This green home will heat itself by heat pump technology-news-293
In category: Heating knowledgeBOSTON–For all the complex solutions proposed to lower building energy use, Simon Hare has a project to demonstrate the power of simplicity in green buildings.
The design-builder earlier this year began reconstructing an 1850 cottage in Boston’s historic Roxbury Crossing neighborhood to be so energy efficient that it wouldn’t need any mechanical heating.
His work is inspired by the Passive House standard, which is based on a set of principles for building energy-efficient homes that took root in Germany in the late 1980s. But Hare has another goal: to show that net-zero, or very low, energy homes are within reach of everyday building professionals.
“The Passive House approach is very techie, which I think is its Achilles Heel–it appeals to geeks but not the layman, the lay builder,” Hare said standing in the half-finished home last month. “We can prove we can do this without hiring consultants and using software to do the energy modeling. We’ll just use precedent and established rules of thumb.”
The Pratt House project is an example of a burgeoning movement in the building industry. With the growing concern over the environment and energy, builders and architects are devising ways to dramatically cut the energy use in people’s homes, for both new construction and retrofits. In the U.S., all buildings represent about half of greenhouse gas emissions, according to the U.S. Green Building Council.
A more high-tech approach to super-efficient homes could be control systems that optimize a home’s mechanical systems, such as heating and lighting, or demand-response appliances that can take advantage of off-peak electricity prices.
Photos: A Passive House in the making
By contrast, many builders like Hare are starting from the ground up by taking steps to lower the energy load that a home needs to operate. In practice, that boils down to constructing an air-tight building with lots of insulation and energy-efficient appliances.
When you add distributed energy, such as solar panels for hot water and electricity, to a well-sealed and insulated home, homeowners can dramatically cut utility bills and potentially get to net-zero energy use. Following the Passive House guidelines, for example, can lower energy use by 60 percent to 70 percent and drop the heating load by 90 percent.
Trickier than it looks
Hare’s project in Roxbury, which happens to be his family’s house, started out as a retrofit. The building, which once was a workshop for a 19th-century gunsmith, had been abandoned for 20 years before he acquired it. After construction began, the crew at his firm, Placetailor, discovered serious structural problems and the cottage was torn down and rebuilt with the exact same dimensions.
Since it’s a new construction, they were able to take particular care with the air sealing. The frame of the home is built using structural insulated panels, which are 12-inch layers of foam insulation sandwiched between two sheets of plywood.
A layer of one-inch rigid insulating foam on the exterior walls brings the r-value (a measure of insulation) to 50, many times more than a typical home. The joints between the wall building blocks and the floor were taped or sprayed with foam to make the building more air tight.
Hare borrowed a fog machine from a local DJ during two blower door tests to see where air was escaping from the building, viewing it both from the outside and from the basement. Many tweaks made the house very tight with 0.6 air changes an hour, which for a building that size translates to 60 cubic feet per minute.
In an unusual twist, the floor is made of concrete and the interior walls, too, will be made of similar material. That material acts as a “thermal mass,” able to retain heat in the winter or absorb heat from the air in the summer to maintain a comfortable climate, Hare explained. By adding solar panels, the Pratt House could easily be a net zero-energy home, he said.
Although it’s rarely done, builders have been converting existing homes into these sorts of superinsulated buildings for decades. One way is to put foam board insulation on a home’s exterior walls and roof while another option is to spray insulating foam onto the exterior and add shingles on top of that layer.
Adding that exterior layer of insulation is expensive upfront and, in practice, tricky when dealing with windows, doors, and drainage considerations. But a bigger barrier to better-sealed homes is simply inertia given that most contractors don’t pay attention to how air flows in buildings.
“The basics are simple–a lot of insulation, a very tight building, and efficient appliance,” said Hare. “But a regular construction crew would just put up a wall quickly and never stop to think about sealing cracks and leaving places where you want air to go through.”
Homes that are extremely air tight need a heat-recovery ventilator which brings in outside air mechanically, while heating the air as it enters.
Miles per gallon for homes
In parallel to the push for more efficient homes, there are more calls for benchmarks and performance standards. Right now, builders and homeowners are largely “flying blind” when it comes to making efficiency improvements, according to Paul Eldrenkamp, of design and build firm Byggmeister, who is the first Passive House-certified consultant in New England.
The original structure, an 1850 gunsmith’s workshop that had been abandoned.
(Credit:Placetailor)
At the Pratt House, the approach is to build first and analyze later. Hare went that route because he’s concerned that exhaustive up-front analysis, which might involve modeling software and complex spreadsheets, is too intimidating to most builders.
“People in this whole movement say that there needs to be standards but they are so far removed from established building practices–it’s just another barrier in adopting these things,” he said. “Passive House is a very strict standard. You could fall short by some fraction but still have an excellent building.”
The target is to have the Pratt House nearly complete by October, which is when the big test for his experiment begins. In the winter, the goal is to maintain a temperature between 63 and 65 during the winter without supplemental heat. So far, in the summer, the home has stayed cooler than the outdoors, thanks in part to the concrete floor which absorbs heat and makes it feel cooler.
This sort of home is clearly not for everyone. It’s very small at 750 square feet and the people inside will need to manage the temperature more actively than simply setting the thermostat. For example, to gain heat in the winter , they will need to open the south-facing shades during the day and close them at night to retain heat. In the summer, too, they open the windows at night to cool off the thermal mass of the building.
On the other hand, the materials involved are cheap or recycled. “You don’t need fancy expensive windows from Germany,” said Hare. And many of the construction techniques can be applied elsewhere. Placetailor, in fact, is involved in another local project called the JP Green House which aims to convert a 100-year-old home to be carbon neutral.
“We had this feeling that if efficient homes with air source heat pump, are going to be widespread, you should be able to do it with common sense,” Hare said. “Otherwise, it’s just a geeky pursuit or it’s rocket science when most people just want a house.”
