The anchor of the passive house principle

It’s been some time since I posted the first blog — “15kwh Is Dead, Long Live 15kwh.” It sparked a lively debate and discussion. That’s really good.

It also is clear that there is at least some confusion about what the core principle of passive house is. So, I will give it another shot before we move ahead with the tech committee and workgroup efforts on fine-tuning the standard for North American climates later this month. Again, as outlined in a webinar last month, once workgroup topics are established, everybody is welcome to join the discussion in a workgroup of their choice.

One common viewpoint voiced is that we like the clear-cut, definite nature of a single numerical standard. Yes, we do indeed. We are proposing that there might not be only one clear-cut certification criteria, but a select few, all derived from a single target.

That target – the universal anchor, if you will — is not 15 kWh. It is actually 1 W/ft² peak load. It has been the basis of all things passive house since it was first scientifically defined in terms of an energy metric in the early 1970s. That’s when scientists and engineers in the United States and Canada collaborated to formulate a response to the oil embargo and the threat of energy dependence. This peak load principle was cemented then in the very first model energy code created in 1975 and it still remains in the IECC 2012 edition.

Today’s IECC commentary explains it very simply: If you have a room that is 100 ft² big, and that room has a 100 Watt light bulb in it, you are meeting your peak load requirement and don’t need a separate heating system.

This is a very simplified way to describe the conceptual target of passive house: If you can minimize your losses so that the internal heat gains match those losses, you can get rid of the heating system. Respectively for cooling, it works similarly: minimize gains through whatever measures and with it the need for cooling.

This ideal thermal balance is the underlying physics relationship of passive house. It’s what makes a building passive. This balance of gain and loss is theoretically possible and desirable anywhere. Can it be achieved anywhere for any type of building? That of course depends on the climate, the max delta T and the pocketbook.

There are climates where one would need infinite amounts of insulation if using affordable insulation (approx. R-4). Of course, if one works for NASA or can afford 6+ inches of vacuum insulation at an R per inch of 60, sure, we can built a passive house in the Arctic, what say I, on the moon!

In practice, the climate sweet spot for this principle is the relatively narrow belt of the “moderately cool climate.” This zone is heating dominated only. That means no complications having to optimize cooling and heating against each other; there is not too much sun, and humidity is not an issue. The balance of losses versus gains in this climate pencils out to almost exactly 1W/ft² peak load with an approximate insulation level of 12-14” for a well oriented, small, compact single family home and essentially no cooling needs.

Bingo! By using superinsulation, middle of the road Solar Heat Gain Coefficients, very good windows (the now well-known design recipe for components in this particular climate), we achieve an annual heating demand on average of 15 kWh/m²yr or 4.75 kBTU/ft²yr. No rocket science. Simple energy balancing.

15 kWh/m²yr or 4.75 kBTU/ft²yr are not part of the “functional definition” of a passive house; it is a consequence of applying the passive house peak load target and the related strategies in a climate where it works almost perfectly.

To summarize: the peak load target of 1W/ft ² is the anchoring principle, since 1975. It is a goal, one more practically reached in some climate zones than others. That’s why it’s become clear that the idea of refining the annual figure to climate zones will result in a tightening of the standard in some climate zones – not a relaxation, as some generally understood the proposal to do.

Climate variation make things a bit more complicated — but not that much. More on that in a future blog post.

Thanks for reading,


10 thoughts on “The anchor of the passive house principle

  1. The PHPP program is a great tool and seen by those who enjoy building science as a most effective means of determining the best efficients. It allow optimizing materials to develop a structure requiring little energy to reach a comfortable human environment. The PHPP plus the other building tools such as WUFI and Therm has democratized the building process. However, it is a tool, and while the analytical goal can be 1watt/foot squared, (w/ft²) it is well to remember these facilities are being built for the fussy human. There is the academic viewpoint of the technically inclined, architects & engineers, versus, what I see as, the laymen who want a place to live. A modern ‘home’, or facility, needs to be functional and desirable somewhat better than a cave. It needs to be your place reflecting your individual desires and needs.

    To do this the PH designers should not make a religion out of reaching a ‘standard’; however, I am seeing a religion such as the one I, and many grew up with, might incorporated a day of rest, which does not seem to be happening with some true believing individuals, and might provide some balance. Watching our latest videos, and attending a good seminar at our local Seattle chapter of the Northwest EcoBuilding Guild I am more concerned with the lack of depth of knowledge of designers and the limited focus of the academically inclined. To broarden the passive house concept to a widening audience the designer needs to understand the scope of products available and the analyst needs to know whether satisfying a test result leaves one with a functional result.

  2. Kat,

    This post about the “Anchor” of Passive House is excellent ! Knowing that you are occasionally teased about your German to English trials and tribulations, I want to compliment you on this anchor metaphor! One watt per square foot peak load is the anchor that holds PH in place against the winds and waves of the environment! Dramatic! Memorable! I have a dozen movie scenes running through my head right now of ships (from wood to steel-sided juggernauts) being tossed about in the storm. I like it!

    I am curious, now that I know you are going to write a novel or two in your spare time, did you consider “foundation”?

    I do have a semi-technical question for you: How does the concept of meeting the peak load through the ventilation air relate to 1W/ft2 ?

    • Lance,

      Thanks, your post made me realize that it indeed has poetic potential :) , ships, storms and such.

      Yes, excellent question in regards to space conditioning through the ventilation air. As we all know, that is currently presented as part of the definition of a passive house. Unfortunately many times impossible to meet in most regions of North America. Most projects go for the annual heat demand threshold instead which unveils a conflict of the definition with the actual certification practice. If you can meet either or for certification, but you cannot meet the peak load criteria, instead you go for 4.75 or 15, then technically following the definition you are no longer a passive house…

      This is where things get a little convoluted and need clarification, in my opinion.

