Concrete floors provide thermal mass, which helps keep a house cool in summer and warm in winter.
Photo By George Austin
Many people who are planning to build or buy a house would like to own one that’s energy-efficient but aren’t sure how to get started. Architect Debra Rucker Coleman gives advice by offering insight into the fundamentals of passive solar design, and explains why it’s always a good idea to design homes (or select house plans) with the sun in mind.
Tell us about your background and how you became interested in passive solar design.
I graduated from architecture school at the University of Arizona. That was in the late ’70s, so before the current green movement, but during the 1970s energy crisis.
The instructors were very sensitive to the environment. So that’s where I got the basics of passive solar design — even though it wasn’t called that and was taught as just one aspect of environmental design.
When did you start actually designing passive solar homes?
It wasn’t until I left Arizona and started working for other firms in the East — where the climate was colder — that I realized, “They’re not even thinking about the way a building should face or about where the sun is!” It was what I perceived as the lack of attention to the sun that made me decide to go out on my own in 1985 and establish Energetic Design, the company I had when I lived in North Carolina. I was intent on making all of the buildings I designed more energy-efficient and working with the sun as much as possible.
What Makes a House a Passive Solar House
What are the basic elements of a passive solar or sun-inspired design?
Orientation of the home is first. You want the longest side to face south.
Window placement within that orientation is next. Most of your windows should be on the south side of the home if you’re in the Northern Hemisphere. Ideally, that is the longest wall.
Third is the overhang for the south windows. Porches or trees can offer shade protection for the east and west windows.
Thermal mass, which helps store the sun’s heat, is fourth.
Does all of this cost more?
Not always. Passive solar design takes the common elements found in most home designs — windows, overhangs and thermal mass — and arranges them in a smart way to improve home performance, decrease energy consumption and increase home comfort. Most passive solar design elements can be incorporated without any extra cost.
How do you add thermal mass to a home?
Additional thermal mass — other than that in typical building wall systems, such as Sheetrock — is not needed in homes where the south glass is less than or equal to 7 percent of the conditioned floor area.
A 4-inch-thick concrete slab foundation is an economical way to add thermal mass. Some homes will have 4 inches of brick or stone veneer on interior walls, or 8-inch-thick free-standing concrete block walls that have both sides exposed and finished with thin stone or tile.
Many people choose to build with insulated concrete forms (ICFs) because of their strength and energy savings. There’s also a lot of inherent mass within them, and now there are ICF floor systems, too.
What kinds of changes to your passive solar house plans do you have to make for different climates?
There is surprisingly little difference in the basic house design requirements. The details may vary substantially, especially in relation to the amount of insulation needed and the type of south-facing glass, but those details are easily changed without major house plan adaptations. Some will say you wouldn’t want to put the exact same home in Minnesota as in Atlanta, for example, but the design requirements are not that different.
Warmer climates may need to have the south-facing glass reduced somewhat. For instance, in the United States, the southern latitudes, which would be Southern California across to northern Florida, need south glass that’s only 5 to 7 percent of the floor area. You certainly wouldn’t want to go above 7 percent.
Because it would overheat if you used more?
Yes, and it would overheat in both summer and winter.
The middle range of climates, starting at approximately 34 degrees north (the latitude of Columbia, S.C., or Los Angeles) and upward to about 40 degrees (the latitude of New York City), would overheat less, allowing 7 to 10 percent glass.
The northern part of the United States — with the exception of the Pacific Northwest, which is not that cold — can handle 10 to 12 percent south glass.
There are very few plans with the higher percentages of south glass, however, because there’s usually not enough available south wall area. The most efficient shape to heat and cool is a cube. It has the least amount of surface area through which to lose and gain heat for the given volume of space. Then, stretching that shape along the east-west axis increases south wall area, which allows for more south windows. Extreme southern climates can also handle more north glass.
And you’d want to minimize north glass because of the heat loss?
Exactly. Because you have heat coming in through the south windows in the winter, the more you can do to keep that heat in, the better. Windows are the weakest link in a home’s “thermal envelope,” so those that don’t contribute to the winter heat gain should be minimized.
However, I am not a proponent of putting all your windows on the south and none on the east, west and north, because that may result in a home that’s uncomfortable in other ways.
Passive-Solar Energy: Serious Energy Savings
Can you estimate how much people reduce heating costs by choosing passive solar design?
It ranges a lot, and it depends on climate. Based on the sun, I would say about 20 to 40 percent with conventional construction. Higher latitudes are going to be at the lower number. However, if you spend 5 to 10 percent more of total construction costs and double the R-value of insulation and windows, you’ve nearly doubled the percent of heat gain the sun is providing. Because there is less heat loss in the house overall, the sun contributes a greater percentage of the remaining heat load.
Most of our clients desire an energy savings of about 50 percent more than a code-built home. Thirty percent comes from insulation and energy efficiency, and a minimum of 20 percent comes from passive solar design.
