Facts and Figures
- Many older homes’ designs were oriented through the use of a heliodon, which is a moveable light source used to mimic the Sun’s path that hovered over a small-scale model of a proposed building. Today, mathematical computer models calculate location-specific solar gain and seasonal thermal performance with precision, and have the added ability to rotate and animate a 3D color graphic model of a proposed building design in relation to the Sun’s path.
- Homeowners may now tap into a specialty market of homes designed to spin on their axis in order to follow the hourly and seasonal path of the Sun. These UFO-shaped homes can spin a full 360 degrees in minutes and are built with unusually tall ceilings and windows for maximum efficiency in powering their solar energy system.
- While some passive solar features are relatively recent innovations, the practice of orienting a home to the path of the Sun is as old as civilization itself. Examples are numerous, from south-facing doors on Neolithic and ancient Ming Dynasty houses, to the astonishing Pueblo ruins in southwestern Colorado.
The Sun’s True Position
Schoolchildren (and most homeowners) will tell you that the Sun rises in the east and sets in the west, and, if this were true, building orientation would be a fairly simple matter. In reality, the sun rises and sets in the east and west only on the autumnal and vernal equinoxes, and something very different happens during the remaining 363 days of the year. The Earth’s tilt causes the Sun to rise and set slightly south of east and west in the winter, and slightly north of east and west in the summer. This slight angle depends on the time of year and the observer’s distance from the equator.
As a result, the winter sun spends all of its time in the southern sky, and the summer sun spends much of its time in the northern sky (the sun crosses over into the southern sky during part of the day, depending on latitude). In the Southern Hemisphere, all of these directions are reversed, so the winter sun rises and sets in the northeast and northwest, respectively, and the summer sun rises and sets in the southeast and southwest, respectively.
How the Sun’s Variations in Position Can Affect Building Design
The relative position of the Sun is a major factor in heat gain in buildings, which makes accurate orientation of the building a fundamental consideration in passive solar construction.
Most importantly, a rectangular house’s ridgeline should run east-west to maximize the length of the southern side, which should also incorporate several windows in its design. For this reason, fewer windows should be located on the northern side of the house, where the summer sun can be intense. A deep roof overhang can shade the few windows in this area, as can different types of shade trees and bushes. Research supports an east-west ridgeline. Homes re-oriented toward the Sun without any additional solar features save between 10% and 20% and some can save up to 40% on home heating, according to the Bonneville Power Administration and the City of San Jose, California.
Builders should note that these directions are given in reference to the Sun and not magnetic north, which can vary significantly from the Sun’s actual position. Magnetic north, as read from a compass, can still be used as a reference if the builder adjusts the figure based on the location-specific magnetic variation, which can be found in publicly available maps.
Building Tips for New Construction
The following tips will also assist homeowners and builders in maximizing heat gain through building orientation:
- Orient the floor plan – not merely the building’s profile – toward the Sun. Design the home so that frequently used rooms, such as the kitchen and living room, are on the southern side. Occupants will appreciate the sunrays in the winter and relief from the sun in the summer. Patios and decks should be built on the south side of the house, where direct sunlight will permit their use for more hours during the day and more days during the year. Likewise, the garage, laundry room and other areas that are less frequently used should be situated at the northern part of the house, where they will act as buffers against cold winter winds.
- Beware of mountains. The north/south sun differential is exaggerated in hilly and mountainous regions, where significant climatic differences can be seen over comparatively small areas. A passive solar house should be constructed on the south-facing slope of a mountain to avoid the extreme shading created where the low-angled sun is blocked by the mountain on the north side. Halfway up the slope is ideal, as the mountain’s peak is exposed to strong winds, while cold night-time air flows into the underlying valley, which is also a natural drainage point.
- Plan for tree shade. Trees are an important factor in passive solar design because they can both provide needed shade on a balmy summer day and starve the house of natural light when it is needed most. Deciduous trees planted on the south side will lose their leaves in the winter and allow natural light to enter the house, while evergreen trees planted on the north side will provide shade from the summer sun. Builders should carefully consider the age, species, growth rate and mature canopy cover of existing trees before deciding where to orient a structure on a building lot. Trees also pose unique dangers, which are covered in InterNACHI’s article on Tree Dangers.
- Install as many windows as possible, but not too many! The exact number of windows required is different for each house because it’s based on – among other considerations – the local climate. A “sun-tempered” house should include enough glazing to equal 5% of the conditioned square footage of the house. Remember, though, that windows allow heat transfer more easily than walls, so too many windows can actually drain heat from the house during the cold winter months. Read InterNACHI’s article on Window Gas Fills and Window Films to learn how to insulate a house’s glazing.
- Stray from the rule on east-west orientation, if needed. The east-west orientation of the ridgeline may be adjusted to accommodate other factors by up to 20 degrees with only a minimal impact on heat gain.
- Driveways can get hot! Driveways and parking lots are made using gravel and asphalt – materials that heat up faster and reach higher temperatures than the rest of the yard. Excessive heat there can spill over to the adjacent house, which is why placement of the driveway or parking lot to the south or east of the building can reduce summer heat buildup in southern climates. During the cold winter months in northern climates, a south- or west-oriented driveway will melt snow faster and provide the home with greater warmth.
- Glass need not be vertical. Custom glass is available that may be tilted to match the angle of the sun and minimize reflection. Angling glass away from the vertical makes it less insulative, however, so builders should balance potential gains in sun exposure with loss of heat to the outdoors.
- Another environmental factor that should be considered in the equation of building orientation and positioning is prevailing winds, which are the winds that blow predominantly from a single, general direction over a particular point. Data for these winds can be used to design a building that can take advantage of summer breezes for passive cooling, as well as shield against adverse winds that can further chill the interior on an already cold winter day, or even prevent snow from piling up against windows and doors. Detailed information about prevailing winds for specific locations are plotted in a graphic tool called a wind rose, which is usually available from airports, larger libraries, Internet sources, and county agricultural extension offices. As a general rule of thumb, cold winter winds generally come from the north and west, which can be limited by using insulating glazing on these sides of the house. Also, remember that coastal areas typically experience breezes from an onshore direction, while cool breezes flow down valleys from mountain slopes.
Ultimately, factors such as street appeal and the property’s lot dimensions may restrict a builder’s ability to orient a building in strict accordance with passive solar techniques. Even while working under these constraints, however, a builder can still create an energy-efficient home through the implementation of energy-saving features, such as low-E windows, adequate insulation, air sealing, and cool roofs.
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Trevor Drinen | CPI Certified Professional Inspector # NACHI16122702