What Airtightness and Energy Numbers Actually Mean
By Brett Sichello ·
Two numbers do most of the work in a high-performance home: ACH @ 50Pa and kWh/m²a. Here is what each one means and why it matters.
ACH @ 50Pa: how airtight the building is
ACH @ 50Pa means air changes per hour at 50 pascals of pressure. It measures how airtight a home’s building envelope is, and it gives a measurable baseline that new buildings are held to. A lower number means less air leakage. The maximums are below, with each Step Code level compared against the Passive House limit:
- Certified Passive House: 0.6 ACH @ 50Pa
- BC Energy Step Code Step 5: 1.0 ACH @ 50Pa, about 67 per cent more air leakage than Passive House
- BC Energy Step Code Step 4: 1.5 ACH @ 50Pa, about 150 per cent more air leakage
- BC Energy Step Code Step 3: 2.5 ACH @ 50Pa, about 317 per cent more air leakage
Why it matters: in a leaky house, every dollar spent heating or cooling the home leaks out. It can also cause damage. In winter, warm air that escapes meets cold surfaces inside exterior walls and attics and can condense into water. Over time that can lead to mould and rot, costly repairs, poor indoor air quality, drafty cold spots and health problems for the people living there.
kWh/m²a: how much energy the home needs
kWh/m²a stands for kilowatt-hours per square metre per year. It shows how much heating or cooling a home needs to stay at a steady, comfortable temperature across a year. It closely determines your annual heating and cooling costs.
We look beyond heating and cooling to hot water, lighting, appliances and plug loads such as computers, laptops and cooking devices. We model all of it in advance. After construction we measure the as-built conditions with individual monitors on every circuit of the electrical panel, so we can compare actual performance with both the energy model and the utility bills.
Why ongoing monitoring pays off
The larger benefit is integration. By combining energy data from individual devices with indoor air quality data, we can build automated smart home systems that monitor performance and diagnose issues before they become problems. With one-second energy data, AI-assisted tools can help flag and diagnose faults early and keep systems running well for longer.
One example is using the data from heating, cooling and hot water systems to tune how they run. Their cycles can follow indoor and outdoor temperatures, weather forecasts (preparing for a storm that could knock out grid power instead of reacting to it) and time-of-use billing, which shifts power use to the cheapest hours. Solar panels and electric vehicles tie into all of this too, and we will cover that in a future article.
For related reading, see Passive House and the BC Energy Step Code and our FAQ.