The Anchoring Passive House Principle: Equal In – Equal Out

We are glad that this topic of local climate-specific refinement of the passive house standard has sparked such a lively debate and discussion. Clearly there is a lot of interest in the topic! That’s really good.

I’d like to take this opportunity to explain why our proposal to make passive house accommodate North American climate variations does not challenge the standard’s core principle.

We consider the passive house standard’s anchoring principle to be its commitment to comfort through near-perfect balancing of losses and gains. To date, meeting this goal has required minimizing peak load (the worst case scenario of heat loss on the coldest day of the year) to approximately 1 W/ft².  At this peak load only a very small back-up space conditioning source is needed to keep comfortable.

The original idea of this balancing act — and of the peak load target of 1 W/ ft²  — pre-dates the European passive house standard by more than 20 years. The peak load target was first introduced in the U.S. inaugural model energy code in 1975 (a code created as a result of the oil embargo in 1973). Today’s IECC commentary explains the principle very simply: If you have a room that is 100 ft² in area, 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.

Of course this is a very simplified way to describe the conceptual anchor of passive house, the light bulb being a placeholder for the sum of the internal energy sources matching the losses through the envelope, including ventilation losses. Equal in – equal out.

Now let me explain why we do not consider the 15kWh metric as either a magic number or an anchoring principle, but rather as a derivative of that peak load assumption. The 15 kWh/m²yr (or 4.75 kBTU/ft²yr) annual heat demand metric is used to identify the amount of heating energy consumed over the period of one year.

That 15kWh figure was derived for the German climate from the peak load target figure (1 W/ft².). It so happens that Darmstadt, Germany is one of the climate sweet spots where limiting heat loss to that 1 W/ft² (10 W/m²) threshold is possible with relatively reasonable and cost-effective amounts of insulation. Germany’s climate is called “moderately cold” for a reason. The delta T is not that great. Heating is the only climate issue that needs to be addressed. That makes the design process — relatively speaking — easy and clear-cut as there are no additional conditions, such as cooling needs or dehumidification, to consider.

We know the design recipe components necessary for building a European passive home envelope that keeps heat loss smaller or equal to our internal gains, or, in other words, meets the 1 W/ft² (10 W/m²) peak load criterion: we calculate the required amount of superinsulation; we use high quality windows, we assume airtightness at 0.6 ACH50…

In Central Europe, we reach the 1 W/ft² (10 W/m²) peak load target with an approximate insulation level of 14 inches of R-4 for a well-oriented, compact single family home. A practical and attainable scenario — in Darmstadt.

From the specs for that same house, we can calculate the total energy usage for heating over the period of one year based on the climate-specific heating degree days. For the Darmstadt climate, that annual heating demand calculates to approximately 15 kWh/m²yr or 4.75 kBTU/ft²yr. No rocket science. Simple energy balancing.

Therefore we do not consider the annual heating demand (15 kWh/m²yr ) as a fixed and given part of the “functional definition” of a passive house,. It is a consequence of designing to meet the peak load criterion of 1 W/ft² (10 W/m²) in the particular Central European climate.

The 15 kWh figure is a good median starting point for passive designs, as it is derived in a median type climate — median delta T, median length of time when heating is required — where the peak load balancing act is fulfilled almost perfectly. But this is only one specific climate with one specific combination of climate characteristics. This 15 kWh criterion will need to flex as the delta T and amounts of heating degree days change and the underlying principles are applied in different, more extreme climates that deviate significantly from the median base line climate of Central Europe.

Aside from heating, the existing standard is limited even further when we factor in additional North American climate issues such as cooling and dehumidification.

To reiterate:

  • We consider the passive house standard’s anchoring principle to be its commitment to comfort through near-perfect balancing of losses and gains.
  • To date, meeting that balance has meant minimizing peak loads to approximately 1 W/ft².  In Central Europe, that happens to pencil out to the 15kwh average consumption figure.

But, we will demonstrate in a future blog post that achieving that peak load goal (and therefore the 15kWh max threshold figure) is next to impossible in some climates, and definitely not practical. Because the peak load of  1 W/ft² doesn’t apply everywhere, neither can the 15kWh.

Other building science experts, Including Marc Rosenbaum, agree that the current standard has limitations, and offer their own ideas about addressing the issue. (Check out Marc’s proposal for New England.) The good news is that despite all these reasonable challenges to the notion of a single standard, the design principles still hold true, and the peak load target remains a useful tool as a benchmark — even if not an absolute in every single climate zone.

