How Do Passive House Retrofits Actually Perform in California?

I recently went to the ACI national conference in San Francisco and heard a very interesting talk by Jeremy Fisher and Brennan Less of the Lawrence Berkeley National Laboratory (LBNL). They are studying deep energy retrofits in Northern California—a topic of great interest to me, because I first became interested in the Passive House (PH) approach as a way of retrofitting existing homes to cut their energy use drastically.

At this talk they presented information on energy use in six of the case study homes that they have so far been monitoring, although they had only just gotten started with some of them. In three of their six deep energy retrofit cases, PH principles were used to guide the retrofit process. One of these case studies was the first certified PH retrofit in the United States; the other two are not certified as PH retrofits. One of their case studies, a retrofit in Berkeley, was featured in my first book, Homes for a Changing Climate, while the certified retrofit case study in Sonoma was covered in Recreating the American Home: The Passive House Approach.

How did the energy use in these PH retrofits compare to the other deep energy retrofits that they are studying? The researchers don’t have a quick answer to this question, partly because on some houses there were only a couple of months of energy monitoring data, and only a couple of weeks in one case, and also because it is sometimes hard to make fair comparisons.  Should you be comparing actual savings pre-and post-retrofit? What if the size of the house changed during the retrofit process? Is comparing energy use post-retrofit on a per-person or a per-square foot basis more useful? Answering the original question, it turns out, is not easy.

The researchers did say that, based on the limited data that they have, the PH retrofits succeeded in sharply reducing heating energy use and, according to client testimonials, making their houses much more comfortable.

In our climate, though, heating energy use is not the only energy use to target for reductions. Water heating energy use and what they are calling discretionary energy use, or plug-in loads, also need to be targeted for reductions. Simple approaches to reducing energy use are better than complicated technologies, because those are more likely to break or cause problems that the homeowner may be slow to notice.  And, what is essential for a successful deep energy retrofit is low-energy user behavior.

Comparing the heating energy use for just one month across five of the houses revealed that the three PH retrofits used 0.16 kWh per square ft., 0.04 kWh per square ft., and 0.11 kWh per square ft.—all tiny amounts. The two deep energy retrofits that had not used the PH approach used 0.19 kWh per square ft. and 0.41 kWh per square ft. Two of the PH retrofits relied only on electric baseboard heaters, which the researchers applauded for their low cost and ease of use and maintenance. The home with the lowest heating energy used a mini-split heat pump. Keep in mind, though, that, without very well insulated, airtight building envelopes, it would be impossible to keep a home’s occupants warm and comfortable, and still use very little energy, with only a mini-split heat pump or baseboard heaters.

As anyone knows who understands the basics of the PH approach, having an airtight building envelope is critical to maintaining comfortable temperatures indoor, while using little energy to heat and cool a house. The researchers reported the following airtightness measurements for the three PH retrofits: 1.1 ACH at 50 Pascals, 0.45 ACH at 50 Pascals, and 2.45 ACH at 50 Pascals. For the three case studies that didn’t use the PH approach, they reported these airtightness values: 5.8 ACH at 50 Pascals, 6.5 ACH at 50 Pascals, and a whopping 11.1 ACH at 50 Pascals. That final house had just been enrolled in the study because it is a LEED Platinum house. They only had a couple of weeks worth of energy use data, so no energy use comparisons could be made but, with that air leakage, I very much doubt the house will win any accolades when it comes to heating energy use.

The details of what strategies were used to cut energy and how each home was renovated were fascinating, but too long to reproduce. My take-home lesson was that the PH approach delivers as promised, yielding comfortable homes with low heating energy use. But user behavior and minimizing plug loads are critically important contributors to the success of any deep energy retrofit—and both of those strategies are very low-cost options. As Linda Wigington of ACI has been saying for several years, we need to figure out local solutions to deep energy retrofits that are cost effective and can be widely applied.

The state of California has a mandate to reduce carbon emissions by 80% from 1990 levels by the year 2050. To get there, deep energy retrofits of many homes will be required—something like 1.5% of the existing stock every year. (Switching to electricity for as much of our energy needs as possible and decarbonizing that supply will also be critical.) This study is very helpful for those of us figuring out how to sharply and cost-effectively reduce energy use in homes in California.