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,

Kat

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!!