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!!

15kWh is dead. Long live 15kWh.

Hi Everybody:

Welcome to my brand new blog! I hope to keep you updated on the progress of Passive House and PHIUS, and to spark constructive discussion along the way.

On that note, I’d like to start this blog by addressing what we at PHIUS think will be the central issue for Passive House in 2012: fine-tuning the Passive House standard’s certification criteria, which were developed for the central European climate and market, to the unique climate and market needs of the United States and Canada.

Having evaluated five years of data from PHIUS’ portfolio — including more than 100 reviewed and/or certified projects of varied building types in the United States and Canada (thanks to the good work of the Passive House community) — we came to the conclusion that it’s time to:

  • Allow for a modification process to the rigid annual heating and cooling requirement of less or equal to 15 kWh/m2yr or 4.75 kBTU/ft2yr for the North American continent’s more extreme climates.
  • Define what has been missing all together so far – a stringent requirement for the third load which is the significant energy consumed in North America for dehumidification.

This idea that we need to adapt the standard to various regions has taken root around the world from domestic energy experts like Martin Holladay, Alex Wilson and Marc Rosenbaum to Passive House groups from other countries, like the Swedes. From our perspective, we do not feel that this adaptation guts the value of Passive House.  Instead, the goal is to introduce a new balancing act into the standard.

On the one hand, we need to let go of the illusion that there is a god-given magical number that can cost-effectively be adhered

Average solar radiation exposure is just one factor that differs dramatically between Germany and the US. (National Renewable Energy Laboratory, European Commission)

to worldwide. (The notion behind “15 is dead.”) In North America, in some DOE designated climate zones (specifically, top 5 and up and bottom 3 and down), the cost of meeting the mark is problematic. The added expense required to hit 15 is not recoverable over the life of the building.

On the other, we need to maintain the value of having a single baseline. (“Long live 15.”) The first 100 projects or so have indeed validated this benchmark as an appropriate design starting point for excellence in high performance building in North American climates.  The baseline builds into the standard a necessary rigor and elegance that gives the brand its strength.

The PHIUS Proposal

PHIUS is proposing to the PHIUS Tech Committee — composed of industry and policy leaders from the United States and Canada — to leverage the PHIUS dataset of 100 buildings, and to solicit feedback from the consultant community to create a new protocol that will allow Passive House professionals to determine practical modifiers to the standard to address climate, small home and retrofit scenarios. This new protocol would be used to determine the acceptable modification ranges for:

  1. Additional peak load allowances (heat, cooling, latent) per climate other than the cool moderate reference climate.
  2. Annual space conditioning requirements (heating, cooling and dehumidification) that follow from acceptable higher peak loads per climate other than the cool moderate reference climate.
  3. Annual source energy requirements. Based on the new annual requirements per climate other than the cool moderate reference climate.
  4. Airtightness criteria that will have to be re-evaluated based on climate; hygrothermal criteria might have to be added as well.

By introducing this protocol into the standard process, we start with the baseline design of 15kWh for a climate dominant space conditioning need (might be heating or cooling or dehumidification). It is business as usual from here. If you meet the governing energy metric in a cost effective way you are done. Congratulations!

If you find yourself in a climate that is more extreme, you will need to benchmark the economic benefit expectations. Using the new protocol, the level of insulation can then be fine-tuned within a small range of above or below the energy metric of 15kWh without sacrificing the quality and comfort criteria. Tuning down the conservation level in return fine-tunes the annual energy requirement specific to each project without having to resort to prescriptive measures, possibly wasting savings potential.

In this manner a new Passive House balancing act is achieved – 15kWh remains the only fixed criteria, while deviations to both sides are allowable within clearly defined parameters.

We envision this to be a simple evaluation process to determine cost-effectiveness that will not add significantly to the consultant’s workload or complexity of the design tasks. The recently announced PHIUS+ certification program — that introduces a level of quality assurance into the Passive House standard — can also be used to assure that all building science and quality criteria are upheld regarding this new protocol.

From Movement to Mainstream

Addressing the issue of modifications is a pivotal moment for high performance building in North America! Its time has come because we now have the dataset necessary to make these adjustments. PHIUS Tech Committee evaluation of over 100 projects represents the first study of an unprecedented volume of data on implemented Passive Houses in North America.

It’s also a pivotal moment because we are proposing a departure from a single one-size fits all international standard. We welcome everybody who would like to contribute to this effort.

This proposed shift of course will not happen overnight. There is a lot of work to be done.  It will be a careful, collaborative process driven by the experience and data gathered from real-world experience. I’ll be providing updates here, and PHIUS will provide updates via its e-newsletter and Web site in the next several months.

This progress is very exciting to me! I have been waiting for this for a while. We are finally tackling the contradictions that we all have been struggling with and it seems we are about to solve them.  In the process, I look forward to your comments and input — I hope you’ll use the comments feature (just click on the little bubble below) to weigh in!

Katrin