Passive solar design is one of the most attractive strategies available for energy-efficient construction and green building. The sun provides free heat, daylighting, and a better connection to our outdoor environment. It does this for the life of the structure.
If you follow these priciples, your house will offer passive survivability, meaning it will remain livable through winter power outages. The passive elements of your home design will have no moving parts, and the only maintenance need is occasional window cleaning.
The best part: passive solar design can be done with zero extra upfront costs.
Start with a site with good solar exposure
Not every building site or lot is suitable for passive solar design. Strategies will vary depending on latitude, material selection, design, and site conditions. These methods are adjustable to fit one’s site and needs.
Just as shoppers need to be alert to the exaggerations of greenwashing, homeowners should be alert to the liberal use of the phrase “passive solar” by builders and designers. Just because a builder claims that a house has been designed according to passive solar principles, doesn’t mean it has.
Passive solar design requires stretching a building’s shape from east to west while prioritizing southern windows. Here are some important details of passive solar design, in order of importance (after assuring that the house has a tight, well insulated building envelope).
1. Calculate the area of south-facing glass as a percentage of the home’s conditioned floor area:
2. Specify high-solar-gain glazing
For south-facing windows, the type of glass you choose is extremely important. Specify high-solar-gain glazing.
The higher a window’s solar heat-gain coefficient (SHGC), the better. Be aware of the difference between the glazing-only SHGC and the whole-window SHGC. A whole-window SHGC (the number on the NFRC label attached to the window) should be at least 0.40; a better target is 0.56. Check out the new Cardinal low E 180 glazing; it is a replacement for the Cardinal 179 glazing. The manufacturer rates this product (these are glazing-only specs) at U-0.26 with a SHGC of 0.69.
It’s possible to use a higher glass ratio with a lower SHGC and get similar results to a lower glass ratio with a higher SHGC, but this approach increases your material costs. To be sure you get the energy performance you expect, check your glazing plan using RESFEN or another type of energy modeling software.
3. Design an appropriate shape and orient the house correctly
Most designers agree that the more you stretch a building’s shape from east to west, the better it will perform on sunny days. This helps by providing more wall area for southern windows. It also minimizes the impact of intense east and west sun angles during the cooling season. There is a downside, however: such buildings lose more more heat at night than a cube-shaped building. Good energy-modeling software will help a designer optimize a building’s shape.
Perfect orientation is not always possible, since the building site often dictates the design. Facing within 20 degrees of true south is ideal, but some designers feel that up to 40 degrees is still advantageous. In a climate with cooling needs, it’s better for the house to face southeast than southwest.
Overhang design can be customized to compensate for undesirable design and orientation conditions. For more information on orientation-specific window design, see In Search of the Most Energy-Efficient Windows.
4. Design the roof overhangs
Generally, passive solar design requires overhangs that are sized to fully shade the south-facing windows on the summer solstice (June 21) and to allow full sunlight to enter on the winter solstice (December 21). A strict implementation of this guideline is not necessary, but be aware that some state tax credit programs and green certification points may require it.
Some designers take a contrarian view of passive solar design, having concluded that overhangs are not necessary. However, most designers prefer them.
Correctly sized overhangs are especially important in climates where homes need air conditioning. Since the global climate is warming and the use of AC is dramatically increasing, it’s safer to include overhangs in most cases. Since they help protect siding from the weather, they provide maintenance benefits as well. In some designs, like the one pictured above, the overhangs cost us nothing extra.
Getting overhangs sized correctly seems to give designers a lot of trouble, especially on houses with multiple stories. Each floor level needs its own overhang. A rule of thumb that works for most U.S. latitudes is that overhangs should be 12 inches above the window and 18 inches deep. This is an easy variable to adjust depending on window height, latitude, orientation, and design situation.
The best Web-based tool to help designers of passive solar homes with overhang sizing is the Overhang Design Tool on the Sustainable By Design website. Sketchup is another good option.
5. Include thermal mass
How much thermal mass is required in a passive solar house? This is perhaps the most controversial element of passive solar design, and probably the least important. Yet it could easily be argued that thermal mass is more important than overhang design, especially the further north you go.
Most people feel that a designer who includes a lot of thermal mass can see the cost-effectiveness of the project drop dramatically. However, there is plenty of evidence showing that thermal mass inside the conditioned envelope can improve a building’s thermal performance. The question is, how much more are you is willing to spend in time, materials, and labor to install additional thermal mass to achieve the supposed benefits the mass provides? The question is debatable.
