Achieving Desired R-Values

In addition to making sure that all membranes are properly sealed and that the drainage system is adequate, avoiding thermal bridging is crucial. The biggest problems involve elements that support the cladding, which run the greatest risk of becoming thermal bridges that can significantly compromise R-value.

The traditional approach has been the use of Z-furring, with insulation placed between the Zs. This design renders the insulation non-continuous, which violates current code requirements and re- duces R-values by as much as 50%, according to Jonathan Baron, AIA, LEED AP, Director, Shepley Bulfinch.

The Façade Group’s Altenhofen agrees: “The use of continuous Z girts aligned with the metal studs of the back-up wall to support cladding is a common solution, but a very poor one.”

Fortunately, newer products are available to reduce bridging. For example, bracket systems that reduce support to isolated points can significantly boost thermal performance by only compromising R-values by a small percentage. Interruptions in the insulation at the wall frame members can also reduce insulation levels; installing continuous insulation on the wall frame exterior is a good strategy.

When dealing with brick veneer, the brick ties only cause relatively small thermal breaks. However, the shelf angle and lintels will create large thermal breaks. “Continuously supported masonry shelf angles should be avoided by using cantilevered shelf angles with continuous insulation behind between the support attachments,” says Blackburn.

For barrier walls, all joinery internal to the system, as well as the building expansion and deflection joints, must be properly detailed, according to Shepley Bulfinch’s Finneral. “Develop details that minimize thermal bridging at veneer support structure, and carefully select and layer underlayment materials to avoid creating multiple moisture/ vapor barriers which could trap water within the wall,” he explains.

When planning for a mass wall, the Building Team should consider the density and absorption rate of exterior materials, the amount of insulation on the interior, and the resulting dew point location within the wall. In addition, the designer should have a good understanding of how the wall will dry out when it gets wet. SpecGuy’s Kabza points out that wall assemblies are complex systems pieced together by perhaps a dozen or more trades, often using products from several dozen manufacturers. Thus there is plenty of potential for all aspects of performance, including thermal performance, to be compromised.

B.R. Fries’ Brody emphasizes that team collaboration can help avoid thermal-bridging problems. “Construction managers have made great strides to coordinate their work and improve upon the thermal anomalies.” In addition, he notes, air barrier installers and inspectors make great efforts to ensure compatibility of their products, and manufacturers have developed thermally-improved fasteners for rainscreen cladding that reduce thermal bridging and moisture penetration.