Freshome shows a lovely house that the architects, Klopf Architecture, call the Net Zero Energy Modern House, built in Cupertino, California. But what is Net-Zero? The Living Building Challenge writes:
Net Zero Energy is quickly becoming a sought after goal for many buildings around the globe - each relies on exceptional energy conservation and then on-site renewables to meet all of its heating, cooling and electricity needs. Yet the true performance of many developments is overstated – and actual Net Zero Energy buildings are still rare.
The owners were very concerned about the environment, specifically about energy and resource efficiency. They directed us to use materials that would last as long as possible while avoiding "food for termites" and designing a high-performance sustainable home.
The house has a lovely modern design that is reminiscent of the great Eichler houses. However the architect makes some choices and claims that I think merit a bit of discussion.
The house isn't small, but the owners work from home and it is not grossly excessive. There is a very generous and dramatic wall of north-facing glass that is perhaps more than one needs if one is really concerned about energy, but is made from what they call high-performance windows.
The house is built primarily of plastic foam, with insulated concrete forms below grade, and structural insulated panels above. While there are many who would question whether we should be building with this stuff, (See here ) it is effective at delivering a well insulated and tight house.
The house is heated and cooled by an air source heat pump, which has become the system of choice for many green builders, and which which the contractor describes:
Traditional furnaces and air conditioners burn relatively large amounts of energy in order to create or remove heat, respectively. Instead of trying to directly create or remove heat, a heat pump uses a relatively small amount of energy to simply move (or pump) heat from one place to another.
This is not strictly accurate. Heat pumps are simply air conditioners that are capable of running backwards, and in the cooling cycle do not operate differently. The heat pump is more efficient than a traditional electric furnace, but in this case the heat from the heat pump is distributed via a radiant floor.
The contractor claims that "Because the heat originates at the floor level and rises, in-floor radiant is far more efficient than ducted forced-air systems that often distribute over-heated air from the ceiling (where it tends to stay)." This is not actually true; Martin Holladay notes, "a Btu is a Btu. The overall efficiency of a hydronic heating system is basically governed by the boiler; the distribution equipment plays only a minor role in system efficiency."
I am not convinced about the merits of radiant floors because of the thermal lag, the time they take to adapt.. (see also Is Radiant Floor Heating a Good Choice For Green Homes?) However the architect makes a good case for their use, noting that it is not designed as a system to be turned on when you need it, but is programmed "using relatively few BTUs to heat up a very insulated envelope for the whole 24-hour period."
The roof is covered in a 13.4 kW solar photovoltaic system that the architect says is "sized to cover all the energy use in the house." The architect notes that " Over the last year of electrical usage, with electric cars charging for part of the year, they had a net-meter electric CREDIT, not a payment due for the year." And this is with the owners working at home and while charging electric cars.
This is a very beautiful house; there is so much about it I love, from the furniture to the natural light to the Eichler-like gray beams in the ceiling. I have previously expressed some concerns about the term Net-zero energy, calling it a "useless metric", but looking more closely at this house and at the criteria being established by the Living Building Challenge, I have been revisiting the issue. This house has the classic charm of an Eichler home, with all the natural light and air and open-ness that are the trademark of the midcentury modern classics, without the energy bill. That's nothing to complain about.