Continuous Exterior Insulation in Residential
As building and energy codes continue to evolve, there is an increasing focus on reducing thermal weak points in construction. One major area of concern is thermal bridging in stud framing. While continuous insulation has long been a requirement in commercial light-gauge metal stud-framed walls, it has now become a more common element in the prescriptive compliance pathways of newer energy codes. This article focuses on the 2021 Washington State Energy Code, but many of these principles are applicable—or will soon be—across other city, county and state jurisdictions.
Thermal bridging occurs when thermal energy passes through a building material with minimal or no resistance. Different materials have different thermal conductivity. Insulation is a common strategy for preventing this energy transfer through a building assembly of materials. By reducing thermal bridging, alongside other strategies, building designers can decrease the size of heating and cooling equipment required and reduce the risk of condensation and mold in building assemblies ultimately making the building more thermally comfortable and durable.
Below is a diagram sections drawings that notes common energy weak points in typical residential construction that needs special attention to detailing:
Diagram section noting areas of vulability of thermal heat loss
Approaches to Detailing Exterior Continuous Wall Insulation
The thermal resistance (R-value) of cavity insulation is only effective where the insulation is located, which is at the in-between studs. This creates a direct pathway for thermal energy to transfer through the studs the most common form of thermal bridging in this project type.
To combat this, an exterior layer of continuous insulation reduces the thermal energy transfer. This approach especially helps at building corners, structural components embed in the exterior wall, or any scenario where packing insulation between framing would be a challenge or impossible.
Plan view of a typical wood framed wall assembly. Left: No Continuous Insulation. Right With Continuous Insulation.
To achieve continuous insulation, we recommend 2 methods:
Traditional Continuous Insulation Approach: This method typically involves an exterior OSB, plywood or exterior rated gypsum sheathing, a weather-resistant barrier (WRB), air barrier (AB), and exterior-grade rigid or mineral wool insulation attached with thermal Z-girts for support for cladding.
Integrated Thermal Sheathing Panels: In this approach, the insulation and OSB sheathing are combined into a single panelized system. An example is ZIP© sheathing, which also includes an integrated WRB layer, or LP NovaCore© which is just insulation and sheathing. These system reduces labor and installation time by panelizing the insulation and sheathing. Exterior sheathings is outboard of the insulation, so the insulation has direct contact with the stud framing - a key difference from the traditional approach.
Key Considerations:
The LP NovaCore© system offers flexibility by allowing the use of a separate WRB system and/or rainscreen, opposed to ZIP© system in which the WRB is integrated with into the sheathing. ZIP© offers a proprietary rainscreen system. Additionally, in some cases, the exterior sheathing may need to be fire-rated from the outside, and the LP NovaCore© product provides fire-rated solutions that are tested and approved by UL standards.
Both integrated thermal sheathing products offer varying insulation thicknesses to meet required R-values, ranging from R-5 to R-15 and higher. These panels can also be used for structural lateral design in the same way that non-insulated OSB or plywood sheathing is used.
The traditional approach is commonly used in commercial applications but can also be applied to residential projects. A key advantage is that the sheathing does not need to be OSB or plywood. If structural lateral design is not required, the sheathing can be replaced with less expensive exterior-grade gypsum, which is often used when fire protection if needed on the exterior. Also better for air sealing and moisture control than OSB or Plywood.
In the traditional approach, since flanged windows are often attached to the sheathing, special attention must be given to how windows and others wall penetrations are trimmed out. This is less of a concern with integrated thermal systems, where penetrations are typically easier to manage since flanges can attached to the exterior sheathing.
Both approaches meet requirements for continuous insulation set forth in the 2021 Washington Energy Code for prescriptive compliance .
Examples of different types of Continuous Insulation (CI). From left to right: LP NovaCore, ZIP, and tradition exterior CI.
Approach to Detailing Exterior Continuous Roof Insulation
There will be a more detailed roof post in the future, but this section will focus on advantages of continuous insulation at the roof.
Proper detailing to ensure that insulation is continuous at the wall to roof transition is paramount to ensure minimal thermal loss - especially critical in colder climates. In the detail example below, we explore using exterior thermal sheathing, spray foam at the parallel chord truss ends, and inverted roof assembly approach to insulation. Inverted Roof assembly consists of exterior grade rigid insulation on top of the roof deck. This minimizes thermal energy transfers at the trusses. The rest of the insulation to make up the total R-Value of the roof assembly is placed under the roof deck in-between the trusses. Spray foam provides continuity from the double top plate to the roof insulation.
Example Detail of an inverted roof assembly approach to insulation that minimized thermal bridging.
