Each successive energy code update increases the stringency of requirements. With higher expectations for energy performance, the emphasis is shifting from mechanical equipment and lighting to the building enclosure. The energy performance of the building envelope is dependent on more than just insulation thickness, with thermal bridging and air infiltration also being critical.
One building envelope enhancement prescribed by energy codes in many climate zones is continuous insulation, which significantly reduces thermal bridging and increases the thermal resistance of wall assemblies. This has challenged the industry to adopt different design and construction strategies to comply with code. The added wall depth and typical location of the continuous insulation results in designers pushing window assemblies out in the plane of the wall to bridge the gap between the cladding or veneer and the backup wall assembly. This detail solves one problem but creates another with air barrier continuity, requiring architects and contractors to mind the gap.
To meet energy code requirements for a continuous air barrier, a project team needs to consider the continuity of more than just the material or assembly designated as the air barrier. The continuous air barrier needs to transition from the foundation to the wall, from the wall to the roof, and most importantly, from the wall across openings for the entire building enclosure. This requires the primary seal of a window assembly to be sealed directly to the air barrier to maintain continuity across openings. The air barrier is frequently installed on the face of sheathing on the interior side of the continuous insulation while the primary sealant joint for the window is often located at the exterior face of the frame, or at the back of the glazing pocket of curtainwalls. This means that the two things that need to connect to maintain air barrier continuity are not in the same plane, with the window frame proud of the air. Something is required to bridge that gap.
To close this gap between the air barrier and window assembly primary seal, design teams often turn to metal, like using a light-gauge flashing in masonry veneer or a transition piece as part of a metal cladding assembly. The detailing of these conditions often breaks the continuity of the air barrier, leading to air or water infiltration issues at the window perimeter, which can be costly to correct after installation. There are two challenges that need to be met to close the gap: making the metal flashing an extension of the air barrier and detailing the upper and lower corners where horizontal and vertical flashings meet to avoid leaks.
Figure 1
Firstly, the metal flashing that bridges the gap at openings must be an extension of the air barrier so the primary seal of the window can be sealed to the air barrier. If metal flashing is placed between the air barrier and the window primary seal it effectively interrupts the continuity. The metal flashing must be incorporated into the air barrier to maintain the continuity. This can be accomplished by sealing the flashing to the air barrier by setting the flashing in a bed of sealant, or by transitioning the air barrier onto the flashing using materials such as self-adhered membrane flashing (Fig. 1).
Secondly, the metal flashing at the upper and lower corners of openings where the metal transitions from horizontal to vertical (and at splices) must be detailed to prevent air and water from bypassing the window primary seal. Failures here cause issues with energy performance and with moisture damage to interior finishes. When metal is used to flash around a window opening, the corner transitions from horizontal to vertical flashing are often not sealed. Corner and splice joints, if left unsealed, create a void allowing air and water to bypass the primary seal. This can be avoided by detailing metal flashing similar to air barrier flashing requirements by incorporating sealed connections and positive drainage (Fig. 2).
Figure 2
Because the solutions to this critical interface involve materials in multiple specification sections and oftentimes execution by different trades, it is important to clearly communicate the requirements for transitioning and sealing in the contract documents, and to verify performance through testing. In detail drawings, this means showing the intent for how flashing is to be incorporated into the air barrier, ideally using isometric details to show how materials are coming together in three dimensions. In specifications, it is critical to include installation requirements that define how flashing is to be incorporated into air barrier and how transitions and seams are to be sealed. Including this condition in a mock-up and conducting field testing of the mock-up and the initial installations on the building is critical to verify the performance. Applicable test methods include dynamic water testing (AAMA 501.1), water penetration testing with interior negative pressure (ASTM E1105), and whole-building air leakage testing (ASTM E779).
As the saying goes, the devil is in the details. High-performance building enclosures, whether complying with the energy codes or supporting zero net energy and passive house standards, require careful attention to the transitions between systems and components. Clearly communicating design intent in details and specifications and verifying performance with testing are part of the quality assurance process to mind the gap.
Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions in Portland, Ore. Brian Davie, AIA, LEED AP, is an architect with 26 years of experience and a senior project manager with Intertek Building Science Solutions in Denver. To learn more, follow Scott on Twitter @alanscott_faia.