Learning from Kranichstein

Passive House in Kranichstein, GermanyHow do you design a building that is quiet, comfortable, and costs substantially less to operate?

One tool is an energy-modeling software called the Passive House Planning Package (PHPP) developed by a German physicist, Dr. Wolfgang Feist. He developed the software as a relatively easy way to determine compliance with the Passive House construction standard for low-energy buildings. The PHPP model for the first Passive House–a townhouse development in Kranichstein, Germany, near Frankfurt–is included with the software. It is easy to change the location of the project using the provided climate data sets. I “moved” the building to New York City to see what modifications to the building envelope would be required. It turns out that it is much easier to build a Passive House in New York than in Central Europe.

I made a few changes to the energy model.

First of all it seemed unrealistic to expect New Yorkers to use a clothesline to dry their clothes, so I included an electric condensing dryer. Maybe it would be even more realistic to omit the washer and dryer entirely, since most New York apartments don’t have them. But, let’s give the imaginary tenants the city luxuries of a washer/dryer and even a dishwasher.

Second, I added air-conditioning. I know there are hippies crying right now that we should just suffer through the summer in order to save Gaia. When it is 90 F/ 32 C and 90% humidity at night, I don’t care about the environment, I want my A/C.

Third, because of the A/C, I decided to change the heat source from a boiler to a mini-split heat pump. That way, one piece of equipment can provide heat in the winter and cooling in the summer. These units are common in penthouse retrofits in the city. They are also much quieter than window A/C units or through-wall PTAC units.

Fourth, I changed the heat recovery ventilator (HRV) to a model that is available from a US distributor and changed the energy model assumption that tenants would open windows at night for cooling. Even when we get cool summer nights, it is often impossible to leave the windows open because of street noise. It turns out that the efficiency of HRV’s has increased since 1991. Instead of 83% heat recovery efficiency, you can now get 92%.

Then, I eliminated the earth tube sub-soil heat exchanger. These are very common in Europe, but many Americans are skeptical about the potential for mold growing in the underground air-supply pipes. Until there are some examples of earth tubes successfully installed in New York, I will hold off on assuming that developers or homeowners will want to use this technology. Although if you want to be the first, I’ll be more than willing to work with you.

[update] I also changed the building from a corner to a mid-block location with shared walls on both sides. This is a much more common situation in New York.

Finally, I drastically reduced the amount of insulation. Instead of relying on rules of thumb or guesswork, the architect can use the PHPP energy-modeling software to determine exactly how much insulation is required for the specific building. In this case, because New York gets a lot more sun and has milder temperatures, a lot less insulation is required. The urban density of the city which makes shared walls more common also significantly reduces the heating and cooling loads.



Walls: 275 mm EPS; Roof: 400 mm blown mineral wool; Sub-slab: 250 mm EPS

New York City

Walls: 50 mm EPS; Roof: 400 mm blown mineral wool; Sub-slab: 25 mm EPS

With substantially less insulation, this hypothetical NYC building uses less than the Passive House limit of 15 kWh/m2a for heat and is below the same threshold for cooling, at 9 kWh/m2a. The electricity use is higher than in the Kranichstein example because of the clothes dryer and the heat pump. This building uses 83 kWh/m2a source energy (called primary energy in PHPP). This is still below the Passive House limit of 120 kWh/m2a.
Total energy cost for the apartment is about $1250 per year for electricity and $50 per year for gas.  That’s $108/month for all of the electric, heat, and hot water in a 2,000 SF apartment.

Given the low utility costs, a developer could easily include heat AND air-conditioning in the rent as a way to distinguish the building in the market.