Uncarpeted concrete slabs make the most sense, since the concrete is a component that is often needed anyway for slab-on-grade and basement construction. Stained and polished concrete floors are increasingly recognized as one of the most aesthetic, low-maintenance, high-performance floor finishes available.
If you are designing a passive solar house for a building site that is ideal for a crawl space, consider building stem-walls filled with dirt or gravel and topped with a slab. A stem-wall foundation with a concrete slab is a common building practice that can cost less than a sealed crawl space. Such a foundation has more thermal mass and less space to condition. A slab is also (arguably) healthier than a crawl space.
Beware of advertised benefits from products that don’t have their thermal mass completely inside the insulated, conditioned space. We’re looking at you, ICFs and ACC (autoclaved aerated concrete). These can be appropriate products, but don’t expect much benefit from the thermal mass, unless you live in a high-elevation desert with wide diurnal temperature swings.
Some designers and builders go through a lot of extra expense and trouble to increase the amount of glazing, and then address overheating concerns with systems to actively or passively move and store heat in additional thermal mass. Lots of experimentation has been done using many methods and materials, with mixed results. These methods can work, but they add cost and complexity, and they can introduce serious problems. Windows and concrete slabs are simple and maintenance-free. They work extremely well and most designs need them anyway.
The most important detail of a passive solar design is an airtight, continuously insulated building envelope. Free heat does us little good if we can’t control where it goes. A building envelope is THE most important component of passive solar, energy efficient and high performance homes and buildings.
Upfront savings and opportunities for energy cost savings when specifying windows
One of the ways that passive solar design can add zero extra upfront costs is by refining the window sizes and placement. By decreasing the windows on the non-south sides of the home, we can stay balanced in costs and can decrease year-round thermal losses while increasing needed solar gain.
Other tricks for achieving cost savings and improved performance:
- Increase the number of large fixed windows. If you specify as many fixed windows as possible, the costs per square foot of window drops dramatically. You’ll also see an increase in energy performance because there is less thermal bridging and more solar gain. Fixed windows are easier and cheaper to equip with blinds and movable insulated curtains than operable windows. For the most part, fixed windows are easier to clean. However, there is a legitimate concern about the difficulty of cleaning fixed windows on upper levels. One good strategy is to place an operable casement nearby to facilitate cleaning from the inside. Be sure it opens the right way.
- Decrease the number of operable windows. It’s common to see designs with far too many operable windows. They are expensive, have more air infiltration, require maintenance, decrease solar gain, and increase thermal bridging. One operable window per room is usually plenty for ventilation, especially if you are choosing casements. A single bedroom egress window is already plenty big. Locating windows opposite doors helps with ventilation and makes rooms feel bigger.
- Avoid extra engineering. Another good reason for keeping your glass ratio in the 9% to 12% range is that one can create large areas of wall that are uninterrupted by window openings, increasing shear resistance. This can eliminate the extra expenses of engineering, materials, labor, and energy costs of needing structural members where your insulation should be. Solid wall area can help with interior furniture placement and is a good location for stairwells. This strategy also helps you avoid the use of pricier tempered glass.
- Avoid muntins and dividers. They cost more, decrease solar gain, interrupt views from inside, and make it harder to clean the window if they aren’t between the panes.
What about trees?
In regards to deciduous shading on the south side of the house, the best practice is to remove trees if possible and never plant anything that could eventually grow to shade the south windows.
We have found that passive solar design can still work, even with deciduous shading. We recently completed two nearly identical passive solar homes using the same materials and achieving the same ACH50 of <1. The home that has considerable shading from deciduous trees performs nearly as well as the home with zero shading.
Should I plan for air conditioning?
Some builders in our area have built excellent passive solar designs without air conditioning or mechanical ventilation, hoping that the extra thermal mass of concrete floors on upper levels would save them during the cooling season. Having been in three of these homes during the summer, however, I can say that this strategy is not working very well.
Asheville is a mixed-humid climate (climate zone 7, near zone 6). Even with excellent passive ventilation practices, passive solar designs with lots of thermal mass should include a means of mechanical cooling in most U.S. climates.
Passive solar design matched with an airtight, building envelope with minimal thermal bridging offers builders and designers a cost-effective way of reducing heating costs by 40% to 90% while providing abundant daylighting and a better connection to the outdoors. Spread the word and share your opinion or experience on one of the most appropriate energy-efficient construction and green building techniques available.
Brian Knight is a builder and the owner of Springtime Homes in Asheville, North Carolina.