Passive House Net-Zero Design is one way to strike the right balance for maximum energy savings
By NATALIE LEONARD
From January to September 2019 there were 653,300 single?unit housing completions in the United States and 45,086 single-family house completions in Canada.
By design, each one is thermally out of balance with its surrounding environment. The imbalance represents the difference between the desired temperature of the conditioned space and the temperature of its surroundings. Maintaining this intended imbalance requires that heat energy be added or removed as needed; how much energy depends on the performance of the building envelope and the sources of heat energy.
Many approaches can be used to manage the thermal imbalance. In the residential sector, using Passive House Design as the basis of a Net- Zero house is a very suitable solution.
Net-Zero is defined in a number of ways: A useful definition says: net zero building is a building with zero net energy consumption, meaning the total amount of energy used by the building on an annual basis is equal to the amount of renewable energy created on the site.
Passive House is a voluntary standard for energy efficiency in a building, which reduces the building’s ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or cooling.
A Net-Zero house is not necessarily built using Passive House Design, and a Passive House is not necessarily Net Zero. Net-Zero is a calculation, one that ensures site energy supply and demand are in balance; Passive House Design results in ultra-low heating-energy demand.
Costs for any Net-Zero project are divided between the building envelope and on-site energy production and storage. Spending money on each is required, but in what proportions? If saving energy is the cheapest fuel, then Passive House Design, with its minimal energy requirements provides an excellent foundation for any Net-Zero project.
The Passive House has one outstanding feature: it uses between 60-80 percent less energy than an average house. Energy savings are achieved by controlling the three modes of heat energy transfer: radiation, conduction, and convection. Specifically, Passive House Design employs the following strategies:
- Siting a Passive Home takes advantage of solar radiation to help with heating and natural ventilation to assist with cooling.
- Building shape is important. Passive Houses use compact building shapes that reduce exterior surface area, thereby minimizing opportunities for air infiltration and conductive heat loss or thermal bridging.
- The envelope is super-insulated to eliminate convection energy transfers and is made airtight to ensure all air exchanged between the inside and outside passes through a high-efficiency energy recovery ventilator. This guarantees consistent indoor air quality without unnecessary heat energy loss or gain.
- Window and door fenestrations are built with special care to, 1) prevent air leaks, and 2) control radiation energy transfer. Passive Houses use triple-glazed windows with special coatings that maximize solar radiation gain during heating periods and minimize it during cooling periods.
- The roof design and orientation of a Passive House anticipates installation of solar photovoltaic panels that can be used to replace purchased energy including the losses incurred by its generation and transmission: The current EIA estimate is that for every 1 BTU (British thermal unit) of electricity used in a home 2.1 BTUs are lost in generation and transmission. (National Association of Home Builders, Special Studies, November 5, 2014)
- Passive Homes avoid fossil fuels and specify electrical systems that have only highefficiency components. For example, heat pump technology gives 3.5 times more heat than electric resistance heaters for the same unit of electrical energy.
The true value of Passive House Design is measured in its Total Cost of Ownership. A Passive House may increase construction costs five to15 percent over code-built alternatives, but it achieves a 75-90 percent reduction in heating costs, with annual heating bills as low as $150.00.
With these drastic reductions in energy consumption, a small array of solar panels brings the home to Net-Zero energy. A grid tie-in with net metering eliminates the need for batteries and greatly simplifies power management. The result is a much less expensive on-site power generation system that can be amortized in five-to-twelve years, depending on local costs and availability of efficiency incentives.
A Net-Zero Passive House is not a compromise. Instead, it provides a number of attractive benefits:
- Significant reduction in heating costs over code-built homes;
- reduced carbon footprint;
- Superior indoor comfort – the bright, draftfree, and consistently warm spaces are what owners most love about their Passive House;
- Less complex operational and maintenance requirements due to simpler mechanical systems.
- Increased safety and security during storms (even without power for weeks a Passive House will maintain temperatures above 10C.)
Net-Zero Passive House Design strikes the right balance among these objectives and provides a simple and economical path to achieving them in most North American regions with heating dominated climates.
Natalie Leonard is an engineer and the President of Passive Design Solutions. She has worked on over 100 Passive House projects that are net-zero ready and provided support to 30+ builders on their first Passive House projects. passivedesign.ca