Tuesday, December 23, 2014

Energy Efficient House Sprouts on Linden Lane

When last we visited the new house sprouting at the corner of Linden Lane and Hamilton Ave, it was a hole in the ground, knee deep to a cicada, with one side of the basement pressed up close and personal to the buried portion of Harry's Brook that flows from downtown Princeton to Harrison Street. The owner's profession has to do with "aging in place", and her desire was to build a house particularly well suited to that purpose. So-called "passive homes" use passive features like abundant insulation, tight construction, and solar heat to combine great comfort with very low utility bills.

Now that the house is two stories high, framed in and sheltered from the storms, the architect and builder, David Cohen and Baxter Construction, have been hosting tours for all to see its sustainable features.

We started in the basement, which was prefabbed in a factory using custom-sized panels of concrete so dense they didn't need any coating to keep the soil moisture out. The copper tubing wrapped around the sewer pipe transfers heat from the wastewater leaving the house to the city drinking water entering the house. These units, which cost $600 at a bigbox hardware store and can also be installed as a retrofit, raise the temperature of incoming water significantly, reducing the energy needed to make hot water.

This nifty panel distributes water to bathrooms and kitchen. There's a direct line--red for hot, blue for cold--going to each room. The lack of branching shortens the distance the water needs to travel, thereby reducing heat loss between the water heater and the faucet or shower.

The water heater is "on demand", meaning it doesn't have to expend a lot of energy keeping a big tank of water hot. One of the two pipes heading from the water heater to the wall brings in outside air for combustion. Otherwise, the unit would need to use indoor air for combustion, which would mean pulling unconditioned air into the house.

Upstairs, the walls are six inches thick, allowing four inches of sprayed foam insulation, leaving room for wires inside the walls.

This panel of insulation on the outside prevents unwanted heat transfer into or out of the house through the studs.

There's a row of windows facing south that passively harvest solar heat in the winter. An overhang shades the windows in the summer. Standard window glass for houses is treated to reduce the amount of solar heat that can enter. For passive solar, special windows are needed that allow the solar energy through. According to the architect, only one manufacturer in the U.S. supplies this type of window, which speaks to the near complete rejection of this free source of winter energy by the building trade.

The house's passive harvesting of solar energy in the winter is one feature of passive homes. There's a great documentary about the passive house concept called Passive Passion that was shown at the 2013 Princeton Environmental Film Festival. Even in our 1960 house, we can enjoy the boost in solar heating provided by the southwest facing windows in the winter.

Upstairs, the architect explained that they had considered geothermal heating, but due to the high cost decided instead to install a heat pump. It's like an air conditioner that can function as a heater in the winter. Instead of ducts, the one compressor outside drives three separate units mounted on inside walls, one for each bedroom and one for the great room on the first floor. The indoor unit, in the box on the floor in the photo, will be mounted on the wall where the tubing is. Each unit has its own thermostat. Because the house is so well insulated, even rooms that don't have heating/cooling will remain comfortable. The air-tight construction allows ventilation to be controlled, with much of the energy in the exhaust air transferred to the fresh air coming in from outside.

There will be a "green roof" over the porch. Solar panels may be added to the metal portions of the roof in the future, but require a track record of energy use so the system can be properly sized. Metal roofs are more expensive, but last a long, long time and do not emit the volatile organic compounds (VOCs) generated by asphalt shingles.

This is a very thoughtfully designed house, combining affordability with longterm thinking that will yield great comfort and savings for as long as the raingarden grows and Harry's Brook flows. Thanks to architect David Cohen and Baxter Construction for a great tour of this impressive new home.

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