Does a Passive House Use 90 Percent Less Energy?
Does a Passive House Use 90 Percent Less Energy?
Examining an oft-repeated claim about Passive House energy savings
Longtime readers of GBAGreenBuildingAdvisor.com know that I get frustrated by exaggerated energy savings claims. A glaring example is the statement that “a Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. building uses 90% less energy than a conventional building.” A variation on this claim: “A Passive House building uses 90% less energy than a code-minimum building.”
It doesn’t, of course. The oft-repeated falsehood is based on a boast made by Wolfgang Feist in the 1990s. Back then, Feist claimed that a new Passivhaus residence needed 90% less energy for space heating than a “conventional” residence in Germany. (It's important to remember that space heating energy is just one small part of the energy-use pie.) The “conventional” residence that Feist was talking about was an average German home, not a new house meeting modern code standards. (Many German homes are decades, or even centuries, old.)
These days, Dr. Feist and the Passivhaus Institut are usually more careful in their statements than many of their enthusiastic followers. That said, it’s easy to find a Passivhaus Institut document online that includes the boast, without any qualifications about space heating or the age of the housing stock that is used for comparison. Here are two sentences from a Passivhaus Institut press release published in December 2015: “Feist built the world’s first Passive House almost 25 years ago. Still today, this terraced house in Darmstadt (Germany) consumes about 90 percent less energy than conventional buildings.”
As I said, it doesn’t. Let’s peel the onion and figure out why.
What two homes should we compare?
Let’s say that a young couple has money in the bank and wants to build a new energy-efficient home. The couple is weighing the added cost of meeting the Passive House standard (either the European standard or the new PHIUS climate-specific standards).
This hypothetical couple needs to keep two facts in mind:
- They shouldn’t compare the predicted Passive House energy bills to those of an “average” house. Anyone thinking of building a new house should instead compare the proposed Passive House residence with a new code-minimum house. After all, no one is legally allowed to build a house with the same specifications as an average American house. Building codes are more stringent now than they were when most U.S. homes were built.
- They need to keep claims about heating energy use in perspective. Heating energy use only makes up a small segment of total residential energy use.
New code-minimum homes don’t use a lot of energy
The most important fact undermining the “90% less energy” claim is that new code-minimum homes don’t use much energy. Why? Codes have gotten a lot more stringent during the last few years.
CLAIMS FOUND ON THE WEB
Inhabitat: “The Palatine passive home in Seattle uses 90% less energy than required by standard building code while providing a modern, comfortable living space."
Green Builder Media: “The result is a home that uses 90% less energy than a comparable 2,400 SF home built to today’s ‘code minimum’ standards.”
Sage Condos: “Passive Houses are ultra-energy efficient buildings that require only 10% of the energy used in a standard home or building.”
Sprarta Capital: "The energy savings in Passive Homes are achieved by using special energy efficient building components and a high-quality ventilation system. ... They are expected to use 90 percent less energy than a typical house."
Daniel Buck Construction: "Passive House construction gives you a home that uses 90 percent less energy than standard home construction at affordable building costs."
A&R Solar: “The German Passive House design standard requires buildings to use 90 percent less energy than code requirements.”
Dwell.com: “He describes with excitement how the house ‘substantially exceeds the standard energy code’ and notes with pride that the house will use 90 percent less energy than its neighbors consume.”
National Geographic web site: “No furnaces are needed, because they're projected to use up to 90 percent less energy than a typical house.”
I’m going to focus on the U.S., where the “90% less energy” claim is alive and well among Passive House adherents. (To read some examples of the claim, see the sidebar at left.)
To delve into this issue, I needed a good source of information on the energy used for new code-compliant homes. I found such a source online: the “Residential Energy & Cost Savings Analysis” documents prepared by the U.S. Department of Energy. These documents were prepared to determine the savings attributable to proposed energy code changes.
If you click the link in the paragraph above, you’ll come to a page with links to most U.S. states. (California, Oregon, and Washington state aren’t included, because these states have oddball energy codes.) If you click the name of a state you are interested in, you can find out the modeled energy use of a new code-compliant home. This modeled home is assumed to have 2,376 square feet of conditioned floor area. (More details on the modeled home are available on this web page.)
