Energy-efficiency requirements and overheating

What is the peak cooling load?

The peak cooling load is the predicted rate of heat removal required at the height of summer to maintain a comfortable temperature in a building. The peak cooling load is also used to size cooling equipment.

How is it modelled?

Energy modelling software uses information provided about the proposed geometry, insulation, airtightness, orientation, glazing and other factors, and data about typical weather at the planned location, to calculate the peak cooling load of a proposed house. The mechanical cooling system selected does not affect the result.

What is the peak cooling load requirement?

The peak cooling load is one of three main criteria in the performance-based energy-efficiency tiers in Part 9.36. of the 2020 National Building Code (NBC). When modelling for compliance with any tier in the performance path, the peak cooling load for the proposed house design may not be greater than the peak cooling load for the reference house.

How does the requirement work in practice? 

CHBA analyzed the effectiveness of the peak cooling load requirement in relation to 447 homes built under the Net Zero Home Labelling Program. The analysis showed that 44% of built Net Zero ready homes had peak cooling loads higher than the reference house. This means that the approach to peak cooling in the NBC does not align with proven net zero performance.

Most of the Net Zero ready homes analyzed were within 15% of the reference house peak cooling load. Adjustments to the designs to accommodate the new peak cooling load requirement in the NBC could have had a significant negative impact on cost, and on homebuyer satisfaction.

What are the implications?

Homes that meet the requirement might overheat. The requirement allows homes to be prone to overheating if the reference house is also prone to overheating. This can happen when a home design has a complex shape and lots of windows or doors.

Homes that are unlikely to overheat could be disqualified. If both the reference and proposed houses have geometries that reduce the likelihood of excessive solar gain, the peak cooling load for both designs would be relatively low.
The peak cooling load for the proposed house design might still exceed that of the reference house due to differences in window specifications, for example, even though the proposed house has a low risk of overheating.

The pass/fail criterion is not climate-zone sensitive. The risk of overheating is not the same in Vancouver or Toronto as it is in Yellowknife, yet the requirement will apply identically across the country. This means that design options will be limited in northern areas where homes are unlikely to overheat.

Mechanical cooling will not help compliance. A home with properly sized cooling installed is unlikely to overheat. If such a home meets the energy and envelope targets when cooling energy is accounted for,
it will still fail to comply if the proposed design exceeds the peak cooling load of the reference house.

The modelling assumption for solar heat gain is not realistic. The requirements for tiered energy performance set a low Solar Heat Gain Coefficient (SHGC) of 0.26 for fenestration in the reference house, which is at the low end of typical values currently used in practice (0.25-0.55). The SHGC value selected may suit a focus on space-heating energy but becomes problematic for cooling energy. Most available fenestration will have higher solar gain than the reference assumption, meaning that the proposed home will be starting at a cooling load disadvantage before other factors are considered.

Passive cooling measures are not modelled. Apart from window overhangs, the modelling assumptions do not account for passive-solar measures such as shading through trees or shutters, or for the impact of existing terrain or neighbouring buildings.

The implications of this criterion for the winter situation may not have been fully considered. In order to meet the peak cooling load requirement, many homes will need to limit the solar heat gain of the building. To make up for this, homes will either need to use more efficient equipment, higher levels of insulation or increased airtightness to meet the efficiency targets. These additional measures come at both a financial and carbon cost. In northern locations, and for homes with low cooling needs, this may be unwarranted.

A better approach

Homes labelled under the CHBA Net Zero Home Labelling Program have proven net zero ready performance. Most of these homes include mechanical cooling, which has been demonstrated to prevent significant overheating issues. When mechanical cooling is included, it is accounted for in the energy use modelling. If mechanical cooling is not provided, modelling needs to result in an annual cooling load below 2 MJ/m³ or show that the home would still comply if a standard air-conditioning unit were installed later by the homeowner. This approach helps address the space-cooling load while limiting the risk of overheating.