How to add passive cooling to your home by adjusting the windows, vents, and doors or adding awnings or overhangs.
There are several ways to use your existing windows, vents, and skylights to help keep cool this Summer without adding to your electric bill. In the past, most homes had higher ceilings and transoms installed over the doors. This was not an architectural feature as much as a way to use passive cooling to improve comfort.
By opening the transom and opening a window on an opposite wall, you were able to drawn in cool night air and exhaust warm at a high level out of the room into the large central staircase and out through the top of the house through a cupola. Many homes in the South had large porches to help shade windows and allow occupants to sit outside. Now most homes have air conditioning, but you can still use these techniques to reduce your electric bill.
Waverly Mansion - West Point, Mississippi (Notice covered porches with doors allow cooler air into the room and warmer air exits the room through transoms above the doors in the stair hall)
Operable louvered shutters were also used in the past to shade windows yet allow air movement.
Use of Overhangs or Awnings
Overhangs and awnings have made resurgence recently with the boom in sustainable or green design. In most cases the easiest option would be to add an operable awning to south-facing windows to alleviate excessive solar gain during the hot Summer months. Fixed overhangs can be incorporated into new homes or renovations to accent the architecture of the home. Operable awnings offer a more cost effective option and eliminate the requirement for solar path calculations.
When an awning on a south facing window protrudes to half of a window's height, the sunlight will be blocked during the Summer but will enter the house during the Winter.
Overhang for 40 degree N latitude
If you have a lot of East or West facing windows, you can use landscaping to help block the amount of sunlight coming into the home in the early morning and late afternoon. Deciduous shrubs and trees are best since the leaves will fall and allow sunlight to warm the space in the Winter through passive solar heating.
Finding the Position of the Sun
NOAA has a very good online calculator for determining the elevation of the Sun for your location on any date and time.
Another old technique being used in commercial buildings is convective cooling. This is where cooler outside air is brought into a space as warm indoor air is vented. This is also known as the chimney or “stack” effect. Hot air exits the building through a high vent, and cooler air is drawn in through a low vent.
You can use existing double-hung windows to create this effect. Open the lower sash on first floor windows on the windward (facing into the prevailing wind) side of the home and open second floor windows on the leeward (facing away from the prevailing wind) side of the house. The windows on the second floor should be opened about 1 ½ times as much as the windows that let the outside air in.
Windows at the ends of hallways or at the top of stairs also work very well.
If you have a sunroom, you can add vent low in the wall between the sunroom and the rest of the house and install vents or operable skylights in the sunroom to vent the warm air. As the sunroom heats up, the warm air will rise and exit through the vents or skylights and draw in air from the living areas. Windows that are crack open on the opposite side of the house will let in outside air.
Sunroom Venting (Williams College)
Another way to utilize the chimney effect is to actually build a thermal chimney. These are typically black metal chimney built on the sunny side of the house with a rotating turbine ventilator of the top. The turbine and hot air increases the draft through the chimney and the structure is thermally isolated from the house by a layer of insulation. Low vents are opened on the opposite side of the home to complete the system.
Another way to increase the amount of air into a room is to construct wing walls adjacent to windows which will deflect the wind into the opening. It is best to have at least two windows in the room to allow the room air to exit. You can also open windows on the second floor to recreate the chimney effect.
Wing walls have a similar effect as the porch walls in the Waverly Mansion.
Splayed Skylight Openings
Most skylights are large and installed in straight openings. This increases the amount of heat gain in the Summer and heat loss in the Winter through the glazing. By installing the skylight in a splayed opening, you can install a smaller skylight and get more sunlight in the room.
Splayed Skylight Opening
Resources and Links:
Energy Saving Links and Calculators
NOAA Solar Position Calculator
Sustainable By Design - Tools
Designing with Google SketchUp