Ten years ago we stood on the outside looking in at a phenomenal new building program called the Passive House standard. We were intrigued, and immediately signed up to get the training and became Passive House Consultants. Today, having completed five residences using the Passive House approach and now employing it in multi-family housing, we can see the benefits first hand. No matter what your perspective—cost of ownership, comfort, health, durability, zero energy— the Passive House approach to building is outstanding. The answers in the FAQ’s below will give you some sense of why.
What is the Passive House standard?
The Passive House program is both a certification standard (like EarthCraft, Green Globes or LEED) and an integrated approach to building design and construction. Contrary to the implication of its name, it is not limited to houses; it is used in houses, schools, hotels and high-rise office buildings. It is at once the most aggressive and the most affordable program for producing low-energy buildings. This is because it focuses on the demand side of the energy equation rather than the supply side;
It focuses upon simple and economical ways to reduce the amount of energy needed rather than upon technologically complex and expensive ways to generate renewable energy. Because it addresses only energy issues, it should be considered a complement to—not a competitor with— other green building standards.
The Passive House program was developed in Europe over the past 25 years, and there are now more than 12,000,00 sf of certified Passive Houses worldwide. The standard is now written the requirement for all new construction in parts of Germany and Austria, and is scheduled to be included in the District of Columbia’s 2020 building code for residential construction. Our Bethesda house was the 24th certified Passive House in the US when it was built in 2011; now there are over 1,000 and the program is entering the steep phase of its “S curve.”
There are no prescriptive standards for the Passive House in terms of materials, form, or design. The architect is limited only by his creativity and the bottom line requirements of energy use. Yet there are several essential principles:
- A continuous air infiltration barrier around the entire volume.
- A continuous insulation envelope with no thermal bridging.
- A balanced ventilation/dehumidification system using a high efficiency energy recovery ventilator.
The requirements to meet the standard vary with the five climate zones of the US. They are are simple, yet strict:
- A maximum number of air changes per hour at 50 pascales (varies between .5 and 1.2).
- A maximum specific space heat demand in kBTU / sf/year
- A maximum specific space cooling demand in kBTU / sf/year
- A maximum primary energy demand in kBTU/sf/year (heating, cooling, hot water, and household electricity)
For further information on the Program, visit http://www.passivehouse.us/passiveHouse/PHIUSHome.html.
What makes these houses different?
The differences are mostly hidden. They are primarily in how the building envelope—the walls, roof, floor slab— is put together and how the house is ventilated.
- Insulation: In our DC climate the amount of insulation is about double that of a code-minimum house.
- Windows are a part of the building envelope too and must also be high-performance. This means triple glazing, insulated frame and sash construction, and very very tight seals. This was originally the one big extra expense in building a Passive House, as US window manufacturer’s didn’t make these windows an we had to go to Europe for them. But today more affordable European and American windows are available.
- Thermal bridge-free construction: The most hidden of all of the differences in a Passive House is its thermal bridge-free construction. A thermal bridge is basically a short-circuit in a house’s insulation envelope. In a standard house built with wood studs, with insulation in the cavities between the studs, each of those wood studs serves as a thermal bridge, interrupting the continuity of the insulation blanket. When we design a Passive House, we find ways to prevent these energy flows.
- Airtightness: Not only are the walls better insulated, they are essentially airtight. Every joint, every penetration is taped and sealed. Once the house is closed in with all windows and doors in place, it is tested with a blower door fan to verify compliance. The end result is a house that is roughly 10x as tight as an Energy Star home.
- Ventilation: With a house this airtight, good fresh air ventilation is a must. Passive Houses achieve this using an energy recovery ventilator (ERV). This is basically a big fan that brings in a constant flow of fresh air and constantly exhausts stale air. The two air streams pass each other inside the ERV and humidity and energy are exchanged. The efficiency rate for energy is around 85% and for humidity around 55%. Fresh air is continuously delivered at an imperceptible velocity to all the living spaces and stale air exhausted from all the damp air spaces—baths, laundry, kitchen. Of course, on a beautiful spring or fall day, when no heating or cooling is needed, all of this ventilation is unnecessary, and you throw open the windows as in any standard house.
- The heating and cooling system of the house warms or cools the air as required. Because the needs are so small, a small mini-split heat pump – what people use normally for a small addition – is usually all that is required.
Does it really reduce heating and cooling costs that dramatically?
Yes. The average Passive House can be heated by the equivalent of hair dryer and cooled by the equivalent of a room air conditioner. Heating and cooling costs of our Bethesda Foursquare come in within 5% of the number predicted by the Passive House energy modeling software –at around $425/year. And that is for a 4100 square foot house! The heating and cooling costs for our most recent 1600 square foot modular Passive House are $82/year. Total energy costs for that home are $65/month.
What about construction costs?
A Passive House costs between 7%-12% more to build than a house built to minimum building codes, depending upon size.
What about architectural fees for creating a Passive House?
There is no increase in our architectural fees to build a house to the Passive House standard since that is all we do. We have been through the learning curve and come out the other side. If you choose to have the house certified, just as with LEED and other certifications, it does take extra time to go through the certification process, and we charge hourly for that. Architectural and PHIUS fees for certification by the Passive House Institute US have run between $5,000-15,000, depending upon project size and complexity.
Why are they so healthy?
Passive Houses have been extensively tested in Europe for interior air quality. They have been shown to have lower CO2 levels, lower radon and a number of other toxins, and fewer asthma triggers. There are three primary reasons for this:
First, mold does not grow in Passive Houses. Because of the lack of leaks, the consistently heavy insulation envelope, and the lack of thermal bridges, there are no cold spots on any walls in the winter. Thus there are no surfaces for warm humid inside air to condense against in winter—the leading cause of mold.
