Understanding EnerGuide Rating
By Christopher McLellan
Natural Resources Canada (NRCan) has launched its newly revised EnerGuide Rating System (ERS) with a staggered rollout across the country. The new ERS v15 brings some significant changes, and builders will need to become familiar with some new terms and energy metrics.
Under the “old” version of the EnerGuide Rating System, houses were rated on their energy efficiency using a dimensionless numeric value ranging from 0 to 100. Houses built to energy code standards are typically within the range of 77 to 80, with ENERGY STAR for New Homes coming in at approximately 80 to 85, R-2000 at 86, and houses requiring little purchased energy at 91 or higher.
This older system provided an energy efficiency rating of the house. Factors impacting energy consumption such as house size and climate were normalized in the calculation, making comparison of different sized houses across the country simple. Energy efficiency programs could simply state a threshold, such as 82 for their eligibility criteria.
The new ERS v15 no longer provides an energy efficiency rating, but rather is an energy consumption rating. As such, a lower rating value now indicates lower energy consumption and superior energy performance. Elements such as house size, climate, and energy efficiency all affect the rating. The new ERS is no longer dimensionless and is now expressed in terms of gigajoules per year (GJ/year), with the joule being the SI unit for energy. [To convert to the more familiar kilowatt-hours (kWh), divide GJ by 0.0036.] This consumption-based rating provides more transparency, as the contribution of energy consuming (and producing) elements to the rating becomes more apparent. One challenge to this approach is in comparing houses of different sizes and locations.
Both a large house in a warm climate and a small house in a cold climate can have the same rating if built to the same energy specifications. Similarly, an energy efficient house in a cold climate can have the same rating as an inefficient house in a warmer climate.
Comparative Energy Consumption
To overcome this challenge, users of the ERS have some new metrics that give the rating greater context. One is the comparative energy consumption of a reference house, which is displayed on the label rating scale as “A typical new house.” The reference house is a theoretical replica of the evaluated house as if it were built to the energy requirements of the National Building Code of Canada (NBC). To do this, the software copies the evaluated house and modifies the mechanical systems and insulation levels to conform to the NBC 9.36 requirements.
The location and geometry remain unchanged, although the fenestration and door to wall ratio (FDWR) will be raised to 17% if the evaluated house has an FDWR lower than this, and lowered to 22% if higher. This allows houses with a lower than typical FDWR to receive some credit for design. Houses above 22% FDWR will experience an energy “penalty” when compared to the reference house. Houses within the FDWR range of 17% to 22% will receive neither a credit nor a penalty.
Glazing in the reference house is equally distributed in the four cardinal orientations so that it is neither the worst performing orientation nor the best. This allows builders to take advantage of good design principles to orient the glazing for energy performance. Some houses can have a rating that is higher (worse) than the reference house, while others can be lower (better) based strictly on the orientation of evaluated house. It will be important for builders to understand the reasons a new house can get a rating that is worse than “A typical new house” to avoid any perception by the homeowner that their new house is not up to par.
Another reason that a new house could rate lower than the reference house is if the tested airtightness of the house is lower than the assumed airtightness of the reference house, currently set at 2.5 air changes per hour at 50 Pa (ACH@50). In many regions, builders consistently meet or exceed this target and will see a benefit in their house ratings.
Expectations Going Forward
It is expected that many energy programs will set new performance targets as a percentage lower than the reference house. For example Canada Mortgage and Housing Corporation has now allowed its Green Home mortgage loan insurance premium refund to be based on a specific percentage lower than “A typical new house.” Builders with an understanding of the technical details of the reference house may be able to find cost effective ways through design and construction to meet energy targets under ERS v15.
Christopher McLellan, M.A.Sc., is Director, Technical Services, with Canadian Home Builders’ Association in Ottawa.