Adding exterior continuous insulation such as, Halo® Exterra®, to an above grade wall assembly helps to keep the air behind the foam board insulation at a higher temperature.
The benefit of air at a higher temperature means it can hold more moisture. This attribute is clearly illustrated when reviewing various air temperatures and the amount of moisture those temperatures of air can hold on a psychrometric chart (Figure 1). Since the air behind the continuous insulation is at a higher temperature there is less of a chance for condensation to occur (dew point temperature reached) within the walls above grade due to air leakage across the assembly.
Figure 1: Comparing Amount of Moisture in Various Dry Bulb Temperatures of Air
Exterior continuous insulation is an effective measure to help control the air temperature between the studs and thus reduce the likelihood of condensation formation or if condensation were to form it is likely to dry at a fastener rate. Visually this can be seen in Figures 2 and 3 when comparing a 2×6 R24 above grade wall assembly to a 2×6 R19 with R5 exterior continuous insulation wall assembly.
The opportunity for the development of condensation can be significantly reduced with just an R5 exterior continuous insulation and almost eliminated with an R10 exterior continuous insulation (Figure 4). Note the calculation of potential condensation depends on the exterior temperature of the climatic location, interior temperature, and the interior or exterior relative humidity.
Figure 2: Controlling Cavity Temperature – 2×6 R24 Batt Insulation with Wood Sheathing (Toronto)
Figure 3: Controlling Cavity Temperature – 2×6 R19 Batt Insulation with R5 Continuous Exterior Insulation (Toronto)
With respect to exterior continuous insulation, often the question arises, “Does the permeance of the insulation matter in context of affecting the assembly’s durability?” The concern being, does a low permeance exterior insulation combined with a low permeance interior sheet membrane (i.e., 6 mil. poly) create an opportunity for moisture to be trapped inside the above grade wall assembly?
Figure 4: Controlling Cavity Temperature – 2×6 R19 Batt Insulation with R10 Continuous Exterior Insulation (Toronto)
This concern was analysed in the NRC research document, “Report on Properties and Position of Materials in the Building Envelope for Housing and Small Buildings.” Four geographical locations were reviewed that included Ottawa, Edmonton, Vancouver, and St. John’s and five types of exterior insulation with a thermal resistance of R4 were analysed that had vapor permeances of 2 ng/Pa•s•m2, 60 ng/Pa•s•m2 , 90 ng/Pa•s•m2, 300 ng/Pa•s•m2, and 1800 ng/Pa•s•m2.
The findings concluded:
- More cavity insulation increases the risk potential for condensation at the exterior sheathing interface
- Exterior insulation reduces condensation risk potential at exterior sheathing interface
- No significant difference in condensation risk potential between the five insulations
These finding resulted in the National Building Code (NBC) of Canada lowering the trigger point for the inboard and outboard calculation to a vapor permeance of 30 ng/Pa•s•m2 for any exterior insulated sheathing (i.e. continuous insulation).
In conclusion, the research highlighted that the amount of thermal resistance outboard of the framed structure is a more important variable than the permeance of the exterior insulation.
Although the tested wall assembly in the NRC research report utilized an interior vapor retarder with a permeance rating of 60 ng/Pa•s•m2, best practice from a building science point of view suggests that the assembly should be able to dry in at least one direction, which represents a margin of safety for the assembly should it become wet.
This is the rational for those builders who adjust the interior vapor retarder to a membrane that has a variable vapor permeance. If the moisture content behind the membrane were to increase the vapor permeance of the membrane increases to allow the moisture to escape towards the interior. Conversely, this is also the rationale for builders to use a vapor permeable exterior continuous insulation layer when a regular interior vapor retarder is employed (i.e. 6 mil. poly.).
Halo® Exterra® – A Continuous Insulation Designed Specially For Above-Grade Walls
The key product performance attributes of Halo® Exterra® include:
- Proprietary perforation technology which allows vapor to escape helping to further increase the durability of the wall assembly
- Conforms to the requirements of a water resistive barrier when joints are sealed (additional membranes on top or behind are not required but can be added without affecting performance)
- Provides a thermal break for components in the above grade wall assembly eliminating thermal bridging
- Significantly more flexible than other foam board insulation products, which offers higher levels of resilience during handling on the job site.
Tyler Simpson, MBSc.
Manager of Technical Services & Building Science (Canada) for Logix Brands
Tyler is passionate in the design of building enclosure systems that are durable, safe, efficient, and engage material selections that limit advancement of carbon emissions. His educational background provides a solid foundation of field research, hygrothermal modeling, and forensic investigation of building failures.