Many people are starting to aim for energy savings 80 percent better than a code-built home without the use of active solar. That level of savings requires extreme attention to air sealing and additional insulation. The percentage of energy use attributed to leaks in a home is much higher than many people realize.
Unfortunately, just meeting code really isn’t very energy-efficient, because building codes point to minimum requirements.
Some people will say they want the current Energy Star minimum standard, which is at least 30 percent better than code. Scoring well for Energy Star standards can result in 90 percent savings. Some clients will tell us they intend to build to the Passive House Institute US standards, which are variable in their relationship to code based on the climate, but in cold climates can be 90 percent better than code.
Do you make suggestions on heating systems?
We can help the clients size their heating and cooling systems and work with the heating subcontractor as one of our services. They may not need a super-high-cost system for their house because heating loads can be so low.
What People Don’t Know About Passive Solar
What are some of the biggest misconceptions you run into about passive solar design?
Fear that the house is going to overheat in summer. But what can actually be more of an issue is overheating in winter. If there is not a properly sized overhang based on the south wall orientation (ideally between 0 and 10 degrees east or west of true south) and latitude, it could overheat.
The second misconception is that it’s going to cost a lot more. In many ways, there are really no extra costs for the passive solar design elements. You’re taking most of the windows that are typically in a home and putting them on the south side. There is a little more cost as you get into the higher percentages of glass because of extra window costs and extra thermal mass needed to store the extra heat. Extra insulation is not particular to passive solar, but to any low-energy home.
What else can you tell people about designing an energy-efficient, passive solar home?
I like to emphasize the hierarchy of thinking about what you want your home to be.
- First, determine your needs according to your lifestyle and land, and consider a compact floor plan that allows for passive solar and extra insulation.
- Next, think about a really low-tech, simple, auxiliary heating system.
- Finally, consider active solar, such as solar hot water and photovoltaics, or other forms of renewable energy that create electricity, such as wind generators.
I would also emphasize the importance of a smaller footprint and a smaller home. Extra square footage costs more to build, maintain, heat and cool. The savings in initial construction costs, let alone ongoing operating costs, can far exceed the architectural fees.
If you’re looking at a two-story home, think about reversing that. Instead of your additional floor space being on the second floor, place it in a day-lit basement. If you have land that slopes to the south, you can build it so that you don’t even know you’re in a basement. And that’s a very energy-efficient space (see a photo of a passive solar house designed by Debra Rucker Coleman).
Do you work with people on retrofitting their home for passive solar design?
Yes, and every home is different! We suggest that the client first have the home evaluated by a home energy rater for basic home energy efficiency, as it would be impractical to add passive solar — or any kind of heat energy — without putting on a thicker jacket and zipping it up. Having a builder involved can help identify structural issues that may not be evident in photos. As with new design, the focus should be on the ability of the south wall to house more windows. If the client also wants to add on to the home, passive solar can be designed into the addition.
During the ’70s and ’80s, did you ever think passive solar would eventually be everywhere — that by now almost everyone would be doing it?
Yes, so I’ve been surprised by how slow the construction industry has been to change. Many green building programs still do not adequately allow for passive solar credits.
I find myself constantly wondering, “Why would you not integrate something into a new design if it saves you money, costs very little extra, is good for the environment, and makes you comfortable and happy to be home?”
Passive Solar House Plans and Sun-Inspired Services
My company, Sun Plans, was established in 2002, and my goal was to offer affordable architectural services, especially related to sun-inspired/passive solar design.
I’d had a lot of questions from people asking why there weren’t more passive solar house plans available. So I thought, well, we’ll make them available on the Internet. With our custom energy specs that we prepare for each house plan order, we make recommendations to adapt the design to the climate in which the home will be constructed.
My second goal with Sun Plans was to make it affordable. Typically, architects will charge 6 to 15 percent of construction costs and provide a proportionately high amount of service. We developed an affordable service that keeps the fees — even for a new design — closer to 2 to 3 percent of total construction cost. That’s only about half of what you’d pay a realtor if you were buying a home. Clients do not always need the extra services and often enjoy helping in the design process of their home.
The typical cost for buying a passive solar house plan we’ve developed is about $1,200. If we adapt a plan, those fees vary, but on average it costs about $3,000. To create a completely new, custom plan is about $8,000, although we can sometimes create them for less. Some clients want us to provide more services, especially if they plan to build the home themselves, so the fees can be higher, too. I enjoy helping clients with what may be the biggest investment of their lifetime.
— Debra Rucker Coleman
Architect Debra Rucker Coleman’s book The Sun-Inspired House outlines her passive solar design ideas, and you can learn more about her company and designs at Sun Plans. She lives in Citronelle, Ala.
Megan E. Phelps is a freelance writer based in Kansas. She enjoys reading and writing about all things related to sustainable living including homesteading skills, green building and renewable energy. You can find her on Google+.