Here comes the sun
By Brian Knight on 03/06/2012
Passive solar design is one of the best techniques available for green building and energy-efficient construction. It can be done for zero to little extra upfront expense and is capable of reducing heating costs by 40 to 90 percent.
Other benefits include abundant daylighting, a stronger connection to nature, and passive survivability — meaning the home remains livable through winter power outages. Like the building envelope, these features are permanent and will last the life of the structure. There are no moving parts, and the only maintenance required is occasionally cleaning the windows.
The most important component of passive solar design is a building envelope that is airtight and minimizes thermal bridges. Free heat does little good if it quickly escapes through the building envelope. Obtaining great blower-door test results and using insulation details that minimize thermal bridging should be prioritized before passive solar design.
Here are five ways to get the most out of passive solar design.
Calculate the area of south-facing glass as a percentage of the home’s conditioned floor area
• 7 percent or less is considered “sun-tempered.”
• 9 to 12 percent is the ideal range that most passive solar designs aim for.
More than 12 percent puts the house at risk of overheating unless the design includes extra thermal mass. Other concerns with higher glazing ratios include increased costs, glare and excessive thermal losses.
Specify high solar-gain glazing
For south-facing windows, the type of glass you choose is extremely important. The higher a window’s solar-heat-gain coefficient, 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.
Bright solutions: A good passive-solar design takes into account such factors as which way the house faces, what kinds of windows are installed and what size overhangs are used.
Shape and orientation
Stretching a home’s shape from east to west will create more room for southern windows and minimize the effects of intense sun angles striking the home’s sides during the cooling season. This approach needs to be balanced with the fact that a squarer house loses less heat at night.
Facing within 20 degrees of true south is ideal. Many designers still incorporate passive-solar features up to 40 degrees off true south. Facing southeast is better than southwest due to less overheating. Currently in Asheville, true south is 6 degrees east of what a compass indicates due to magnetic declination.
Overhang design
Getting overhangs sized correctly seems to give designers a lot of trouble, especially on houses with multiple stories. Generally, overhangs should provide full shading on all southern windows on June 21 and full sunlight on Dec. 21 (the dates of the highest and lowest sun angle each year). A strict interpretation of this is not necessary but highly advised.
The rule of thumb that works for Asheville’s 35-degree latitude is that overhangs should be 18 inches deep and 12 inches above the window. This is an easy variable to adjust depending on window size, orientation and design considerations.
Thermal mass
Thermal mass is the most controversial element in passive solar design. Much evidence shows that thermal mass inside a home’s conditioned space can decrease heating and cooling costs. The question is, how much more are you willing to spend in time, materials and labor to install additional thermal mass to achieve the benefits the mass provides? The question is debatable.
Uncarpeted concrete slabs are the most obvious mass to include since they are often needed anyway in typical construction. Stained and polished concrete floors are increasingly recognized as one of the most aesthetic, low-maintenance, high indoor-air-quality floor finishes available.
If you are building on a site that’s ideal for a crawlspace, 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 crawlspace. Such a foundation has more thermal mass and less space to condition. A slab is also (arguably) healthier than a crawlspace.
For thermal mass to perform, it must be entirely within the conditioned space. This eliminates most of the thermal-mass benefits attributed to insulated concrete forms and autoclaved aerated concrete.
By installing fewer windows on the non-south sides of the home, owners can balance costs and decrease year-round thermal losses while increasing needed solar gain.
Here are other tricks to save money and improve performance:
Increase the number of large fixed windows. If you specify as many fixed windows as possible, the cost 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-percent 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.
North Carolina is one of the only states to offer a tax credit for passive solar design. It falls under NC-478: Investing in Renewable Energy Property. By following the advice above, it’s possible to avoid any additional expenses associated with implementing passive solar. The credit allows one to offset several components of passive solar design, including additional southern windows, thermal mass and overhangs.
The credit covers 35 percent with a $3,500 maximum for any improvements attributed to passive solar. Be sure to better research this credit. Tax policy and required details are subject to change at any time. Some of its notable requirements:
• Southern glass must have an SHGC of .70 or higher (glazing only).
• Orientation must be within 15 degrees of true south for new homes (20 degrees for existing homes).
• Overhang design must fully shade windows on June 21 and allow full sun on Dec. 21.
Brian Knight is the owner of Springtime Homes, a green builder specializing in energy efficiency and indoor air quality, serving Asheville and Western North Carolina.