Beyond Windows Walls Need Some Attention, Too

By Helen Senders and William Green

Windows are often considered the weakest points in the thermal building envelope. Several studies show that we consistently overestimate walls’ performance by a wide margin. This is because thermal bridging is largely ignored, especially when assessing the assembly R-value of rainscreen wall systems.

Thermal Bridges

Thermal bridges are localized areas in the building envelope where high conductance materials, such as aluminum, penetrate the continuous insulation. These penetrations typically occur at interfaces between envelope components or by fastening mechanisms.

These create “bridges” that provide a thermal short in the building envelope, allowing heat to freely flow from inside to outside and vice-versa, degrading thermal performance. Cold areas also are created inside walls, which may lead to condensation and, consequently, the growth of potentially harmful mold and microorganisms.

Rainscreen Wall Systems

Rainscreen systems help increase wall systems’ thermal performance and longevity. A rainscreen system is a type of double wall consisting of a main structural wall, which usually has outboard insulation with an exterior air and water barrier applied. A second (outer) skin of wall cladding—the rainscreen itself— is then mounted to that structural (inner) wall, creating a ventilated cavity between the insulated structural inner wall and the outer skin.

The outer skin acts as the primary water barrier, limiting the interior skin’s exposure. The cavity is critical as it provides the drainage and airflow necessary to support evaporation of any water penetrating the outer skin, keeping the interior skin dry. The cavity also helps improve thermal performance. For example, it produces a stack effect in the cavity
in summer, moving hot air up to vent. Outer skin materials can include metal, stone, timber, composites, terra cotta, stucco and more.

Degradation of Performance

In principle, rainscreen systems should have very high thermal performance, but thermal bridging caused by the attachment mechanisms can degrade thermal performance by more than 50%, sometimes as much as 80%. The Building Envelope Thermal Bridging Guide (BETB), prepared by Morrison Hershfield, is an excellent resource for understanding the relative performance of more than 300 wall and glazing assemblies.

Solutions

Continuous, highly-conductive metal Z-girts most commonly are used to attach cladding, creating large thermal bridges around each panel’s perimeter. Several solutions are available that replace continuous metal girts with point attachment mechanisms, also known as clip-and-rail systems. The clips penetrate the inner wall and are often thermally-broken, further reducing thermal bridging. These thermal clip systems generally have similar thermal performance, differing installation speed and complexity.

Value engineering often targets these high-performance attachment systems, risking their replacement with poor-performing Z-girts. Thermal modeling is highly recommended to demonstrate the impact of a substitution on U-factor and condensation risk.

Helen Sanders is the general manager and William Green is a market team member at Technoform North America Inc. based in Twinsburg, Ohio. Read Helen’s blog each month at usglassmag.com/insights

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