As we develop the specifics of our proposal, we look forward to discussion and debate among all interested and knowledgeable parties. Combined with the growing body of data we’ve accumulated from passive house projects that have been built around the continent, we believe we can introduce the flexibility that will make fundamental passive house principles mainstream practice.

In the meantime, look for more on lessons learned, climate complexity and how to possibly refine annual heating and cooling demands while maintaining the underlying physics principles in upcoming blog posts. Stay tuned!!

14 thoughts on “The Anchoring Passive House Principle: Equal In – Equal Out

  1. Don’t understand what’s different, then: PHI standard currently says: either consumption requirement <=15 kWh/(m2 a) OR load <=10 W/m2. If you intend on keeping the load requirements: does that mean there are no changes to what PHI advocates, after all?

    • Kara,

      You are exactly correct! The basic passive house principle of approx. 1 W/sqft that was defined already in the 70s in the US and that the PHI refers to as well is just physics, can’t change that. That relationship is pretty crystal clear. It can’t be easily achieved in every climate, but that is a different story and we will talk about that and what to possibly do about it in the next blog post.

      The discussion is about REFINING the 15 appropriately according to climate with more granularity, as it is a derivative of the peak load criteria based on climate. Using 15 as an etched in stone criteria in other than the European climates is approximate at best if the climate is very different.

      No intent on changing it all together, its a great baseline and generally in the ball park (I think that “changing” language was actually introduced by the petition people).

      We intend to propose improvements, democratic tech committee sanctioned, to inform the greater discussion about it and make a proposal to the community solidly based on science. Said so in the very first blog post.

      Lets make a great standard even greater and have this discussion, that’s all!

  2. But how is an either or condition “etched in”, and not already flexible enough? If you are not questioning the 10W/m2 requierement: what aspect of the PHI standard – apart from the lack of democracy in its maintenance most seem to agree on – do you see wanting?

  3. Kara,

    In some climates you cannot meet either comfortably, in some climates you can exceed both easily. It depends entirely on climate characteristic combinations. To take a median climate baseline relationship of peak and its annual derivative and declare it appropriate for all climates imposes economic penalties in very cold climates (and slows adoption) and in warm climates it misses savings potential (and is perceived as not much better than standard and as possibly not worth it).

    I will get into it in more detail in the next post.

  4. To me, the defining or differentiating feature of Passive House is that separate requirement on space conditioning energy, it’s not just about total site energy or source energy. I perceive a certain elegance in its three-hurdle structure: First, you must get the energy for space heating and cooling below a certain level using only passive envelope means – insulation, shading, air-tightness. And the only “active” system that counts is heat recovery ventilation. Second, you must get the total source energy below a certain level with your mechanical system design and appliance selections, and the only renewable system that counts is solar hot water. Finally you can optionally go net zero on electricity, site total, source total, or CO2 using PV or other on-site renewable electricity.

    As I understand it we are going to retain that defining feature of limits on space conditioning energy, to be met by passive means. We are talking about a couple of tweaks. The main question we are reopening is how good is good enough, is that 10 W/m2 peak heat load threshold really the economic optimum? We are giving consideration the criticism that it forces uneconomic levels of insulation or makes PH impossible in certain climates. We have been going around telling people an economic justification for that peak heat load performance level, that this is the point where you can deliver the space conditioning by ventilation air, simplify the mechanical system, and tunnel through the cost barrier. That you can afford more insulation than you think, because you can take cost out of the mechanical system. It’s my understanding that in Europe this is readily demonstrated, as hydronic heating systems are typical and you can get rid of a lot of expensive plumbing. The problem with this economic story is that it, let’s say doesn’t always hold up that well in the US. If I take out a $6000 gas furnace but put in a $6000 heat recovery ventilator, I haven’t so much tunneled through a cost barrier as walked over a mushy flat spot.

    So what we are talking about doing is rerunning the economics on where is the point of diminishing returns on envelope measures, or, at what performance level is that cost barrier tunnel, in different climates. Personally I’m not convinced much of a change will be needed but it seems like a fair question.

    I have argued repeatedly that on fairness grounds, building performance criteria should be independent of climate, because the atmosphere is a commons and CO2 blows all over the place. I still believe this is true when it comes to the Primary Energy criterion.

    I have also argued that even the uniform peak heat load requirement is fine, and that things only become “difficult” or “impossible” if one insists on continuing a single-family-detached pattern of development – that the appropriate response is to huddle up in those extreme climates with duplexes and fourplexes, to change our cultural habits of design. Typical response is “that will doom us to a market penetration of zippo,” so, I’ve decided I’m okay with compromising on this particular point, on the uniform space conditioning requirement. Let us recheck the economics, with a good long time horizon, and see what makes sense to do.

