Expectations with performance
With these four preconditions under the specifier’s belt, attention turns to the impact of CI on building performance, as well as its ramifications for cladding system selection. It turns out continuous insulation is a design mindset as much as a widely prescribed wall feature.
Among the primary benefits of continuous exterior insulation is it maintains the enclosure and framing elements at temperatures closer to those of the building interior. With additional R-value at the exterior, the dewpoint moves toward the outside and, in some cases, exterior to the insulation in the framing cavity, says Slone. This effect can also eliminate or reduce condensation in the enclosure—a pernicious source of moisture that can prematurely degrade structural materials.
Beyond helping with this situation, CI protects against the thermal bridges where structural components, substructure, anchors, and other penetrations reach through to the exterior. Uninsulated steel-stud framing in contact with exterior sheathing, for example, is an efficient conduit for heat regardless of how much insulation is packed between the studs. Adding CI across all the steel frame members dramatically cuts heat and cold bridges, boosting overall R-value and reducing the U-factor.
Other penetrations through the façade can cause thermal bridging and compromise the CI layer, due to inadequate detailing or misalignment of the thermal control layer. If the structure includes steel shelf angles without stand-offs, it will transfer heat. Exposed concrete floor slabs and steel penetrations for balconies or canopies can compromise the CI layer.
Other bridging challenges are windows and doors with thermal breaks that do not coincide with the opaque wall’s thermal control location, or where structural members hold off their lintels. Sometimes, parapet walls are incorrectly detailed, becoming a building-wide perimeter heat sink. Details matter—the enclosure design team should track possible thermal bridging paths. Properly designed, the CI layer cuts U-factor considerably.
Another effect that can be reduced using CI is moisture accumulation due to transport of water vapor through envelope materials such as brick or CMU. This mitigation is especially effective when a properly specified and installed WRB is also employed. Attention to climate zone, the type of wall system used, and the building’s intended use will help ensure proper enclosure function.
This raises a related point—the CI layer can also serve as part of an air barrier system and moisture barrier protections. For example, extruded polystyrene (XPS) insulation boards can be an effective air-barrier material, typically with taped joints and sealed penetrations using silicone- or latex-based sealants, which are compatible with XPS. To determine if the CI systems employed will perform as a code-compliant air-barrier assembly, specifiers can refer to manufacturer data per ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies. (Specifications sometimes also refer to ASTM E2178, Standard Test Method for Air Permeance of Building Materials, or ASTM E1677, Standard Specification for Air Barrier Material or System for Low-rise Framed Building Walls.)
Using the CI layer as part of the air and moisture protection systems is an efficient double use of a building material—a sustainable combination.