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Why a New Standard for Passive House?


There’s this thing called passive house. It’s a terrible name. Almost everyone agrees on that. Look at how its adherents are driven to a frenzy and you’ll see that the name doesn’t really fit. The houses themselves aren’t passive either, but let’s leave that aside for now. Let’s talk about numbers and objectives. The North American Passive House program of PHIUS grew out of the German Passivhaus program, which has one set of targets for the whole world. Now PHIUS has adapted the German standard to suit North America. It’s an important change. And the frenzy of the adherents has been fun to watch!

There’s this thing called passive house. It’s a terrible name. Almost everyone agrees on that. Look at how its adherents are driven to a frenzy and you’ll see that the name doesn’t really fit. The houses themselves aren’t passive either, but let’s leave that aside for now. Let’s talk about numbers and objectives. The North American Passive House program of PHIUS grew out of the German Passivhaus program, which has one set of targets for the whole world. Now PHIUS has adapted the German standard to suit North America. It’s an important change. And the frenzy of the adherents has been fun to watch!

The Passivhaus requirements

First, let me say a few words on terminology. When I use the word “Passivhaus,” I’m referring to the German program promulgated by the Passivhaus Institut. When I use the term “Passive House,” I’m talking about the North American program of PHIUS (which, by the way, stands for Passive House Institute US). And if I use the term “passive house,” in lower case, I’m talking about the whole concept.

The two basic Passivhaus requirements put limits on a home’s heating load (either peak or annual) and on the primary energy (also known as source energy). Here they are, as shown on the PHI website:

Heating:  Annual load ≤ 15 kWh per square meter (4.75 kBTU per square foot) of net living space (what PHI calls treated floor area) or peak load ≤ 10 W per square meter (3.2 BTU/hour per square foot)

Primary energy:  ≤ 120 kWh per square meter (38 kBTU per square foot) of treated floor area per year

They added cooling load requirements similar to the heating loads for places that need cooling. The PHI standard also has an airtightness requirement (≤ 0.6 air changes per hour at 50 Pascals, ACH50) and a comfort requirement based on temperatures in the house. Then you look a little further and find other measures for the efficiency of the ventilation system used and the U-values of the windows installed.

Yeah, there’s more detail in there when you really dig in, but it’s really a simple, elegant standard. It’s really appealing to think all you have to do is meet a few requirements that are the same the world over.

But will it work for North America?

Professor John Straube wrote a critique of the Passivhaus program in 2009. He compared homes designed to meet Passivhaus requirements with homes in the US Department of Energy’s Building America program. It’s a great read and a good starting point to understand why the Passivhaus standard needed modification for North America. Here’s his conclusion:

Achieving the specific Passivhaus target of 15 kWh/m2/yr for heating on site energy use, results in investment of materials and money that often will exceed other less costly and environmentally impactive solutions. Achieving the equally arbitrary 120 kWh/m2/yr has more direct environmental benefits than the heating target, but may best (i.e., with least cost and environmental damage) be achieved using some on-site or renewable off-site power generation.

As new clean, local, and renewable energy sources come on line over the next 25 years and become more affordable than current PV prices, it is unlikely that the extreme conservation measures taken by Passiv Haus to meet the specific requirements will be considered an optimal deployment of resources for cold climate housing.

The following year, Alex Wilson of Building Green also found the Passivhaus program unsuited to the needs of North America. He found three main deficiencies, and gave corresponding recommendations.

  1. “In climates with very high heating or cooling loads, the Passive House standard right now may be too difficult to achieve.” His recommendation was to focus on the source energy rather than the heating and cooling loads in really hot or cold climates. After all, homes in a really cold climate won’t need much (or any) allowance for cooling load so “why not allow some of that energy to be used for heating?” The key is the source energy.
  2. Passivhaus targets are based on floor area. Wilson noted the implication: “larger houses can use more energy and meet the standard, and it’s harder to certify small houses.” The airtightness requirement is based on ACH50, which also is lends a bias toward larger homes.
  3. Give some incentive for existing buildings to do passive house. “Solving our climate crisis will require a huge focus on existing buildings, and a strong standard like [Passivehaus] could be a tremendously important tool in getting there. But it’s just too hard to achieve right now.” He suggested convening a panel of experts to “come up with a more reasonable Passive House Retrofit Standard for North America.”