    Lest anyone hope/fear that this will represent a relaxation of the standard, my understanding is that the primary energy factor for electricity in PHPP, of 2.7 or now 2.6, is not correct for the North American power mix and should probably be 3.0 or higher. That would represent a tightening of the standard. If we do both tweaks then we should have a by-and-large pain-and-virtue-neutral adjustment.

    Brand identity politics are not my forte, but this seems like top-freezer vs. side-by-side to me. It’s still a “fridge”. It should retain the essential features and logic of Passive House and ground it more firmly in regional reality.


    Graham S. Wright, Ph.D.

    Principal, Wright On Sustainability
    Junior Pre-certifier, Passive House Institute US
    Chair, PHIUS Technical Committee
    Hygrothermal Analyst, Small Planet Workshop

    • Graham: most certainly it should be possible to question aspects of the PHI standard!
      However: I strongly doubt that economic considerations can be a sound basis for this. Foremost, because most factors in an economic prediction are rapidly fluctuating, prominently the cost of energy and rates of interest and inflation. And – while not being extremely fluctuating – the specific cost of measures like insulation, HRV, windows are by no means a constant. They typically come down with critical mass. There is a lot of guessing going into economic considerations for the long run – even when you correctly refer everything back to present value.
      I also wonder if cost are indeed a factor preventing further market penetration. It’s funny how the cost argument always pops up with energy efficiency, and never with granite bench tops! There are other factors far more prominent that decide on the affordability of housing than some added cost for efficiency.
      I agree that the cost tunnelling effect is not transferable into other cultures (the cost for a heating system in the budget of NZ houses are zilch – until later, a heat pump and some oil filled electric radiators are bought). Passive Houses have never sold on affordability, though, but always on better performance (and occasionally: status).
      While I do see some value in economic considerations, as often times what looks not viable on the face of it turns out to make good economic sense after careful considerations using the right methodology, a great idea has never penetrated the market based on it being cost effective.

      Yes, we should question the benchmarks – but not regards their economic appeal, but regards their merits in achieving measurable goals. The difficulty with any strategy in this regard is that we build now for the unforeseeable future (at least I find it extremely hard to predict the word in 100 years from now). The only promising strategy I can see in this light is doing the technically possible within the budget available – and I’d be wiling to compromise the granite bench top for better insulation, but others may not.
      I suggest that in cases where money is really tight, like social housing, it does not matter much whether we put a PH certificate on the wall, as long as the very best value for the money is obtained. If the project then falls through the certification cracks on needing 10.2W per square meter and consuming a calculatory 15.4 kWh/(m2a) – it’s still a really good house.

  5. Thinking about the marquee-level Passive House criteria, it seems to me, the Source Energy criterion is derived from moral and environmental considerations – what is your fair share of the earth’s capacity to absorb CO2 ?

    The Space Conditioning criteria are derived from a cost/performance consideration, subject to comfort standards, with a priority on conservation-first, and a drive to high performance (so high that as Katrin put it, the passive energy gains almost equal the losses, the building is nigh-self-conditioning.)

    The air-tightness criterion is derived from considerations of quality and durability.

    If, in considering adjustments to these criteria, we return to those same source considerations, then we are faithful to the spirit, story, and logic of Passive House and not subversive. This is mend it don’t end it, repair not replace, debugging not innovation.


    Graham S. Wright, Ph.D.

    Principal, Wright On Sustainability
    Junior Pre-certifier, Passive House Institute US
    Chair, PHIUS Technical Committee
    Hygrothermal Analyst, Small Planet Workshop

    • Graham,
      From what I’ve read, as you move to warm climates, the requirements of a Passive Building become “Passive” in a different way. Whereas air-tightness may not play such an important role because the parameters for what is “comfortable” begin to change. What are specific drawbacks you see with loosening air-tighness in the south?

    • I agree in principle with this approach. However, as previously established, the cost/performance considerations were rooted in a specific culture, where they continue to make sense: so what are you proposing to put in the stead of the European style central heating system? Again, for New Zealand, this will typically be: nothing, as people are happily living at below 14 degrees indoor temperatures (whether this is healthy is another story altogether; it’s certainly not comfortable, yet: there are no actual cost for a heating system of any kind with the typical house). Now: would I go and calculate the cost that occurred if houses were heated to a comfortable level? The cheapest option then may be a hot air system, decentralised or ducted – but can a high level of comfort be achieved that way with a poorly insulated envelope? Are we ending up with a European style central heating system out of comfort considerations after all?

