Siting with the Sun: Passive Heating and Daylighting
Design Around the Sun to Lower Heating and Lighting Needs
Siting can keep a house warm and cool
How a house is oriented to the sun has a dramatic impact on heating and cooling costs — the largest energy load in most homes.
As promising as photovoltaic (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) and solar hot water collectors are for reducing our reliance on fossil fuels, passive solar design alone can lower heating costs tremendously. Much of the reduction is available without spending an extra dime.
Use concrete wisely and choose windows well
Specify different types of window glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. for different sides of the house. While west-facing windows require low-solar-gain glazing, south-facing windows (in all but the hottest climates) require high-solar-gain glazing. Although high-solar-gain low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. glazing is available from major glass manufacturers, it is often unavailable from U.S. window manufacturers. Canadian window manufacturers are much more likely to stock high-solar-gain glazing.
Dense materials like stone and concrete soak up the sun's energy during the day and release it at night. High thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. helps smooth out swings in temperature even without the benefit of mechanical heating and cooling. Rooms that have higher thermal mass can afford larger window areas because the heat generated by the sun has somewhere to go. Rooms with low thermal mass and large windows can overheat and suffer wider temperature fluctuations.
Designing for daylighting has its own unique issues
Pay attention to critical areas: kitchen, office, reading area. In many households, the occupants are gone for a large part of daylight hours. This, combined with the fact that code requires regularly inhabited spaces in the home to have windows, may mean that "design as usual" will provide adequate results for daylighting purposes. Areas that may benefit from more attention are those where critical visual tasks take place — kitchen workspaces, offices, hobby and reading areas. Other daylighting opportunities not to overlook include utility rooms and walk-in closets. In these spaces, quality daylighting can significantly reduce the need for artificial lighting. “Solar tubes” (narrow, tubular skylights) may be particularly useful in these types of locations.
Flashing a curbed skylight
A skylight is a great way to bring light deep into a house but can be a leaky mess if it's not installed well. Whether your flashing comes as a kit or you make it yourself, follow these basic steps to get it to work right.
More windows mean more insulation
Lots of passive solar heat can mean that you have to be better than code. Chapter 11 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. sets the baseline for energy efficiency in building envelopes — but if you're considering passive solar heating, you'll probably have to do better than code. Directly using the sun's heat usually means plenty of south windows. If glass covers more than 15% of your exterior walls, you'll need to meet the stricter insulation requirements prescribed by the IECC International Energy Conservation Code..
Even though the IECC sets the bar a bit higher, a passive solar home has some unique needs that might encourage you to up your R-values by as much as 40% or 50%.
Thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night.
can help, if it's insulated
Many passive homes rely on thermal mass of masonry floors or walls to hold some of their heat into the night. If these large heat sinks are only insulated to the code minimum, they may actually be fighting your passive gain instead of helping it.
Windows don't work when the sun isn't shining
The IECC tries to compensate for all that extra glass with higher R-values in the rest of the building envelope. But even if you superinsulate your walls and ceilings, the best windows will still be the weak link when it comes to heat loss. Movable insulation is not often thought of as a building envelope component, but maybe it should be. Simple accordion shades or insulated curtains will require routine manual operation, while automated window insulation systems can be triggered by light or temperature sensors — providing convenience along with a potential maintenance headache.
ABOUT PASSIVE HEATING AND DAYLIGHTING
Start your design with a compass.
Think about how the sun moves through the day and through the year when you start designing your floor plan. Selective siting, shading, and construction strategies can save money on lighting and heating bills. Also, a house with plenty of natural light is more pleasant to be in.
Design floor plans to use sun all day long. Think of a house as four distinct quadrants, each with its own potential for daylight and free heat, depending on the sun's position during the day.
Morning sun is dominant in east-facing rooms. Locating the dining room or breakfast nook and the kitchen on the east wall makes the most of light potential early in the day. It's also a good place for a dense floor that can soak up some solar heat for the day. Bedrooms with east-facing windows will be great for early risers but terrible for people who like to sleep in.
Sunlight is strongest on the south wall. This is the right quadrant for the living room and other spaces that will be used throughout the day. For heat gain, rooms with south-facing windows are another good choice for dense materials like stone, brick, or concrete.
Early evening light from the west is at a low angle. Because the sun is so low in the sky, west-facing windows get direct sunlight blazing through them. This makes west-facing rooms a bad choice for TV rooms because strong light makes screens harder to see.
