Until now, anyone planning to build to the Passivhaus performance standard had a single set of rules to follow. Whether you lived in San Diego or International Falls, Minnesota, buildings could use only a certain amount of energy for heating and cooling, and were allowed a very specific amount of air leakage in the building envelope.
The rule-writing agency in the U.S., Passive House Institute US (PHIUS), has now launched a new set of certification guidelines that for the first time link building performance to climate. The change moves PHIUS further away from the Passivhaus Institut in Germany, where the certification standard originated, and gives builders in North America much wider latitude in adapting buildings to local conditions.
For some builders, meeting the standard will get a little easier. For others, it’s likely to get just a bit tougher.
“We were trying to make things more fair, more realistic,” says Graham Wright, PHIUS senior scientist and chair of its technical committee. Wright describes the process of rewriting the certification standard with PHIUS Executive Director Katrin Klingenberg and Betsy Pettit of the Building Science Corp. under a Department of Energy grant in a blog posted at the PHIUS website. Their work resulted in a report published in March.
Recently, Wright discussed the thinking that went into changes for all three “marquee pillars” of the standard — airtightness, space conditioning criteria, and limits on source energy consumption. The changes also are covered at length in an article by GBA senior editor Martin Holladay posted in January.
There ‘was something funny going on’
The original Passivhaus standard was developed in Darmstadt, Germany. Early on, architects of the Passivhaus building standard came up with two ways of measuring how much energy was used for heating and cooling. One was the annual heating demand, set at 15 kilowatt hours per square meter per year. The other was the peak heating demand — the amount of energy it would take to satisfy heating needs when outdoor temperatures were at their coldest — established at 10 watts per square meter.
The rules allowed buildings to meet either limit as part of its certification but did not require both.
Wright said it became obvious to PHIUS co-founder and executive director Katrin Klingenberg that “there was something funny going on” when those limits were applied to two different climates.
“PHIUS started certifying to that and kind of noticed in different climates the relationship between that peak load and the annual demand varies a lot because some places have long heating seasons but they don’t get very harsh,” he said. “The minimum design temperatures are kind of mild but their heating season might be long.
“In other places,” he continued, “you have kind of the opposite. If you’re designing for a low peak load and you have a really cold outdoor design condition that’s going to be really difficult. And that turned out to be the case pretty much away from the coasts.”
For builders in California, New York, or the Pacific Northwest, getting to the 10 watt/meter threshold was feasible. But it was proving a lot more difficult for builders in the middle of the continent. There, many builders found the annual energy limit easier to meet.
“The way the criteria were written, an annual demand as an alternative was easier to meet in most places,” Wright said. “We went through the database for the final report and looked, and sure enough in Zone 3C and 4C, people certified on the peak load criteria maybe half the time. But outside of those zones, people went for the annual demand 92 percent of the time because it was easier.”
Eliminating the ‘either or’ rule
Some builders were holding down the annual demand for heat by designing buildings for a lot of solar gain. That, Wright said, drove builders toward classic passive solar designs with a lot of glass, and what they got was an inherent risk of overheating — the same problem that plagued many passive solar designs in the 1970s and ’80s.
Plus, the default assumptions for internal heat gains generated by lights and appliances were too low.
“It was an interaction between the way the criteria were written and the climate that led designers a bit astray,” he said. “We wanted to fix that, and also revisit the question of how low to go.”
So the authors settled on two revisions to the classic Passivhaus standard. The first was to require builders to meet both annual heating and cooling limits and peak heating and cooling demand. No longer would builders be able to choose one or the other.
Second, the standard varied by climate. For instance, a building in Duluth, Minnesota (Climate Zone 7) could have an annual heating demand of no more than 8,400 Btu per square foot per year (KBtu/sf/yr) and a peak heating load of 4.6 KBtu/sf/hour (there are separate requirements for annual cooling and peak cooling demand). In Santa Barbara, California (Climate Zone 3C), annual heating demand was pegged at 1.8 KBtu/sf/yr and peak heating load at 2.9 KBtu/sf/hour.
Those values are available for more than 1,000 locales in the U.S. at the PHIUS interactive website.
Make certification a simple ‘moon shot’
Will the revised, climate-specific standard — dubbed the PHIUS+ Passive Building Standard — make it easier for a builder in Duluth and other very cold areas to win certification?
“The short answer is yes,” Wright said. “They should be able to meet our standards without doing something crazy in terms of passive solar design or a hot passive house design. That’s definitely our expectation. We’re trying to make things more of a moon shot for people in Duluth as opposed to a Pluto fly-by.”
In “very rough numbers,” the new PHIUS standard is twice as generous as PHI, Wright said, but in keeping limits on both the annual and the peak demand, the idea was to prevent designers from going for whichever option was easier “and being led astray and getting their designs out of balance.”
