Thermal control layer
The final control layer for this discussion is the thermal control layer. This component reduces the amount of thermal energy (heat) from passing through the building enclosure. Not only does heat loss waste valuable energy which must be provided to the building in the form of electricity, natural gas, or a similar energy source, but it can create cold spaces inside the building which have a negative effect on occupant comfort. In the worst cases, localized heat loss can create spots where the interior surfaces can be cold to the touch, and even result in condensation at the interior. This moisture, sourced from the interior air and not a leak from outside, can cause water damage, mould, and/or rot to building components.
As building codes have evolved, the requirements for the thermal control layer have become more rigorous. Modern codes require continuous insulation (ci) in most cases, not just insulation placed between structural members like wood or steel studs. Typical glass fibre batt insulation at 150 mm (6 in.) thick has a nominal insulating value of approximately R-19, but when installed between wood or, worse, steel studs, the building’s thermal energy can bleed through the studs which form thermal bridges through the insulation.
In a 150 mm wall with steel studs at 406 mm (16 in.) on centre, the effective R-value drops from a nominal R-19 to an actual R-7.1. Just 38 mm (1.5 in.) of extruded polystyrene (XPS) insulation, continuous outside the stud wall, provides a higher R-7.5 insulation value. Adding a continuous layer of insulation is the better design, but the best design attempts to eliminate all thermal bridges.
One often finds these thermal bridges at areas where structural supports need to span to the exterior, such as at window supports or cladding fasteners. These details are more difficult to get right, but more manufacturers are bringing to market specialty components such as fibreglass cladding supports to mitigate this additional heat loss path.
Each year building product manufacturers are introducing new and innovative products capable of functioning as more than one of these control layers, either intentionally or unintentionally. Some rigid board insulation, for example, can act as a vapour barrier, but if placed on the exterior of interior insulation it can create a second vapour control layer within an assembly, whereby trapping moisture within the construction. Hygrothermal analysis can determine where the dewpoint is and if it will be a problem.
One tried-and-true method for assessing the design of the building enclosure is to take the design—plans, sections, and details—and to draw on with different colours each of the control layers. If one could draw each layer without lifting the pen, then it should be continuous in practice. If there is a gap in the air barrier line, then it may have a hole allowing air to pass through the building envelope. If one lifts their pen on the thermal control layer, it may have thermal bridging. All four control layers are critically important to any high-performance building enclosure design. Getting the details right is vital.
Ian Miller is a building science engineer with 18 years of experience in the design, assessment, and repair of various buildings and their components. He is a graduate of the University of Waterloo and a licensed professional engineer. He is a partner at Pretium, a building science engineering consulting firm, and works in the technical role of a project principal, taking lead roles on many of the firm’s most important projects. Project work has included assessments as well as design, tendering, contract administration, and construction review for various building construction, maintenance, and repair undertakings. Miller served on the board of directors for the Ontario Building Envelope Council from 2011 to 2021, including as the organization’s president from 2017 to 2019. He is the organizing chair for the 16th Canadian Conference on Building Science and Technology, to be held in Toronto in October 2022.