Homeowner’s Guide to Passive Solar Energy
Every new home should be designed to take advantage of free heat and light from the sun. Passive solar homes collect the sun’s heat through their windows. You live in the collector. You don’t need anything complicated or expensive, as long as solar features are included at the design stage.
The basic design principles used to capture the sun’s heat are not new. Many of the world’s oldest structures were orientated to let the sun’s heat in (in colder climates) or out (in warmer climates). Closer to home, many older Maritime farmhouses include solar features such as living areas and windows that face south, utility buildings and evergreen trees positioned to block cold winter winds, and hardwood trees planted to provide summer shade.
Solar Heating… in Nova Scotia?
Studies have found that mild winters and cold clear winter days combine to give Halifax the third best solar climate in Canada. On sunny winter days, even cold ones, the sun’s rays are strong enough that you shouldn’t need to run your heating system during the day.
Passive Solar Basics
Passive solar houses don’t need to be complicated or expensive to perform well, but there are some basic guidelines that must be followed in a conventional home to strike a balance between desirable heat gain and potential overheating problems.
In its simplest form, passive solar involves carefully choosing the size and location of windows, and developing a floor plan that positions primary living spaces (family room, kitchen, eating areas, etc.) on the south side of the house. When combined with good insulation, efficient windows, and tight construction, passive solar heating can supply as much as 30 percent of a conventional, low-thermal mass home’s annual heating needs, with little or no increase in construction costs. Unfortunately, there aren’t any specific products that can guarantee effective passive solar performance. A building’s solar performance depends on its design, orientation, level of energy efficiency, and construction.
Solar Homes Must Be Energy Efficient
Today’s better windows and energy efficient building systems are the reason that today’s passive solar homes
perform as well as they do. After all, what’s the point of collecting the sun’s heat if you can’t keep it in your home? Build an air-tight, well-insulated home, and let the sun’s energy do its work.
Windows Should Face Within 30° of South
To collect the sun’s energy efficiently, windows should face within 30° of true south (not magnetic south – see
explanation on page 3). Windows to the east allow for morning light, while too much west-facing glazing can
lead to overheating in the summer. Try to minimize the amount of glass that faces north.
More Glass Means More Mass
Some solar homes receive 50 – 60 percent of their heat from the sun. These homes contain heavy building materials, that can soak up excess heat during the day for use at night, called thermal mass. Attractive and cost-effective ways to add thermal mass to a solar home include tiled floors, masonry walls, slab-on-grade foundations, and extra layers of drywall. Thermal mass works best when directly warmed by the sun. Stored heat can then radiate directly back into living areas. Mass located under carpets, wood, or other types of insulating floor coverings will be less effective. Radiant floors can’t store much solar heat, since they are already warm. In slabs heated directly by the sun, heat storage occurs primarily in the top few inches. The performance of a thick floor slab can be improved by circulating air through ducts cast into the slab. This type of thermal storage slabs have worked well in many Maritime solar homes. Thermal mass must be insulated to slow heat loss to the outside or the ground.
More Glass Is Not Always Better
Oddly enough, overheating in both summer and winter is one of the greatest dangers of “solarizing” a conventional house. In this type of home, south-facing glass areas should not exceed 8 – 10 percent of the floor area of nearby rooms. If it does, the building’s design must include thermal mass, materials such as concrete, masonry, stone, etc. that can store heat, as well as a system to distribute heat throughout the building in order to moderate indoor temperatures on sunny winter days.
Vertical Glass Has Many Advantages
Solar designers found that vertical windows have many advantages over sloped glass. They cost less, don’t require complicated flashing, and don’t let much heat in during the summer. During the winter months, when heat is desirable and the sun is low in the sky, vertical glass is only slightly less efficient (about 10 percent) than glass set at the optimal angle. In the summer, when the sun is high in the sky, vertical glass reflects most of the sun’s rays and keeps heat out. Generally, good solar designers avoid ceiling glass since it collects the summer sun’s heat when it isn’t needed. Overhead glass systems are usually more expensive, especially if tempered glass is required. Both condensation and leakage are more likely to occur on overhead glass than on vertical window units.
Small, carefully placed skylights can be an effective way to provide even light levels for plant growth or to light a wide room evenly. Skylights in the middle of a room tend to cause glare and uneven light distribution. For good
light dispersion, light from the skylight should reflect off a wall or ceiling surface.
Overhangs Aren’t Always Needed
As a general rule, south-facing windows don’t need overhangs to control overheating, because most of the light from the high summer sun reflects off the window’s surface. At the sun’s maximum height, an overhang will shade just over two and a half times its horizontal projection.
Windows facing west can cause summer overheating because the sun’s arc takes it low enough in the sky that the window collects the sun’s heat when you don’t want it. Because of the low sun angle, shading a west facing
window from the summer sun requires a very large overhang, a tree, or some kind of trellis.
Choose Your Building Lot Carefully
From a passive solar perspective, an ideal building lot has an east/west street and slopes to the south. Most
homeowners prefer to have the rear of the house facing somewhere between southeast and southwest to create
a private backyard with good solar exposure for a deck or other outdoor feature.
Use Landscaping To Your Advantage
Raised earth banks or mounds and evergreen trees can shield you from cold winter winds. Hardwood trees on the east, west, and south help to shade your home during the warmer months. If your building lot has large trees in a good location, plan your project to save them. It takes decades to grow them back.
Consider the Sun When Designing Your Home
Good designers consider the natural features and contours of the building lot in relation to solar south. Nearly all lots have at least some solar potential. In addition to the heat obtained, light levels will be high, and
spaces will seem “bright and cheerful.” Floor plans should group living spaces such as the kitchen, eating areas, family rooms, etc., to the south to take advantage of the sun’s heat and light. Garages, stairwells, bathrooms, and utility spaces should be kept to the north side. Generally, homes should be longer in the east-west direction to maximize southern exposure.
Annual Heating Impact of Window Location
A low-e, gas-fi lled window on the north side loses more heat than it gains, while a low-e, gas-fi lled window on
the south side gains more heat than it loses. Therefore, planning windows on the south side instead of north side
can save you money year after year on your energy bills.
Magnetic South vs True South
The needle on your compass points to magnetic north; however, there is a diff erence between magnetic north and true north. This is caused by variations in the magnetic field of the earth. The variation differs with location and is called magnetic declination. In the Maritime region, the magnetic declination is approximately 23° W from true north. If you look at the compass, this means that the magnetic north needle will be pointing to 337° when the compass is pointed to true north. In Nova Scotia, add 23° to the compass reading to determine the direction of true north.
What If I Have A Great View That Doesn’t Face South?
When that happens, invest in high-efficiency windows and carefully size and locate your windows to avoid making them too large. Your windows don’t have to be exactly perpendicular to the view for you to be able to enjoy it. Often, windows can be angled somewhat towards the sun to increase solar gain, without spoiling your view.
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