Lloyd Alter of Treehugger saw Wilson’s article and wrote about it as well. In his 2010 article, Passivhaus: Too Rigid and Inflexible for America, he summarized Wilson’s critique and ended with, “Good points all.”

And they were right. Passivhaus was a boutique program and was destined to remain a boutique program with the PHI requirements.

Introducing the PHIUS+ 2015 Passive Building Standard

PHIUS was an independent organization working as the agent of PHI in North America when Straube and Wilson wrote their critiques. In 2011, PHI canceled the contracts it had with PHIUS. In 2014, PHIUS introduced its new climate-specific passive house standard.

In the standard adaptation report, they wrote of the problems with the one-size-fits-all approach, “In some climates this has led to very costly projects; in others it has produced significant thermal comfort issues.” [p. 5] I’m likely to be accused of denigrating PHI by writing this, but the truth is the Passivhaus standard needed modification. Here’s another quote from the report:

Almost every project offered a new climate challenge. These circumstances sometimes resulted in projects that had the same overheating problems as early “mass-and-glass” designs from the 1970s. In other instances—in Louisiana for example—the German-derived standard did not account for humidity loads and predicted cooling demands inaccurately. [p. 5]

As a result, PHIUS made some changes while keeping the overall structure and intent of Passivhaus. It’s still passive house, but now it has diverged from Passivhaus a bit. Here are the main changes:

Airtightness:  Go from 0.6 ACH50 to 0.05 cfm50/square foot of gross enclosure area.

Source energy:  (1) Change from per square foot basis to per person; (2) Change the source energy factor to the US average of 3.16; (3) Base the lighting and miscellaneous electrical loads on American numbers (from RESNET); (4) “Set the source energy limit to 6,200 kWh/person/year and tighten it to 4,200 kWh/person/year within a few years;” and (5) Account for photovoltaic (PV) inputs or other on-site renewable electricity the same way solar hot water is.

Space conditioning:  The annual and peak heating and cooling load targets vary by climate.


There’s a whole lot packed into those three items! The first one is something that some of us have been calling for for a long time. I wrote about the problem with ACH in two articles (here and here) back in 2010. As Wilson noted in his article, ACH is biased toward larger houses.

The source energy changes are mostly about making the program fit the reality of North American buildings and giving PV a fair shake. The more fundamental change is going from a target based on floor area to one based on the number of people. Exploring that last part is probably worth its own article.

And finally, making the standard climate specific is only one of the big changes, and that happens with the heating and cooling load targets. Now instead of 4.75 kBTU per square foot or 3.2 BTU per square foot, you use the targets developed for the climate where the project is located. Go to the PHIUS+ standard page and click on a location on the map to find out what your numbers are.

A lot of science that went into developing this new passive house standard. PHIUS worked with Building Science Corporation using a US Department of Energy grant. I’m tempted to jump into explaining the optimization process right now, but let’s save that for another time. I’ve already thrown a lot at you in this article. If you want to understand the science behind the changes, download and read the standard adaptation report (pdf).

PHI has made some tweaks to Passivhaus over the years, and it’s better than it used to be. That’s great. The changes made by PHIUS are more extensive and, I believe, more suitable to the realities of construction in North America.

Stop complaining; we’ve got work to do!

One of the main goals of passive house is to decrease our impact on the environment, right? We’ve got some serious problems that we can help solve by reducing energy use. We ought to be looking at the best way to do that. It’s ludicrous to think that the number 4.75 was handed down from God. (I heard it was written on the back those tablets Moses brought down from the mountain.) Our goal is to design, build, and renovate buildings so they’re comfortable, durable, healthful, and energy efficient. Passive house could, and should, be the best way to do that.

