This supper-energy-efficient garage uses straw bales for insulation and features a vegetated "green" roof.
Before most people were interested in energy conservation, David and Laura Sill were green pioneers. In 1998 they built a Victorian-style house in Reisterstown, which was designed to one day accommodate solar panels, and in 2004, a set of photovoltaic panels big enough to provide about 30 to 40 percent of the family’s energy needs, was installed.
A year later, their eco-friendly home incorporated more green innovation, including thermal panels on the garage roof, which then became the main source for hot water. In addition, the family lawn tractor is powered by propane, and a small windmill generates additional electricity on windy days.
When the Sills started to plan for a combined detached garage and screened porch project, it was no surprise that their plan included pushing the green envelope even further—key elements of the project would include straw bale construction for insulation and solar thermal panels for heat.
The goal of the building was to be as energy efficient as possible, which meant harnessing any energy that would be needed on site. The garage also had to stay warm enough for working on cars in the winter without the use of a furnace or heat pump. To reduce the amount of heating and cooling that would ultimately be necessary, the Sills wanted to use straw bale walls.
First used in the United States in Nebraska in the late 1800s, straw-bale construction is highly-insulative and made from a quickly renewing resource. The material is also quite structurally sound—some of the original 19th-century Nebraska straw bale houses are still standing today.
To begin, the Sills contacted Albert Nunez of Capital Sun Group in Cabin John, Maryland about what options were available for heating the detached structure. Because the primary goal of the garage was to function without a furnace or heat pump, the entire design essentially started with figuring out how that could be accomplished.
Nunez recommended solar thermal panels to heat glycol, which would circulate from the solar panel tubes to a heat exchanger that sits inside the tank. The heat exchanger transfers the heat to 120 gallons of water in the tank, and the water then circulates through the tubes in the floor. The two pumps running the solar radiant system use about the same amount of energy as an 80-watt light bulb.
A ladder provides Laura Sill easy access to the green roof for occasional weeding until the sedum grows in more fully.
To design and build the garage, Nunez recommended me, because we had worked together before, and he knew I had experience with straw bale construction. But before I determined how to build the walls, I first had to consider the roofline, which would support the solar apparatus used to generate heat and electricity.
The front roof, which faces south, is pitched at a high angle, so that the solar panels are as perpendicular to the sun as possible—in Maryland this would be a 39-degree angle in the summer. In the winter, however, the sun stays lower in the sky, so the Sill roof is pitched at over 50 degrees, to make the solar panels more efficient when they are needed most.
Two side roofs, also facing south, slope up gently to hold green roofs planted with native sedum. The green roof reduces the cooling load in the summer and manages rainwater runoff. Windows in the east and west walls provide cross ventilation, while operable north facing clerestory windows can be opened in summer to let out warm air, creating what is known as a “solar chimney” effect –a technique that has been known to native peoples of the Southwest for thousands of years.
The high sloping roofline that accommodates the solar thermal panels as well as the height needed for a hydraulic lift was also perfect for sheltering the screened porch, which was Laura’s dream. The porch is situated not only to take advantage of east-west breezes, but also so that Laura can keep an eye on her three children when they play outside. The screened porch was also designed to stay dry in the rain, bug free, and provide a variety of electrical and lighting needs so that the design could be changed for different seasons and furniture arrangements.
In the interest of using as little lumber as possible, we wanted the straw bales themselves to provide the building’s structural support. The south wall contained three garage doors, so it did require a wood structural frame, but the rest of the building was constructed using nothing but straw bales, lime, and sand. And because straw bales compress under pressure, we estimated how much compression would occur once the roof trusses were set and then stacked the walls about six inches higher to allow for the inevitable settling in place.
In addition to the straw bale construction, green roof, and solar panels, the project included such other interesting green features as a 900-foot GrassTrac driveway, additional spray foam, recycled blue jean insulation, and a waterless toilet, which uses a small amount of electricity to incinerate waste into a compostable material.
The garage has now been in use for more than a year. The straw bale walls are monitored by permanent sensors. I am happy to report that they are dry and sound. Even in the darkest and coldest season, temperature never drops below 45, even at night.
Today, using modern technology and Old World ideas, a 960-square-foot building is heated and cooled on the power of the sun, using only the energy require to illuminate one 80 watt light bulb!
Polly Bart is the owner of Greenbuilders, Inc.
Contacts:
Capital Sun Group, Ltd.: capitalsungroup.com or 301-229-0671
Greenbuilders, Inc.: greenbuilders.com or 410-833-4814