“There’s still going to be a cost premium,” Wright said of building meeting the PHIUS standard. “If you compare it to the 15 kWh/square meter target, oh, gosh, we’re allowing twice as much. But we’re talking about the difference between a 70 percent reduction from code minimum and an 80 percent reduction from code minimum. So why are we having this fight?
“And if you’re worried that we’re backing off too much, and endangering the comfort story or something like that,” he continued, “in the study we incorporated maximums on the U-values for windows to make sure in the winter design conditions the inside surface temperature stays up there about 60 degrees.”
Changes in the airtightness rule
Under the old Passivhaus rule (which still apply in the recently revised German standard), air leakage is limited to 0.6 air changes per hour with a pressure difference between inside and outside of 50 pascals — the familiar and tough-to-meet 0.6 ach50 rule.
While the standard is clear, it allowed larger buildings to be as much as seven times as leaky as a single-family home, PHIUS said, because the standard is based on building volume. PHIUS abandoned that requirement in favor of one based on the “shell area” of the house. The new requirement is 0.05 cubic feet per minute (cfm) at 50 pascals and 0.08 cfm at 75 pascals per square foot of gross envelope area.
“For a normal sized residential house, it would represent a bit of a relaxation in ACH terms, from 0.6 to more like 1,” Wright said. “In a nutshell what we did in the airtightness study was we set this criteria of 0.05 cfm50 per shell area and looked at what that does to the moisture situation in a wall, and then looked to see if gets a whole lot better if we tighten it more.”
After running a number of computer simulations, the authors didn’t see significantly lower risks from moisture between 0.05 cfm50 and 0.02 cfm50, so they set the standard at 0.05.
The new rule brings the PHIUS passive standard into line with recommendations on how leakage is measured from both the Air Barrier Association of America and the U.S. Army Corps of Engineers, Wright said.
Limits on total ‘source’ energy
The last hurdle was the limit on “source energy,” the amount of energy consumed to produce electricity distributed over the grid and used at a building. In the new PHIUS standard, the residential limit is 6,200 kWh per person per year, and it all has to do with global carbon emissions.
As much as 80 percent of the energy we consume comes from fossil fuels, Wright said, and according to the Intergovernmental Panel on Climate Change, there’s only so much fuel that can be burned if global temperatures are to be kept in check.
“So, we’ve got a problem there,” Wright said. “The idea of a cap on what a building’s source energy uses is consistent with a global emissions limit. For residential, anyway, you’ve got an individual share of that global carbon budget, and the reason for that is because the atmosphere is a commons. The C02 that’s emitted anywhere affects everyone. It’s intellectually appropriate to do a fair share calculation of that emissions budget.”
The math turns out to be 1 ton per person per year, after taking out carbon emissions due to transportation, embodied energy in the goods we buy, and other non-building related sources.
PHIUS also has set the source energy factor for grid electricity at 3.16, which means that every 1 kWh or electricity taken from the grid is the equivalent of 3.16 kWh of electricity when the inefficiencies of generation and transmission are included.
Measuring source energy on a per-person basis, rather than a per-square-foot basis as the PHI standard does, creates what Wright calls a “McMansion penalty”: The new rule won’t allow certification of a huge house built for only one or two people.
PHIUS sees the 6,200 kWh limit as a “temporary shock absorber” and would like the standard tightened to 4,200 kWh per person per year at some unspecified point in the future.
Finally, the revised standard allows credit for onsite renewable electricity generation as long as the power is used as it is produced. This puts power from photovoltaic arrays, for example, on the same footing as solar hot water in the old standard. “It’s another bullet to meet the source energy limit,” Wright said.
Reaction from the building community
Klingenberg announced three years ago that PHIUS would undertake an examination of the standard, so designers literally have had years to chew on the impact that climate-specific requirements would make. Predictably, the change finds people on both sides of the divide — those who claim a loosening of the standard is a bad idea as well as those who think it could be positive, as Richard Defendorf first wrote in an article for GBA back in 2012.
Dozens of people expressed similar ideas after Holladay’s detailed post appeared earlier this year.
Even as interest in passive house building continues to rise, as Klingenberg and Mike Knezovich of PHIUS describe in an article for Environmental Law in New York, it’s still of interest to only a tiny target audience when compared to the million or so housing starts in the U.S. annually.
Performance-based energy requirements widely embraced in Europe have yet to take root in the U.S. For now, whether builders chose the German Passivhaus standard or the PHIUS standard may seem beside the point in a country where an Energy Star home is still a step up for lots of buyers.
Still, Wright is “cautiously optimistic” that the revised PHIUS standard will move building in the right direction and be accepted by builders and designers.
“We expect that it’s more practical or more realistic over a wide range of climates,” he said. “The economics have been looked at in as a nuanced way as we could.”