One of our many in-house research efforts is a large, collaborative project. We investigated into the Thermal Performance of Façades through the AIA Upjohn Research Initiative.
Thermal bridging in building construction occurs when thermally conductive materials penetrate through the insulation creating areas of significantly reduced resistance to heat transfer. These thermal bridges are most often caused by structural elements that are used to transfer loads from the building envelope back to the building superstructure. Though design professionals generally understand that thermal bridging is a concern, few can quantify the extent of its impact on building performance.
Small changes in designs can still lead to dramatic improvements in performance. With careful detailing and attention to the issues of thermal bridging, the design and construction industry can improve the performance of our building envelopes.
Today we’re sharing our findings regarding masonry veneer walls.
Masonry veneer wall systems are common for many building types in North America. Because they are rarely load-bearing, they are dependent on shelf angles and a grid of tie-backs to structurally stabilize the assembly. Unfortunately, these supports and attachments form substantial thermal bridges and can dramatically decrease the overall thermal performance of the facades. In our observations with the thermal camera, we found masonry veneers generally performed at a 25-60% decrease in R-value when compared to theoretical calculations.
While masonry veneers can be supported without shelf angles by bearing on the foundation for limited heights, continuous shelf angles are typically required to support heights over two stories and supporting every story is common in order to minimize deflection joints. These shelf angles typically run from close to the face of the masonry back through to the superstructure, passing through the insulation layer. Taken alone, these steel shelf angles account for an approximate 35% decrease in the R-value. That figure would be far worse if the steel was protected with highly conductive coated copper flashing as it may have been several years ago. Today, we might consider using a membrane flashing and making the entire angle out of stainless steel, which has one-third the conductivity of carbon steel. This sort of change could reduce the performance impact of the shelf angle from 35% down to 29%.
In order to truly minimize the thermal impact from the shelf angle, however, we investigated an option, advocated by Building Science Corporation (Lstiburek 2008), of supporting the shelf angle with evenly spaced blades or brackets that allow the shelf angle to remain entirely outboard of the insulation, thereby creating intermittent rather than continuous thermal bridges. Providing a thermal break between the brackets and the shelf angle, and then conservatively assuming these brackets are spaced at 48” inches on center, results in a substantial improvement in performance. In this system, we saw only a 12% decrease in the R-Value from the support structure. This could be reduced down to 3% if the blades were made of stainless steel.
Traditional Shelf Angle, Left; Improved Shelf Angle, Right
In addition to the shelf angle, metal ties are typically required in masonry veneers to provide lateral support. Surprisingly, though these installations are discontinuous, they occur so frequently that they can have a significant impact on assembly R-values. With typical spacing somewhere between 16 and 24 inches on center, horizontally and vertically, ties can contribute up to a 15% decrease in the thermal performance. Because spacing, material conductance and type of tie all impact the R-value for masonry walls, we looked at a matrix of three types of ties: a screw-on tie, a barrel tie and a thermally-broken tie. We looked at these options at both 16- and 24-inch spacing in steel and stainless steel. The choice of steel or stainless steel proved to have the biggest impact on performance, with the R-values at an average of 6% improvement, whereas the larger spacing of the ties and the choice of tie type both showed an average of a 4%. Stainless steel ties spaced 24 inches on center, which have minimal diameter of material penetrating the insulation, were shown to have a negligible impact on the thermal performance, decreasing the R-value by only 2%. Combined with the shelf angle held off by the blades, the thermal performance of masonry veneer façades can be improved substantially from the traditional approach.
Thermal Performance of Facades: Final Report
Thermal Bridging Research: Curtain Walls
Thermal Bridging Research: Investigating Insultation Thickness for Renovations