Why now, more than ever, all new building should be Passive House.
A recent post, European engineers' recommendation: Stop recirculating air in buildings, got a lot of pushback from American readers. Most American homes and office buildings use moving air to transfer heat, with a bit of fresh air added to keep the CO2 levels manageable. In Passive House (or Passivhaus), the heating and cooling is kept separate from the “hygiene ventilation” system that exhausts stale air and delivers fresh air. As engineer Ted Kesik noted in response to the post, you need a well-insulated building and a heat recovery system to do this efficiently.
As for the claim this would increase energy consumed in buildings 4X, the example that was given in one of the reader responses dealt with the business-as-usual, code minimum, energy pig types of buildings that are prevalent in North America. If we build buildings that rely more on passive rather than active systems and design them to high levels of energy efficiency, the conditioning of ventilation air is not such a big deal.
Essentially, Ted Kesik and the Passivhaus people are saying that you can't separate a mechanical system from the building design. In fact, the building design is almost a result of the mechanical considerations. As Frank Sinatra sang of love and marriage, they go together like a horse and carriage.
Reyner Banham said this, too, in his 1969 book The architecture of the well-tempered environment, noting that historically, Americans have taken a different approach to building, and to heating or cooling. In Europe, many buildings were built out of heavy materials with thermal mass, and the structure was a prime controller of environment. In North America, buildings were newer and had “regenerative” mechanical systems that ran on fuel to keep warm, and soon after, electricity to keep cool. Banham wrote about how American houses and buildings developed differently:
… the abundant timber of which lightweight American houses were built provided abundant fuel for the high-performance Franklin stoves and Rumford fireplaces that heated them… and the skimpy thermal performance of these timber buildings made the invention of high-performance, quick-heating stoves environmentally necessary. The history of environmental management by the consumption of power in regenerative installations, rather than by simple reliance on conservative and selective structures, is thus a predominantly American history.
I went back to my Banham in an attempt to understand the Twitter discussion that followed my post, which was pretty much a Passivhaus bashing or, at best, disparaging session. These are all people in the HVAC business, including people like Nate Adams, AKA Nate the House Whisperer, whom we have mentioned before in our Electrify Everything discussions.
This what Banham described as the alternative American tradition, “a reflection of the unusual problems and advantages of US conditions. The problems were those lightweight structures in extreme climates wherever Americans built in wood, and the advantages were those of the relatively lightweight culture that many Americans took westward with them into a zone of abundant power.”
This attitude still prevails: Passive House is too expensive (and compared to light and fast code-compliant level of American construction, it is more expensive; but compared to the “pretty good house” approach, it is not that far off), so the answer is to throw a heat pump at it, a high-tech solution, and barely mention the building fabric or envelope. And you can't blame them for thinking this way; as one reviewer of Banham noted,
Banham traces the events that led to the architectural profession's abdication of responsibility for the indoor environment. Specialized fields emerged to take over this responsibility, and today's mechanical engineers who design and build and operate HVAC systems in buildings large and small are isolated from the construct of the building as an integrated system require a comprehensive view of the building's functions, structure, operation and use.
In the discussion of the previous post, Nate Adams recommended that I watch his very entertaining video, Bad Ass HVAC, to learn how he designs a system that “can deliver the 6 Functions of HVAC: load matching, filtration, dehumidification, fresh air, mixing, and humidification.” It is impressive but does raise many questions. Nate promotes Mixing: “If your home has a central duct system, you can mix the air in your house by turning on the fan for your HVAC. We’ve found mixing to be helpful with comfort and air quality problems.” Benefits of mixing:
- Even out surface temperatures and reduce hot/cold spots
- Even out temperature differences between rooms
- Reduce air quality differences namely CO2
- Provide filtration
- Provide fresh air if you have a system installed
But as noted in the previous post on air quality, we don't want mixing. In Europe, engineers are recommending that recirculating air systems be shut down. We don't want to move the virus all over our homes, we don't want to depend on filtration, and we do want fresh air. And as for surface temperatures and hot or cold spots, that's the job of the building fabric. Nate mentions that mixing air will raise the Mean Radiant Temperature (MRT) of the walls (responsible for about half of our feeling of comfort) but I am not convinced; Robert Bean and Alison Bailes have taught me that MRT is a function of the building fabric, and is not an issue in a Passive House design.
Nate shows this ASHRAE chart, which recommends keeping humidity between 40 and 60 percent, but writes, “Our recommendation is to stay in the 30 to 50 percent relative humidity range. Aim for between 30 to 40 percent in winter (less if in climate zone 6 or above), and between 40 to 50 percent in summer.” That's because he is worried about condensation and mold, moisture leaking through cracks. He says, “If you need more humidity than a water saver model can deliver, you might need to consider air sealing your home.”
But here again, all of these are problems that come from having a crappy building envelope. That's why you should take a “fabric first” approach; air seal your home first. Get window inserts. He is acknowledging that even his wonderful Bad Ass HVAC cannot solve every problem.
Fabric before furnace
I want to be clear here that I am not being critical of Nate Adams, who is primarily working with those existing houses that as he notes, “are literally 98 to 99 percent of the problem.” His clients are getting a Bad Ass HVAC system.
The problem is the American Way of Building, as Banham described it: fast and light, and if you have a problem, throw smart tech and cheap fuel at it. And of course, the failure of architects and designers, who have abdicated their responsibility for indoor comfort, designing without consideration of the consequences for the indoor environment, and just handing the whole thing over to the engineers and contractors to solve it for them.
This is why I like Passive House and believe it is the future of healthy, efficient building; it puts the building fabric first.
- The engineering for energy performance and comfort are inseparable from the architecture.
- The H and AC are separate from the V, so that there is no recirculation. Instead, as Bronwyn Barry described it, “Direct exhaust from wet rooms & fresh supply to living spaces is going to become an essential feature of EVERY building.”
- The walls and windows are good enough so that you don't have MRT or condensation issues.
Then you have more than a bad ass HVAC system, you have a bad ass home.
Why now, more than ever, all new building should be Passive House.