Serious Energy Savings With Passive House Design
By Ed Welch
Chances are you’ve already given some thought to energy efficiency at home. You may even live in an Energy Star home — this label is the Environmental Protection Agency’s standard for energy-efficient houses. But as more people realize the value of saving energy at home and having a smaller carbon footprint, some green builders are raising the bar. Enter the Passive House standard.
For example, an Energy Star home is already 20 to 30 percent more efficient than typical building code standards. In contrast, a certified Passive House will use an estimated 90 to 95 percent less energy for heating and cooling and 60 to 70 percent less overall energy than a typical code-built home. Although some of the elements of Passive House design add to the cost of the home (think super-efficient windows), that investment pays off through ultra-low energy bills over the life of the home.
Not to be confused with passive solar — a set of design principles focused primarily on capturing heat from the sun — the Passive House standard focuses on the house as a complete, airtight, highly insulated system that uses a very low level of energy per square foot while also improving the home’s indoor air quality. As Katrin Klingenberg, executive director of the Passive House Institute U.S. (PHIUS) says, “Passive House is strictly an energy metric and performance-based certification that works complementary to all other sustainable rating systems already in the marketplace.”
Another exciting thing about the Passive House standard is that it can be applied to existing homes as well as new construction. Certified Passive House consultant Graham Irwin of Essential Habitat recently completed the first Passive House certified retrofit in California (see photo). Retrofitting an older home to meet these high energy standards is no small task, Irwin says. “A Passive House retrofit is a significant and comprehensive lifetime upgrade to the performance and quality of a home.”
Passive House Principles
The Passive House concept was introduced in 1988 by German physicist Wolfgang Feist and Swedish professor Bo Adamson. Feist founded the Passive House Institute in Germany in 1996. The U.S. branch of this organization is PHIUS in Urbana, Ill., which was authorized to certify projects and train Passive House consultants in 2008. Worldwide, there are about 15,000 buildings certified to the Passive House standard, but only about a dozen in the United States, where Passive House is just beginning to catch on.
An important part of the standard is the Passive House Planning Package (PHPP) that Feist developed. This energy-modeling tool enables designers and consultants to manipulate design elements and building components to see how different options would affect energy performance. For example, they can see how changing the amount or type of insulation — or the type, size or location of windows — would affect the home’s overall efficiency.
The Passive House standard requires that a home have a heating load no greater than 1.4 kilowatt-hours (kwh) per square foot per year, the same standard for a total cooling load, and a total annual source energy consumption not exceeding 11.1 kwh per square foot. To get a better idea of what that means, let’s take a closer look at the numbers for space heating.
To meet the Passive House standard, a 2,000-square-foot home would need to use less than 2,800 kwh for space heating a year. The average U.S. home uses the equivalent of 11,800 kwh for space heating, according to the most recent figures from the U.S. Energy Information Administration.
Altogether, the Passive House concept represents a comprehensive building-performance standard that enables homes to use much less energy for heating and cooling than do conventionally built houses. To achieve that high level of energy efficiency, Passive House design centers on the following building strategies.
Reducing Air Infiltration. In most houses, air leaks occur in numerous places, including cracks in the basement, walls, floors and ceilings; through gaps around windows and doors; and through leaks in the ductwork. Air leaks in and out of the house, and within the house air moves between insulated and uninsulated spaces, so that heated (or cooled) air is constantly being lost.
Trained home-performance contractors can measure the air infiltration of a home using a blower door test. The International Residential Code’s current energy code allows a maximum 7 air changes per hour (ACH) at 50 pascals of pressure. Energy Star allows 5 or 6 ACH depending on the climate. In contrast, the Passive House air infiltration standard is no greater than 0.6 ACH, meaning Passive Houses are almost 10 times more tightly sealed than Energy Star requires!
Another important benefit of reduced air infiltration is increased building durability. Moisture travels through air in the form of humidity, so as air infiltrates building cavities, the potential for condensation, mold and rot increases dramatically. Less air infiltration means less building degradation.
Superinsulation. Of course, air leaks aren’t the only way a home loses heat in the winter — heat also travels through solid materials, such as your walls and roof. The better insulated your home, the more you can slow down that heat movement. Passive Houses rely on thick, properly installed wall and ceiling assemblies. The exact requirements are specific to the climate, site and project — whatever is necessary to meet the Passive House standard for energy use per square foot.
For example, in San Jose, Calif., a moderate climate, builders may only need R-14 under-slab insulation, R-22 walls and R-50 ceilings to reach the Passive House standard. In Salem, Ore., the standard may require R-50 suspended floors, R-44 walls and R-95 ceilings. That’s certainly a lot of insulation, but keep in mind that insulation is one of the least expensive building components.
Eliminating Thermal Bridging. Conventional insulation typically only slows heat movement between studs and joists, but what about the rest of the house? When heat is able to pass through those other uninsulated areas, it’s called thermal bridging. Most thermal bridging occurs through the house framing, but other uninsulated areas may be found around concrete foundations, combustion appliance venting, fireplaces and plumbing vents. These are all direct conductive links to the outside. Brrrr! Strategies that builders might use to minimize thermal bridging include insulating under all interior concrete slabs and using exterior rigid foam assemblies and double wall framing.