The Passivhaus program as it existed when it came over from Germany 15 years ago was flawed. Martin Holladay at Green Building Advisor is another critic who’s been pointing the way to a better program over the past decade. Here’s a sketch he made clearly illustrating that the program had problems. Which scenario do you think will save the most energy?

We don’t always have the ability to spread out those seven layers of insulation among seven houses, but we’ve got to keep our eyes on the bigger picture. Unfortunately, a lot of people seem to have myopia when it comes to this subject. If PHIUS gives a rationale for its modified standard, we’re accused of “denigrating PHI.” Because we’ve modified the standard, we’re told we shouldn’t call ourselves passive house. If we say, “Less filling,” they say, “Tastes great!” We just can’t win.

Get over it already! PHIUS does passive house. We have a modified standard. We’re here to stay. Now, we’ve got work to do. Anyone who’s interested, come along and join us. Or do it your own way. If you’re curious to find out what PHIUS is up to, come to Philadelphia in September for the 11th annual North American Passive House Conference.

Let the frenzy begin!


Related Articles

How Much Insulation Do You Need in a Passive House?

The Evolution of Passive House in North America

Passive House — The Only Place Where Real Innovation Is Happening?


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This Post Has 11 Comments

  1. I cut my teeth on a SEO
    I cut my teeth on a SEO finance program for existing homes. It was a great learning experience. I saw a lot of different houses and their problems. I learned some great lessons quickly and some great lessons that took time to figure out.

    Quick Lesson: Slab on Grade 1971 with recessed lighting. Walk under the lights with the blower door going – you will never forget that one.

    Time to Figure out Lesson: People have a really hard time figuring out the why to implement efficiency recommendations and doing so, without grief.

    The program required energy model and implementation of items of low cost high return before high cost low return on the Simple Payback method. If you want to replace your HVAC, you have to air seal and insulate.

    The grief I reference could be: **I need a new furnace so I have to add insulation and do some air sealing to get the financing.** There are many variations. Most of them do not involve saving money or being efficient. Solving a comfort issue for example.

    My education during all these audits was varied and hit many little things that add up. Mostly I keep learning what the next question is. I also learned how inexpensive it is to do it right the first time; and how resistant to change people are.

    My point here is that people doing something to lower to energy use in a house is a good thing. Reducing it more is a good thing. The name of what you are doing is not important.

    An Energy Star certified home is nice. A Zero Energy Ready Home is nice. A PHI certified home is nice. A PHIUS certified home is nice. All will be more comfortable, more durable and more efficient that the non-certified home.

    What does the home owner or buyer want? What will they buy? That is what we have to do, not worry about labels. Yes, I like the PHIUS allowances for climate. That is part of why I got my PHIUS+ Rater Certification.

  2. I believe the retrofit issue
    I believe the retrofit issue is the biggest problem that is usually completely overlooked by the building scientists and trade magazines. Sometimes the HGTV “reality” home shows do a better job at suggesting practical retrofit solutions, especially those that don’t require gut jobs that can cost more than new construction. In the same vein, authors describing passive home standards to the public should illustrate the “typical” existing home and the typical costs versus the improved design.

  3. I am a building science nerd
    I am a building science nerd – I admit it. I am in the minority when it comes to home buyers. There are 2 huge issues facing any advanced building standard or program – existing building stock and the perception (for some reason only applying to residential buildings) that residential buildings appreciate in monetary value without needing any continual investment; second – builders have their customers’ best interests in mind. The truth of the matter is that all residential buildings have an engineered life span of about 50 years (with sub-assemblies (like HVAC systems, kitchens, bathrooms, etc.) even less), so every residential building that hits that age should be gut/rehabbed to meet the current building codes, or razed so a new structure can take its place. This will provide all home buyers (who are not building science nerds) with the best possible living conditions – but runs completely contrary to the real estate and mortgage lending communities who are very powerful lobbyists. And home builders are in business to make money just like every other business. If the building codes say “do this”, they will. And if the customer wants a certain – additional – level of technology/attention/complexity/certification, they will do it. At significant additional cost because it is not their normal, optimized, code-compliant method. But increase building codes to match best industry practices and then all the builders will figure out how to make profit margins using new products/methods and all non-nerds who are having houses built will benefit. If these 2 areas are not addressed, then all these programs in this article will remain the boutique programs they are today. [It would be informative to see how many conventional building starts were performed in the past 5 years compared to the building starts for the programs mentioned.]

