Reducing Overheating
Impact of proposed Canadian limits on SHGC
by Amy Roberts, FGIA’s Director of Canadian and Technical Glass Operations
(Editor’s Note: Amy Roberts leads FGIA’s Canadian codes and regulatory initiatives with a focus on building and energy codes, standards development, and industry advocacy. In her role, she also supports the FGIA FENBC Region, working closely with stakeholders to represent industry interests at both regional and national levels. With over 25 years of experience in glass and insulating glass manufacturing, as well as residential and commercial window production, Roberts brings deep technical expertise and strategic insight to advancing the Canadian fenestration and glazing industry.)
As climate change intensifies, extreme weather events are expected to increase. Heat waves, severe storms, and droughts will become more frequent. According to the United Nations, environmental factors are killing approximately 13 million people annually. Heat can make living conditions unbearable. In 2021, 619 people died in British Columbia alone due to extreme heat events, which are a growing public health risk.
In 2020, the National Building Code of Canada (NBC) introduced five tiers of energy efficiency requirements for residential and small buildings. Tier One is the minimum, as defined in NBC 2015. The higher you went, the greater was the reduction in energy use, ranging from 10% at Tier Two to 70% at Tier Five. While this is promising from an energy-efficiency perspective, the system doesn’t take into consideration the impact of energy conservation measures on a building’s tendency to overheat. This is especially concerning during power outages and in buildings without air conditioning.
Currently, NBC allows building professionals to reduce overheating by limiting the peak cooling load of houses. However, as it was explained in NBC’s Effect of New Canadian Tiered Building Codes on Thermal Resilience of Housing conference, there’s no strong correlation between peak cooling load and overheating. Further, many builders use the prescriptive compliance path instead of the performance compliance path and are unable to use the above method to limit overheating.
Proposed Code Changes
The upcoming 2025 edition of NBC proposed two new approaches to reducing overheating. The first would limit solar heat gain coefficients (SHGCs) of windows and doors based on the climate zone and the fenestration-and-door-to-wall area ratio (FDWR). For example, SHGC caps range from 0.50, FDWR ≤17%, down to 0.26, FDWR >30%, for Canadian climate zone 7A. For FDWR >30%, SHGC is capped at 0.26 across all zones.
The second approach would mandate a maximum dry bulb temperature of 78.8°F in at least one room of a dwelling. This approach would require simulation tools for design purposes, which residential building professionals typically don’t use, and hence would be impractical. If builders decide to adopt the performance compliance path, they will use the HOT2000 monthly bin method, which cannot determine hourly interior conditions. Thus, there would be no way to verify if the design succeeded in limiting the indoor temperature to 78.8°F. Additionally, a strict metric could lead builders to install air conditioners needlessly.
Studying Code Compliance & Cost
A study was conducted to help model the lowest capital cost, code-compliant house for each tier of the proposed 2025 NBC. Results also were presented for a single detached home in Quebec City, which is in climate zone 7A. The study found that the relationship between the FDWR-SHGC product and FDWR is almost linear for Climate Zone 7A, until a FDWR of 0.325, where the value for SHGC is lower.
The study determined that a FDWR of 0.26 may receive the most solar gains overall in Climate Zone 7A despite having the second-largest window area. Cooling energy requirements were highest for a FDWR of 0.26 and lowest for 0.13, indicating that cooling energy demand is largely driven by solar heat gains entering through the windows.
The modeling found the living room and bedroom zones remained below the proposed 78.8°F code requirements between 4.6% and 6.5% of the time. Additionally, the second floor tends to overheat more than the first floor due to buoyancy effects. The study also noted that besides the SHGC-FDWR product, FDWR itself is a factor in reducing overheating because larger window areas mean more natural ventilation.
The study’s results indicated that cooling energy is strongly influenced by solar gains through the window area and SHGC product. Overheating is minimal in the cool season when natural ventilation is employed. The study found overheating in approximately 5% of the hours. However, when simulating a severe heat wave combined with a blackout, significant overheating in all tier/FDWR combinations was found with indoor temperatures even soaring above 91.4°F at times.
If these proposed changes are included in 2030 NBC, there will be a significant impact on how windows contribute to whole-building energy performance. Fenestration manufacturers will be called upon to develop glazing systems that not only help conserve energy, but also to safeguard the comfort and health of residents in all seasons.
