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Thermal performance and meeting net-zero building envelope targets
Building operations are estimated to contribute 28 per cent to GHG emissions.4 Canada’s cold climate results in many days per year that buildings must be heated for occupant comfort and building performance. A thermally efficient building envelope can result in a significant decrease in heat lost through the envelope and corresponding reduction in the heat input from HVAC required to maintain occupant comfort. Even with high-efficiency HVAC systems, a thermally efficient envelope significantly reduces GHG from heat energy.
Canada’s commitment to reduce CO2 emission has led to a target of net-zero energy (NZE) or passive house requirements for all new buildings by 2030. CMUs can be used to create building envelopes that achieve net-zero energy building requirements and help reduce the carbon footprint from building operations. When trying to achieve a thermally efficient envelope, it is important to ensure accurate thermal values and latest technologies are being used for the thermal modelling. For example, using surface-mounted brick ties (Figure 6a) rather than embedded brick ties (Figure 6b) can increase the thermal performance of the wall by R-4.2. Currently the online thermal catalogs5 only have thermal values for concrete block back up walls with embedded brick ties.
Using knife plate standoffs at the foundation (Figure 7) and at floor levels can also be used to increase the thermal performance of the wall by R-4.
The use of vermiculite or other insulation to fill the empty cores can increase the wall performance by R-4 as well. However, if these details are implemented in the construction, but are not captured by the thermal modelling because a thermal value cannot be readily found by the modeller, additional insulation without benefit will likely result.
The additional insulation, if unnecessary, can be detrimental, as it can increase the embodied carbon of the building by using more material than necessary to achieve the thermal target. It is estimated that 11 per cent of CO2 produced globally6 is by building construction and materials. When the insulation being used is extruded polystyrene (XPS), expanded polystyrene (EPS), or spray applied polyurethane foam (SPF), the impact is amplified; as these products are fossil-fuel based, are not always recyclable, and can take years to break down in landfills.
In partnership with the University of Alberta, thermal values were obtained and placed into a spreadsheet application (Figure 8) that captures items not currently found in the online thermal catalog and increase the accuracy of thermal models for wall assemblies using CMUs. This masonry thermal catalog application can be used to complement the existing comprehensive online thermal catalog.
Another method to improve thermal efficiency in the envelope is to reduce the number of window and doors (glazing) and increase the amount of opaque wall, resulting in a decrease in the fenestration and door-to-wall ratio (FWDR). It is easiest to illustrate achieving a net-zero energy building for the opaque wall required for passive house standards with an example on a single elevation of a brick veneer with a CMU backing on a two-storey warehouse/office.
Figure 9 provides a 3D rendering of a hypothetical two-storey warehouse/office building located in Calgary, Alberta.
Figure 10 provides the 2D details for the front elevation of the building. The relevant dimensions of the building are as follows: 14 m (46 ft) long, 7.8 m (25 ft) tall at the roof transition, with a 0.6 m (1.9 ft) parapet. There are four 1.83 x 0.810 m (6 x 2.6 ft) windows and two 2.13 x 0.810 m (7 x 2.6 ft) doors on the front elevation.
To achieve passive house (net-zero energy building) requirements, Passive House Canada recommends a target of 0.15 W/m2 K (R-37.8) for opaque walls and 0.8 W/m2 K (R-7.1) for windows and doors.7
When using a concrete block with a brick veneer, two steps are required. The first step is a reduction in the opaque wall insulation demand for using a low glazing ratio that is typically associated with low-rise masonry warehouse and light industrial buildings.
Using a low glazing ratio benefits the overall performance of the walls system. The thermal requirements for fenestration and doors even by Passive House standards are five times worse than that of opaque walls. Therefore, a fully opaque wall with a lower insulation value, say R-22, will typically have an equivalent or better performance than a wall that has R-37.8 (0.8 W/m2 K) in an opaque assembly at the maximum 33 per cent FWDR permitted (based on the Heating Degree Days or HDD) in many parts of Canada when using fenestration and doors at R-7.1(0.15 W/m2 K).
