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,