and the tube steel support frame at the balconies and the free-floating panels. THERM allowed the team to investigate a variety of materials and details to identify the best strategy for each condition.