The modeling is fairly sophisticated. Its heating appliances and fuel prices are state-specific; for example, some states have an average electricity rate of 11 cents per kWh, while other states have an average electricity rate of 16 cents per kWh. The annual energy use figures include heating, ventilation, air conditioning, domestic hot water, and lighting, but (as far as I can tell) the figures exclude plug loads.
RESNET software assumes that residential plug loads (excluding lighting) average 4,500 kWh per year for the average U.S. household. In order to account for plug loads in the table below, I’ve added the cost of 4,500 kWh (for plug loads) to the total energy costs listed in the “Residential Energy & Cost Savings Analysis” documents cited above.
Here’s what we learn from this table:
- Most new code-compliant homes have relatively low energy bills.
- Energy costs for space heating represent less than 40% of total residential energy costs for most U.S. households.
Passive house data
I reached out to several people, including Katrin Klingenberg, the founder of PHIUS, requesting monitored energy use data for single-family Passive House residences. Klingenberg responded by email, “We have a PhD on the job who has collected measured data for all projects in the [PHIUS] data base (for those who responded to the request). The data is being analyzed right now and I believe the project is supposed to be done by the end of May. That’s when it will be available.” I’m eager to see the data; for the time being, however, we’ll all have to be patient.
In the meantime, I found five single-family Passive House residences in the U.S. with published estimates or monitored data for annual energy use. The data are presented in the table below.
Here are the sources for the energy use data or energy use predictions used to create the table:
- Jason and Stephanie Specht house
- Daniel Ernst house
- Smith House
- New American Foursquare
- Matthew Beaton house
Next, I calculated the estimated annual residential energy cost (including the cost per square foot) for a new code-compliant home in each of the four states under consideration. The annual energy cost that I calculated ranges from $1,710 in Virginia to $2,295 in Massachusetts. The annual energy cost per square foot ranges from 71.9 cents per square foot in Virginia to 96.7 cents in Massachusetts.
To see if these numbers pass the smell test, I compared them to published data from the 2009 Residential Energy Consumption Survey (RECS) published by the U.S. Energy Information Administration. According to the 2009 RECS, the average residential energy cost per year is $2,024, or $1.03 per s.f. It’s only logical that a new home complying with the 2015 IECC would have a somewhat lower energy bill, so my numbers pass the smell test.
Yes, the method of analysis is imperfect
The table above includes only five homes. That’s a very small sample. I’d be delighted if the sample were larger, and I invite GBA readers with monitored energy use to share their data with GBA readers. I’m especially interesting in learning whether any U.S. Passive House uses 90% less energy than a new code-minimum house of the same size.
The five homes may not be representative, of course. My assumptions about the energy bills for a typical code-minimum house can be criticized and improved. I’m aware of all the imperfections of my method of comparison. But it’s as good a comparison as I’m able to make until better data appear.
The ballpark calculations support my premise. Of the five Passive House residences I looked at, the best performing house (the Jason and Stephanie Specht house) used 54% less energy than a code-minimum house. That’s impressive. Bravo.
The house with the worst performance (the New American Foursquare house) used only 24% less energy than a code-minimum house.
So I think that it’s fair to say that a Passive House residence in the U.S. will use between 24% and 54% less energy than a new code-minimum house. That’s good, but it’s not 90% less. So please — let’s finally bury the 90% claim.
Why bring this topic up?
In a recent letter to the editor published in Fine Homebuilding, a reader named Sarah Cobb wrote, “Some of us build Passive Houses because it’s a phenomenal technical challenge and because it’s reassuring to know that our energy costs will always be a tenth of most of the homes built today.”
Fine Homebuilding published my response to Sarah Cobb, in which I noted that “the argument that your energy costs are a tenth of most of the homes built today is based on a myth.” My response then outlined the argument that is presented in more detail here.
My published comments brought a strong response on Twitter from Andrew Michler (whose Colorado Passive House was described in a GBA article by Scott Gibson). Michler tweeted, “It is painful to read Martin Holladay’s selective and biased interpretation of Passive House. Flat out misrepresents its energy and costs.”
When I saw Michler’s tweet, I wasn’t sure what I had written that pained him. So I asked him, and he responded, “FHB May response.”
I look forward to more data on this issue, from Andrew Michler or others, so that we can all get a good handle on a basic question: How much lower are the energy bills of Americans who live in a Passive House than Americans who live in a new code-minimum house?
Martin Holladay’s previous blog: “Two Views of Double-Stud Walls.”
- Image #1: Martin Holladay
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