      The space conditioning criteria are the weak link in the chain, and while they were derived from the economic context of a certain time and place, I question whether they can be maintained on this basis. I see room for putting them to the test, but on a more solid basis than situational economics. Even if every locale came up with their own criteria based on local specifics, this would have to be adapted constantly and in dependence of not only one but many factors. I do not think it sensible to have this sort of moving targets. I thus propose to rather looking in realms other than economics for clues on how to set these.

      • I see what you are saying Kara. Anyone who went to the latest PH NorthWest conference came away with a feeling that maybe Passive Houses aren’t delivering quite what they promise (in economics). But we easily see that Americans have an appetite for electricity. I was harvesting at a vineyard in Italy this past fall and the owner, when showing me my room, gave me thorough instructions on how to turn off a light switch when leaving the room.

        Our climate cut-off point for PH on the west coast seems to be San Francisco where the economics of comfort begin to fail. If Passive House was more appropriately called “Passive-Comfort,” then we’d ask…What is the most economical way for a building to interact with natural forces in order to provide excellent comfort of its inhabitants? I’m going to guess that relaxing airtightness south of San Francisco will have economic benefits without compromising comfort.

        This is an area where research in Europe has maybe gone astray. The CEPHEUS project may be limited in that it is focused on how to deliver the Passive House package in a warm climate, instead of asking, “what is the Passive-Comfort ratio for warm climates?” Fooling around in PHPP I can see that relaxing airtightness to 1.5 is easily offset by 1″ of envelope insulation. Whats more, it seems quite easy to get the heating load down to 8 w/m2.

        My question is…what are the adverse effects to relaxing airtightness? In more humid areas will there be problems?

  6. I immediately had the same reaction as Graham to Marc’s suggestion of looking at Primary Energy Consumption as the paramount criteria, for exactly the same reasons he (Graham) outlined. The result would be an unfair placement of mechanical efficiency before envelope efficiency in the efficiency hierarchy. Heating requirements do a much better job at reaching optimal gain levels in building components, the pride of the Passive House movement.

    Then, to further complicate, if we alter Primary Energy factors, we have to accept the reality of what should follow. Each region is going to look at their power intake and derive their own values.
    British Columbia, for instance would demand that it be 2.0 (just a guess) because of their dependence on Hydro. I don’t think that its a bad idea to adjust primary energy factors per region, but only if PE is not the paramount value in PH certification. The beauty of PH is that it should be very easy for two homeowners from different parts of the country to compare efficiency notes without a calculator, access to local utility data, a map, nor handbook. The more complicated the conversion of standards between standard and between regions, the more people will be confused.

    Next, I really have a problem with Marc’s per person suggestion. The obvious problem is, again, translation….Energy use intensity has become fairly commonplace in the low-energy movement. It is an excellent way of comparing notes between buildings, especially different types of buildings. I see his argument as valid on social grounds, but can’t see a solid rationale. Some people live in small houses without walls. There should be minimal roadblocks to people’s right to live creatively.

    Further, I don’t see the advantage to using Design Heating Demand. Again, I think Marc’s approach has amazing value as social change strategy, but might be failing to accommodate basic realities. Specific Heating Load is simple and universal. I know that PH’s in America are single standing homes, sometimes bigger than they should be, sometimes not sitting right in the sun. But PH is supposed to encourage smart developments, not be apathetic. You find this sentiment widespread in the PH community, so maybe best left alone.

    Katrin, I really love the central focus on the Heating Load. This really seems to be the key. Define what heating load is sensible for a what climate, with 10 w/m2 (1 w/ft2) as the base standard. This is a good solid platform.


    • Hayden,

      My name’s Mike Knezovich and I moderate the PHIUS blog (though I get behind sometimes, apologies for delays).

      We certainly welcome constructive technical discussion and criticism and I hope that’s evident from the string of comments here and in other posts, many of them yours. In my view, the post in question went fairly far afield from the original blog topic, and from building science issues relating to passive house, which is why it was removed.

      I’ll be happy to talk with you if you want to discuss it further. (I’ll email you separately with my contact information.) Meantime, thanks for participating and apologies for any confusion.

      Mike Knezovich

    • Mike,thank you for the response. Clearly, PHIUS can control conversation on its blog. But – correct me if I’m wrong – ’ this post is part of the series, announced in “15kWhr is Dead”, promoting PHIUS’s plan to market its own divergent standard as Passive House. Given this context, and the harm this would cause the high performance building community, it seems cynical and disingenuous to censor discussion of the drawbacks of this plan.


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