  4. RJ – I’ve been watching those
    RJ – I’ve been watching those same shows forever, and I continually see sub-optimized solutions foisted on the unknowing public. In most of those cases, the best option would be a gut/rehab because the actual comfort/cost improvements will not be realized by the quick/cheap “Bandaid” fix illustrated. I just saw a new episode of those guys in Mass. (you know who) talking with their HVAC sub who was going to run flex duct around the entire knee wall area in the third floor to provide conditioned air supplies (see Allison’s previous articles on duct work). This is a gut/rehab project and this is the industry best practice? Or recommending extremely costly geo-thermal HVAC systems for a gut/rehab of an 1800’s multi-story house whose walls have 70% windows and only cavity insulation – in New England. Buyer beware on all these shows.

  5. The PBS TOH guys lost touch
    The PBS TOH guys lost touch with reality twenty years ago. Even Norm does not show up very often anymore.

  6. Great points, John. I
    Great points, John. I especially like your statement: “Mostly I keep learning what the next question is.”

    Will you be at the conference in Philly?

  7. RJ, come to the Affordable
    RJ, come to the Affordable Comfort (ACI) conference sometime and you’ll find plenty of building scientists who are immersed in the world of existing buildings.

    Good suggestion about illustrating how passive house works in existing homes. I’ll do that sometime before the end of this year.

  8. David, gut-rehabs every 50
    David, gut-rehabs every 50 years would be great. I don’t see that happening, though. But you just reminded me of Stewart Brand’s wonderful book, How Buildings Learn. He describes the evolution of buildings as they age and talks about a lot of factors that affect what happens to them, including the insane way real estate values trump many other factors.

  9. Not this year. This is my
    Not this year. This is my off year for conferences. Perhaps next year. RESNET will almost be required with the changes in QA. ACI and/or Passive House would be great.

  10. Glad to know I’m not the only
    Glad to know I’m not the only building science nerd. I definitely missed my calling.

    To your point, I understand where you’re coming from but as we all know location drives the cost of real estate. Gut rehabs happen but the value of the land must make up a disproportionate % of the value of the property as a whole. Ex: 1920’s 2Bdrm dumpy bungalow in LA selling for $700k. Also with upwards of 20% of housing located inside a HOA controlled neighborhood I don’t see how the gut rehab is going to occur on a substantial scale.

    my 2 cents

  11. I’m sure this discussion will
    I’m sure this discussion will continue for, well, as long as the two standards exist. My first general points have to be that 1) the PHIUS standard has as many flaws as the German, and 2) having two standards with the same name adds a whole new level of confusion to the primary goal of getting the N American Public to understand that much more comfortable, efficient, healthy… homes are possible and affordable.
    Let me digress. As a Building Science Nerd of many years, I’ve come to believe that the term “Energy efficient single family home” is an oxymoron. In that same line of thought, I find it very telling that the first European PH was a four family building, while the first N American PH was a single family home.
    As long as the American Dream remains a single family home, especially in extreme climates, not only will the PHI standard be prohibitively expensive to meet, but homes will continue to be a growing contribution to climate change and rapid fossil fuel depletion.
    If we are going to learn to live sustainably, we need a paradigm shift in the way we think of “homes” rather than “making the program fit the reality of North American buildings”. Thankfully, many young people are embracing urban living and indeed even the PHIUS folks are now focusing lots of attention on multi-family projects.
    I hope all the N Americans reading this discussion will pause a moment and consider how your bias towards single family homes affects your view of the broader PH discussion. And then note that the large majority of folks promoting the PHIUS standard are N Americans. Perhaps it’s not only time to embrace the Passive House Approach, but to reconsider the single family home as the heart of the American Dream.

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