      I’d like to offer a different opinion on this. I don’t think it should be part of the definition, it should be a recommendation. This is how it is being handled in practice during the the certification process anyhow as mentioned above. I think it is a fine option if it is actually practical and can be achieved. More on that soon.

      And I assume you already had a sense, of course this sets the stage for the topic of the next blog post installment to continue this conversation. The novel goes on :)

      • Hello Kat,

        You wrote “as we all know ‘that’ is currently presented as part of the definition of a passive house.” ‘That, I assume, being the distribution of space heat via the ventilation system. Currently presented by whom? PHI? PHIUS? I have always understood the definition of a Passive House to be independent of the means used to distribute heat or meet the peak loads, as long as the annual heating demand, annual primary energy demand, and air tightness requirements are met. Yes, the peak heating load target can be used in lieu of annual heating demand for certification, but nothing I’ve ever read suggests that there is a requirement that heat be distributed via the ventilation system EVEN if the peak load is within the 10W/m2. I thought that PH criteria only makes the recommendation about heat distributed via ventilation, not the definition as you say. Am I wrong about this?

        Please clarify. Thank you!

        • Rachel,

          Maybe this is something lost in translation but here is what is published on the iPHA website as the definition of a passive house. It says

          “The global, climate-independent, Passive House definition is as follows:

          A Passive House is a building, for which thermal comfort (ISO 7730) can be achieved solely by post-heating or post-cooling of the fresh air mass, which is required to achieve sufficient indoor air quality conditions – without the need for additional recirculation of air.”

          It is possible that in their minds this also qualifies a point source heater. I understood that it should be integrated into the ventilation system when I read it. If the first is the case then maybe there needs to be a clarification of the language.

          They are also now coming around to saying essentially the same as I was proposing in my initial 15 kWh blog earlier this spring, that 15 is specific to central Europe and that it should be 20 in Stockholm and 10 or so in Rome. This had not been made clear previously as prominently (on home page!) as is now the case. It is interesting – good that we do agree after all. If you like to read up on it, here is the link:

          • Thanks, Kat! Your comments and the link helped. What gets “lost in translation” perhaps, is the fact that if the peak load exceeds the 10W/m2, then the ventilation system alone cannot provide the “thermal comfort” at peak times.

            In my climate, for instance (northern Minnesota), it is very hard to meet the peak load criterion; often harder than meeting the annual demand. PHPP allows this (using demand and not load) to meet the standard. Therefore, either the definition isn’t 100% accurate or there are certified passive houses that don’t meet the definition.

            All that said, as long as the protocol and path to compliance are clearly defined, I’m okay with it.


    • Lance,

      The heat carrying capacity of a ventilation system is a function of the volume of air it carries and the temperature of that air. When a ventilation system is sized only to provide fresh air, the amount of air it carries is relatively small. In order for that small amount of air to carry a lot of heat, it would need to be very hot. However, It turns out that at about 124 F, thermochemical decomposition (pyrolysis)of airborne dust begins. Since pyrolysis creates poor air quality, ventilation air must be kept below that temperature limit. Since the volume of air carried by a ventilation system designed only to meet fresh air requirements is small, and the maximum temperature of the air is limited by pyrolysis, the heat carrying capacity of the ventilation system is limited. For a ventilaiton system designed only to carry the amount of air required to maintain good air quality, the combined limits of ventilation air volume and maximum air temperature correspond to a heat carrying capacity of 1 W/ft2 of floor area. Typical forced-air heating systems are sized to carry heat, not fresh air; they are therefore much larger than Passive House ventilation systems.

      Am I making any sense?


      • Hayden,

        You are most definitely making sense. Thank you for a concise and accurate explanation of the parameters governing the ability to supply space heat via a ventilation system, and the relationship of these parameters to the concept of a peak heating load of 1W/ft2 (meaning, the desired peak load is matched to the functional heat carrying capacity of the ventilation system). Again, I learn more about the science and principles behind the numbers.


  3. Katrin,

    I think It is a mischaracterization to suggest that broad objection to PHIUS’s use of “Passive House standard” for its own certification criteria has to do with confusion about the 1 W/m2 and 15 kWh/m2a criteria of the existing standard. It seems fair to say that most people are fine with PHIUS adopting whatever numbers it wants. As the Passive House Petition makes clear, what people don’t like is the appropriation of “Passive House standard”, a term referring to the Passive House Institute’s well respected and established certification criteria.


  4. Rachel,

    It appears we reached the limit of the replies above :) the text got really skinny….

    So, turns out that in North America most projects don’t certify using the peak load criterion (no surprise in the extreme climates). But even in milder climates such as California it is easier to meet the annual heat demand than the peak load. There they can basically get away with a 2×4 to a 2×6. They could go for the peak load but that requires more insulation and better windows. But they easily hit the 15 so they don’t do it because of cost resulting in missed conservation potential. In the cold climates like yours similar “miss the conservation potential” happens in the other direction by overdoing the insulation when one is too focused on 15. The last inches do so little that the money spent might be better invested in a more efficient solar thermal system. At that point it really is no longer clear that 15 is the optimum. It is the optimum in central Europe if the peak load is met, but in Duluth you can’t meet the peak load to begin with (miss by a factor of almost two) it’s so cold.

    Here is what I think: 10 Watts/square meter peak load delivered through the ventilation air is not climate independent after all.

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