In cold areas, this is the last chance of the day to soak up some sun; in hot areas, it's the most important window to shade with trees. A west bedroom is good for people who like to sleep in because the room is very dark in the morning.
North rooms have the least natural light. They also have the greatest potential for heat loss through windows. This is a good place for bathrooms, utility rooms, entries, and other rooms where natural light isn't as important. That said, painters and artisans might appreciate generous north-facing windows because of the quality of the light.
Of course, cooling a house can be expensive, too.
The sun's orientation is just as important for houses built in the Sunbelt, where controlling heat gain in the summer lowers cooling costs and creates a more comfortable indoor environment.
You can lessen temperature swings in houses in all climates by using dense materials in key areas. Siting a house thoughtfully can make these areas inviting rooms.
MORE ABOUT DAYLIGHTING
Find other ways to let in light.
Not every room in a house can have generously sized windows that admit natural light. Rooms at the interior core of a house as well as those facing north sometimes get shortchanged.
Open floor plans allow light from skylights and windows to penetrate deep into a house. Skylights over stairways, for example, can bring light into the center of a house.
Skylights are a simple way of introducing light to rooms right below roof level. Both fixed and operable skylights are available.
Angled (splayed) walls broadcast the most light, and placing skylights near a wall creates a pleasant light-washing effect on the wall surface.
Skylights have a negative trade-off, however: a high potential for energy loss because even the best have a higher U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. (lower R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. ) than well-insulated walls and ceilings.
Skylights also can produce unexpected glare and uncomfortably warm indoor temperatures unless they have shades. With this in mind, in most climates it is wise to limit skylights to north roof slopes. At least consider the path of the sun throughout the day when picking skylight locations.
Tubular skylights incorporate a plastic, roof-mounted dome, a highly reflective rigid or flexible tube, and a light diffuserIn a forced-air heating/cooling system, the diffuser is a register or grille attached to ducting through which heated or air conditioned air is delivered to the living space. In a tubular skylight or an electric light fixture, the diffuser is a cover plate through which scattered light is delivered. mounted in the ceiling. As long as there is good attic access, a tubular skylight can be easily installed in most existing homes. Tubular skylights are expensive replacements for lightbulbs, but they offer natural light over artificial light, and you won't waste energy if you leave the "light" on. They’re a good option for dark interior spaces like bathrooms.
Ridge skylights, dormers, and clerestories can add natural light to interior spaces where windows are not an option or where more natural light is desired. A clerestory—a band of windows installed between the ridge of an upper shed roof and the ridge of a lower shed roof—can throw light far into a room. Operable windows improve air circulation.
Light shelves, located directly below skylights or clerestories, bounce light back toward the ceiling and provide good indirect lighting to a large room. Light shelves can also block glare from overhead sun when skylight placement options are limited.
Consider interior colors, too. Light colors reflect light more easily than darker shades. Interiors painted in light colors will feel larger and brighter, and make it easier to use daylight as task lighting.
A FEW DAYLIGHTING STRATEGIES TO CONSIDER:
Place a house on the site so that light is managed easily.
Let light in by angling the house properly, using trees or awnings to shade the windows during the heat of the day, and sizing the overhangs to admit winter light while blocking the hot summer sun.
To balance light, shape the house so that light can enter every room from at least two sides.
Four-square styles are a traditional way of evenly admitting light. Adding a courtyard, atrium, or skylight can illuminate an interior from above. Also, an H- or C-shaped house offers many rooms that can let light in from two sides. There are trade-offs to consider, however; H- and C-shaped houses are less compact, and therefore less energy efficient, than compact rectangular houses.
Where light from a second side isn't possible, provide light from above.
Skylights and clerestory windows are two ways to sneak light in from up high. There are many ways to bounce that light around to dim corners such as light shelves and flared skylight shafts. Remember, though, that skylights are a weak link in a home's thermal envelope, and should therefore be used sparingly.
Shape and locate each opening to suit both the climate and the room.
Does the space need bright task lighting or warm ambient light? Are the windows more important for lighting or solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss.? Would large windows be a liability in an otherwise well-insulated space? Such questions that require careful evaluation of the site conditions and the home's needs.
Excerpted from Fine Homebuilding magazine
- Martha Garstang Hill/Fine Homebuilding #172
- Joseph Kugielsky / Fine Homebuilding 166
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