In the northern U.S., designers of high-performance homes often consider the sun a beneficial source of heating energy. Passive solar strategies use south-facing glass to reap heat during the winter.
But in the southern U.S., where cooling loads dominate the energy equation, solar gain isn’t an asset—it’s a load. So in the region extending from Florida across to southern California, designers have to consider all the options for rejecting the sun’s heat. Factors such as massing and orientation, shading and overhangs, natural shading, light-colored and reflective roofing and siding, and properly chosen windows can all have a meaningful impact on a home’s energy demand as well as on the occupants' comfort during sunny times of the day and year.
Trey Farmer is an architect practicing in Austin, Texas. “We are trying to minimize heat gain here,” Farmer says. “Trying to get passive solar gain here is not worth it, because even in the winter, you are still having days when you are going to overheat.”
“Orientation is a big consideration,” Farmer says. “If you can orient your building along the east-west axis, it’s a lot easier to control the sun on the south, because it’s higher in the summer and lower in the winter. You can shade it when you want to and let it in when you want to. But the east and west faces of the building are a lot harder to control, because the sun is coming in laterally, and so it’s difficult to shade.”
“Three o’clock to six o’clock in the afternoon is the really hot time, and when the sun is low, but still high enough that it’s not all bouncing off the atmosphere, you’re getting some serious radiant heat. It’s also low enough that it’s hard to control with overhangs,” says Farmer.
Window placement is another factor that can significantly affect solar gain. To avoid overheating, windows in the south and west walls should be minimized, with north-facing glass preferred.
Overhangs and Shade Structures
Farmer’s firm does a lot of work on existing urban lots, where the building orientation may be dictated by street frontage. But that still leaves some room to maneuver. “Porches are great,” says Farmer. “For an overhang to be effective in the evening on the west side, it needs to get really deep. At that point, you’re cantilevering significantly or adding structure. So why not just make that occupiable space? Then you also get the benefit of having a sheltered entrance and having a nice indoor-outdoor space.”
“You always get oriented with the site,” says Farmer. “You know where south is and you know where your views are, and solar orientation is one of the primary concerns. That will set up the diagram of the building. For instance, maybe you have the garage to the west, if there are no views over there. That shelters the rest of the house. It’s a semi-conditioned or unconditioned space that’s a thermal buffer to the south or the southwest.”
Besides these larger building elements, it's possible to shade windows with awnings or with interior or exterior blinds.
Where possible, savvy designers make use of shade trees. That might not be possible on the development scale, but for urban infill builders, it can be a viable strategy. Farmer just finished a major remodel (almost a teardown) of a house in Austin that is one of the first Passive House projects in Texas. Trees on the site were a major asset, he says: “Most of these neighborhoods have some great tree canopy. We have a big sycamore and a pecan and an oak tree on the west side.”
Roof overhangs shade walls and windows. But what about the roof itself? Dark-colored roofing materials absorb solar heat, and that heat is conducted into the roof structure and radiated down through the attic to the ceilings below, adding heat to the house. This problem has been recognized for decades, and in recent years there’s been a move to require reflective roof materials in the sunniest parts of the U.S. There are now a wide range of roofing products on the market that are listed by the Cool Roofs Rating Council based on their Solar Reflectance Index (SRI).
How does the SRI work? Roofing materials are characterized by their reflectance and by their emittance. Solar energy striking a roof is either reflected or absorbed; if it’s absorbed, it may still be re-emitted to the sky as infrared energy. Reflectance is expressed as a number between zero and one (or as a percent between 0% and 100%). Emittance is expressed the same way — either on a scale from zero to one, or on a scale from 0% to 100%.
The balance of absorption, reflectance, and emittance results in a temperature rise for a material that’s exposed to sunlight. That combined process is expressed in the “Solar Reflectance Index.” Lawrence Berkeley National Laboratory explains:
“The Solar Reflectance Index (SRI) is a measure of the roof's ability to reject solar heat, as shown by a small temperature rise. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. For example, the standard black has a temperature rise of 90°F (50°C) in full sun, and the standard white has a temperature rise of 14.6°F (8.1°C). Once the maximum temperature rise of a given material has been computed, the SRI can be computed by interpolating between the values for white and black. Materials with the highest SRI values are the coolest choices for roofing. Due to the way SRI is defined, particularly hot materials can even take slightly negative values, and particularly cool materials can even exceed 100.”
The 2018 International Energy Conservation Code has a cool-roof requirement for low-slope roofs (less than 2 in 12 pitch) that are directly above cooled conditioned space. For those roofs, roofing material must comply with one of two options: Either the roofing must have a three-year aged solar reflectance of at least 0.55 and a three-year aged thermal emittance of at least 0.75, or the material must have a three-year aged SRI of 64. The IECC has no cool-roofing requirement for steep-slope roofs.
The Energy Star program offers labeling for cool roof materials as well. Low-slope roofs can earn a label with an initial solar reflectance greater than or equal to 0.65, with a three-year aged solar reflectance greater than or equal to 0.50. For steep-slope roofing, the initial solar reflectance must be greater than or equal to 0.25, with a three-year aged solar reflectance of greater than or equal to 0.15.
Studies indicate that reflective roofing can cut cooling costs by as much as 20%, particularly for houses with poor ceiling insulation or with leaky ductwork in the attic space. Given that, researchers have also considered whether reflective walls could have a similar benefit. The Florida Solar Energy Center (FSEC) has investigated this question by studying a set of test buildings in Cocoa, Fla. Painting the exterior walls a reflective white color resulted in a cooling energy savings of about 10% on an annual basis, compared with the original beige-colored walls.
FSEC researchers extrapolated this result using building energy simulations to estimate the benefits of white reflective walls in other locations. According to the modeling, going from dark walls to white walls could save about 12% on cooling costs in cities like Orlando, Miami, New Orleans, or Houston.
The assumptions in the models do somewhat limit the applicability of the conclusions, however. The researchers started with an uninsulated concrete masonry unit (CMU) building. Painting the building white was as effective as adding R-15 insulation; doing both brought even greater savings. But in a modern wood-framed building with code-compliant or above-code insulation, the advantage of a reflective wall cladding might be less pronounced.
Depending on the design and the constraints of the lot, it may be impossible to shade windows, and some windows are likely to receive direct sunlight for part of the day. That’s when window technology comes to the rescue. The ability of a window to hold out the energy of sunlight is expressed in the window’s solar heat gain coefficient, or SHGC. The lower the window's SHGC is, the more of the sun’s heat it rejects. In the far north, SHGC isn’t regulated, and in the midsection of the country, the minimum SHGC is 0.40. But in the deep South, the IECC calls for an SHGC less than or equal to 0.25 — meaning three-quarters of the incident solar radiation is reflected out.
Says Farmer: “Twenty years ago, in order to get a low amount of solar heat gain through the window, it would knock down the amount of visible light that was coming in — it was really tinted and dark. Now, there are a lot of great films on windows that don’t reduce the amount of visible light transmittance the way they used to. You’re still getting a lot of daylight through and you can see through them, and they don’t look like tinted windows. But a significant chunk of the radiant heat coming from the sun is reflected back out. And you can get lower than the code, but it’s a cost concern.”