High-Performance (and Properly Placed) Windows and Doors. These high-performance windows and doors will likely be the highest additional cost in constructing or remodeling to the Passive House standard. Usually, triple-pane windows or heat mirror technology will be required to meet the standard, but double-pane windows may be sufficient in moderate climates. The U-factor rating measures the efficiency of the windows. That number (which is the inverse of the more familiar R-value that we use to measure insulation) always needs to be low in cold climates. To give you some perspective on window performance, the federal government gives tax rebates on windows that achieve a U-factor below 0.30 (R-3.3) in most climate zones. Most double-pane windows are 0.5 (R-2), and single-pane windows have a U-factor of barely 1 (R-1). Passive House windows start with a U-factor of approximately 0.15 (R-6.6), and can go as low as 0.05 (R-20).
A second important window measurement is the Solar Heat Gain Coefficient (SHGC), which is the amount of solar gain a window allows. A good Passive House consultant will advise where to position windows and which types of glass to use to let in or block heat. Here’s where we get into some of the same territory as passive solar design. In the northern hemisphere, south-facing windows are a good way to take advantage of the sun’s heat in winter. However, shading from overhangs or deciduous trees also needs to be considered. It’s important to block the sun in the summer to help your home stay cool.
Super-Efficient, Balanced Ventilation Systems. The final critical element of a Passive House is using a high-efficiency heat exchanger and a balanced mechanical ventilation system to supply occupants with the ideal amount of clean, fresh air. “Balanced” simply means that approximately the same amount of air exits the house as enters the house.
In the heat exchanger in this ventilation system, outgoing interior air exchanges heat with incoming exterior air (the air itself does not mix). During a harsh winter in a cold climate, interior air that’s 70 degrees Fahrenheit exits the building and intersects with and transmits heat to the incoming zero-degree exterior air. In state-of-the-art ventilation systems, that heat is transferred at 75 to 95 percent efficiency, warming the cold incoming air to 52 to 66 degrees.
Another way to think about it is that Passive Houses recycle all internal heat gains, including body heat from the 98.6-degree humans, as well as any heat from the dog, the appliances, the computer, the light bulbs — even lava lamps! With a super-tight shell and a 95 percent efficient ventilation system, much of the home’s heating and cooling needs can be fulfilled passively, and then only smaller and less expensive heating and cooling systems are necessary. Some of the popular heating system choices for Passive Houses are heat pumps, solar thermal systems and electric baseboard heaters.
Final Considerations. Notice that most of the factors that affect the energy performance of the home aren’t visible to the casual observer. That means that a Passive House could potentially be almost any style of home. However, compact designs increase the likelihood of meeting the standard. Energy-efficient appliances and lighting will reduce the internal heat gains in the summer, and choosing healthy building materials and furnishings is always a good idea.
Construction Costs and Energy Savings
As previously mentioned, building to the Passive House standard will slash a homeowner’s long-term energy bills, but when calculating the bottom line, you need to factor in the higher initial construction costs. Costs related to building a Passive House can vary considerably, estimates range from 5 to 20 percent more than a conventional home. “It all depends on the building type, climate, and experience of the builder and designer,” says Klingenberg of the PHIUS.
Experience and knowledge mean everything, which is likely why upfront costs are lower in Europe than they are in the United States. After 20 years of experience with the Passive House standard, the Europeans are building single-family Passive Houses that cost only 5 percent more than conventional construction, and multifamily dwellings at no additional cost. As the Passive House standard becomes more popular in the United States, costs can be expected to come down here as well.
Some builders are already finding effective ways to reduce costs. In Salem, Ore., Passive House consultant Blake Bilyeu recently completed an attractive, 1,885-square-foot Passive House for only 6 percent more than standard construction. “The Rue-Evans house required an additional $18,000 — 6 percent of the total contract price of $300,000 ($159 per square foot) — to meet Passive House requirements,” Bilyeu explains. “The extra cost was primarily a result of higher-performance windows and doors, more insulation, air sealing labor and materials, and a more extensive ventilation system in comparison to our typical projects.” Bilyeu adds that, having now gone through the entire process of designing, building and successfully having this home certified as a Passive House, he is confident he can bring the costs down to even lower levels on future projects. Meanwhile, his clients love their new home, and have calculated that the total energy savings could pay for the extra construction costs in as little as 10 years. (See Passive House Energy Savings for a breakdown of just the heating savings.)
It’s not hard to understand the appeal of owning a super-efficient home. As Alex Wilson, a prominent leader in the U.S. green building community, says, “Passive House standards demonstrate just how far we can go in reducing the energy consumption of homes. Such homes cost far less to operate.”
Life in a Passive House
Passive House owner Sarah Evans talks about her super-efficient home. Get more details on the home in Passive House Energy Savings.
During the colder months of the year, can you feel a difference between living in your Passive House and a past home?
It’s like night and day. In our old house, we constantly ran our heater during the fall and still felt cold. In our Passive House, we didn’t have to turn on the mini-split heat pump until Nov. 19.
Can you sit comfortably in front of the high-performance windows and doors? What about on a cold, windy day?
Yes, most definitely. The high-performance windows and doors do not leak or transfer much of the cold, so sitting next to them feels the same as anywhere else in the house.
Are there any apparent visual differences between your Passive House and a similarly designed home?
Our Passive House doesn’t look much different than a typical home. One visual difference is the foot-thick walls, which make for built-in window seats that our pets love.
What’s it like living in a home with an energy-recovery ventilation system? Are there any special maintenance requirements?
I appreciate having it, knowing we have better control over the quality and temperature of the air moving in and out of the house. We will occasionally have to change the filter, but no more often than we would with a furnace.
Further Reading and Resources
Homes for a Changing Climate by Katrin Klingenberg, Mike Kernagis and Mary James
Recreating the American Home by Mary James
Ed Welch is a general contractor and Passive House consultant in training who desires a cleaner, more sustainable world for his children and all future generations.