Second, the ventilation system in a passive house is innately healthier. A standard house gets its “fresh” air through all the cracks in the building shell, air that moves through some pretty nasty places to find its way in. The mechanical system then churns the same air around and around with a closed loop heating and air conditioning system, so that you are constantly breathing the same air. The air in a passive house makes essentially a one-way trip through the house, delivered from the ERV to the living spaces, and exhausted through the ERV from the bathroom spaces, never to return. Your air is constantly fresh.
Third, Passive Houses use only zero V.O.C., and urea formaldehyde-free finishes and products, so that there is no out-gassing to contaminate the air. Part of the Passive House PHIUS+ certification is meeting the strict requirements of the EPA Healthy House program.
Why are they so quiet and comfortable?
Passive Houses are remarkably quiet. The increased thermal insulation is also increased sound insulation. Triple glazing means that the windows are 1.5 times as effective in blocking sound as standard double glazed units. If you live near a highway or an airport, or if your neighbor likes to crank up his leaf blower on quiet Sunday mornings, this increased acoustic performance will mean something to you.
Around 15 years ago, the EU commissioned the CEPHEUS Project, in which 220 Passive Houses were build in all the climate areas of Europe and then rigorously tested over several years to see if the program lived up to its promises. From an energy use standpoint the houses performed just as the modeling software predicted. The real surprise came from overwhelming owner feedback about how comfortable the houses were.
When you think about it, it should not have come as a surprise. Because there are no thermal bridges, and all wall surfaces are withing 4-5 degrees F. of each other, no convective loops form. In a standard house in winter, warm air against a window or cold wall will cool, lose some of its moisture in the form of condensation, then drop to the floor. This sucks in more air against the window and a thermal loop – or draft–has been formed. This is why you always see radiators or air conditioning grills near windows—to combat those loops. In a Passive House, there are no drafts, no air stratification (think no hot attics in summer and no cold basements in winter) and no leaky doors and windows.
What do they look like?
Because the Passive House standard is performance-based, not prescriptive, an architect has complete freedom of design—from traditional, as in our Bethesda foursquare and our Rockville neo-colonial, to contemporary as in the net zero energy house we recently completed in Fairfax City, and the Arlington net zero house going into construction in fall of 2018. This does not mean that an architect can disregard basic principles of designing compact, efficient buildings. But if there is a design feature you really want — let’s say a lot of west-facing glass to take advantage of a view– that has an energy cost associated with it, but your architect can play with the trade-offs using the energy modeling software to compensate for the increased solar gain.
The design requirements become more restrictive the further north you live, due to the increasing need for winter solar gain to aid in heating. This translates into larger glass areas facing south the further north you live.
About the only feature that gives away a passive house in the mid-Atlantic area is its wall thickness. Or perhaps a trip to the mechanical room where you will not find a furnace.
Can you retrofit a house to the Passive House standard?
Yes. Much of the postwar apartment housing in Germany is now being renovated and upgraded to the Passive House standard or close to it. Matt Fine in our office, directed the first multifamily housing retrofit in the area while he was at Zavos Architects. Retrofitting is more difficult than new construction however, and therefore more expensive. Estimates are that Passive House features costs can increase the cost of a total retrofit by 10-15%. Because of the high cost of new construction and the high energy costs in Europe, Passive House retrofits of apartment buildings in Germany are being done competitively at market prices without subsidies.
We are just completing our first Passive House retrofit — the conversion of a historic stable/carriage house building in DC to a private home. It is the first Passive House retrofit that we know of of a building listed on the National Register of Historic Places.
Do you sell the plans for Passive Houses?
Yes. The Bethesda house was designed as a prototype, and we are offering plans for several versions of the house. In the metropolitan Washington DC area we are offering the house in a design/build package with O’Neill Development as our partner/builder.
What if I don’t live in the DC area and want to buy Passive House plans?
If you are outside the DC area, you can purchase the plans and have your local builder construct the house. Because Passive Houses must be fitted to both their site and their climate zone, some adjustments must be made in all plans we sell. Contact us to learn more about pricing and costs of adjustments.
Can I get a modular Passive House?
We have forged alliances with two manufactured housing companies to produce modular Passive Houses. We just completed our first modular Passive House with Beracah Homes. Go to this link for a description of the project and this link for the blog of construction. Working with Beracah Homes, we have also developed stock plans for several smaller Passive Houses in both traditional and contemporary designs which can be built for a construction cost of $225-$250/sf in the DC metro area excluding land costs and fees.
But I want a Net Zero House. What does that have to do with Passive Houses?
The long-term goal of sustainable design is to build houses that make rather than use energy. There are only a few such houses in the world today. The mid-term goal is to create houses that use no more energy than they create. These are the Net Zero houses, also known as Net Zero Energy (NZE) houses . California has already made it law that all houses built after 2020 will be NZE. The American Institute of Architects and Architecture 2030 have set 2030 as their goal for NZE.
NZE requires generating energy on-site, and not all houses have perfect solar exposure, due either to orientation, tree cover, or roof design. The only way in these cases of creating a NZE house is therefore to build a house with minimal energy needs. The Passive House approach focuses upon reducing energy demand to the absolute minimum so that that only a small amount of solar is necessary to get the house the rest of the way to NZE.
We must also plan our houses in terms of electric vehicles, which will be the norm within a few more years. Providing power for an EV places even more demands on available solar exposure. The case in point is our Fairfax house, which was designed to be NZE and power an EV. The south-facing roof area, housing 20kW of solar, would only meet this requirement by building a Passive House building envelope. You can follow the construction of this house on our blog with this link.
For more info on Passive House construction: http://www.phius.org