Natural Ventilation

Natural ventilation requires a source of air of an acceptable temperature, moisture content, and cleanliness, and a force—usually wind or convection—to move the air through the inhabited spaces of a building. Air flows through a building because it moves from higher pressure to lower pressure areas. Controls are provided for the volume, velocity, and direction of the airflow. Finally, the contaminated air must be cleaned and reused or exhausted from the building.

The simplest system for getting fresh air into a building uses outdoor air for its source and wind for its power. Wind creates local areas of high pressure on the wind ward side of the building, and low pressure on the leeward side. Fresh air infiltrates the building on the windward side through cracks and seams. On the opposite side of the building, where pressure is lower, stale indoors air leaks back outside. Wind-powered ventilation is most efficient if there are windows on at least two sides of a room, preferably opposite each other. The process of infiltration can be slow in a tightly constructed building. Loose-fitting doors and windows result in buildings with drafty rooms and wasted energy.

Depending on the leakage openings in the building exterior, the wind can affect pressure relationships within and between rooms. The building should be designed to take advantage of the prevailing winds in the warmest seasons when it is sited and when the interior is laid out.

Very leaky spaces have two to three air changes or ^Çf more per hour. Even when doors and windows are weather-stripped and construction seams are sealed airtight, about one-half to one air change per hour will occur, but this may be useful for the minimum air replacement needed in a small building. Weather-stripping materials generally have a lifespan of less than ten years, and need to be replaced before they wear out.

In convective ventilation, differences in the density of warmer and cooler air create the differences in pres sure that move the air. Convective ventilation uses the principle that hot air rises, known as the stack effect. The warm air inside the building rises and exits near the building's top. Cool air infiltrates at lower levels. The stack effect works best when the intakes are as low as possible and the height of the stack is as great as possible. The stack effect is not noticeable in buildings less than five stories or about 30.5 meters (100 ft) tall. In cold weather, fans can be run in reverse to push warm air back down into the building. Fire protection codes restrict air interaction between floors of high-rises, reducing or eliminating the stack effect. To depend on convective forces alone for natural ventilation, you need relatively large openings. Insect screens keep out bugs, birds, and small animals, and admit light and air, but cut down on the amount of airflow. Systems using only convective forces are not usually as strong as those depending on the wind.

The ventilation rate is measured in liters per second (L/s) or in cubic feet per minute (cfm). It takes only very small amounts of air to provide enough oxygen for us to breathe. The recommended ventilation rate for offices is 9.44 L/s (20 cfm) of outside air for each occupant in nonsmoking areas. About a quarter of this amount is required to dilute carbon dioxide from human respiration, while another quarter counteracts body odors. The remainder dilutes emissions from interior building materials and office equipment. This works out to slightly more than one air change per hour in an office with an eight-foot high ceiling. Lower ceilings create greater densities of people per volume, and require higher rates of ventilation.

Especially high rates of air replacement are needed in buildings housing heat- and odor-producing activities. Restaurant kitchens, gym locker rooms, bars, and auditoriums require extra ventilation. Lower rates are permissible for residences, lightly occupied offices, warehouses, and light manufacturing plants.

Using natural ventilation helps keep a building cool in hot weather and supplies fresh air without resorting to energy-dependent machines. However, in cold climates energy loss through buildings that leak warm air can offset the benefits of natural cooling. Careful building design can maximize the benefits of natural ventilation while avoiding energy waste.

Attic ventilation is the traditional way of controlling temperature and moisture in an attic. Ventilating an attic reduces temperature swings. It makes the building more comfortable during hot weather and reduces the cost of mechanical air conditioning. William Rose, with the Building Research Council at the University of Illinois, has been conducting some of the first research into how and why attic ventilation works.

Thermal buoyancy—the rising of warm air—is a major cause of air leakage from a building's living space to the attic, but Rose's research shows that wind is the major force driving air exchange between an attic and the outdoors, and that the role of thermal buoyancy in diluting attic air with outdoor air is negligible. Generally, we assume that warmer air rises and escapes from high vents in an attic, while cooler air enters in lower vents. Some ridge vents at the roof's peak may in fact allow air to blow in one side and out the other, without drawing much air from the attic. Ridge vents with baffles may create better suction to draw air out.

Soffit vents, which are located in the roof's overhang, work well as inlets and outlets. There's less problem with rain and snow getting in, because soffit vents point downward. Soffit vents should always be installed whenever there are high vents on ridges or gables, which pull air out of the attic. Without soffit vents, makeup air would be drawn through the ceiling below, which increases heat loss and adds moisture to the attic.

To get maximum protection, soffit vents should be located as far out from the wall as possible, so that rain or snow blowing into the soffit is less likely to soak the insulation or drywall. They should be distributed evenly around the attic, including corners. At least half of the vent area should be low on the roof. The net free area (NFA), which is stamped on the vents, indicates resistance, with higher numbers indicating less resistance and better airflow.

Rose's research shows that a ventilated attic is slightly warmer on a clear, cold night than an unvented attic. In winter, venting maintains uniform roof sheathing temperature, which reduces the likelihood that ice dams will form. Without good ventilation, warm spots form near the eaves that melt snow against the roof shingles, which can later refreeze into an ice dam. Water runs down until it is over the eaves, where it refreezes. This ice then builds up and causes the water collecting above it to seep in under the shingles and into the eaves or the house. More melting snow can build up behind the ice dam and damage the building.

Chronic ice dam problems often lead to the use of electric heater cables or snow shoveling to attempt to clear the snow out of the way. Using self-stick rubberized water and ice membranes plus roof ventilation can prevent ice dams.

Warm air rising up through plumbing, electrical, and other penetrations into the attic will also heat the roof sheathing. Adding ventilation without sealing air leaks into the attic can actually increase the amount of air leaking from the house, wasting valuable heat and potentially making ice dams worse. Air leaking out of air handlers and ducts, and heat leaving the system by conduction can be among the largest causes of heat loss and ice damming.

Heated air escaping into the roof not only contributes to ice dams and heat loss, it is also the primary means for moisture to get into attic or roof framing, where it can condense and cause mold, mildew, and structural damage to the roof. Surprisingly, much of the moisture that rises through openings around plumbing, ducts, and wires comes as water vapor in air vented from crawlspaces. Once in the attic, the air cools, allowing its water vapor to condense on roof sheathing. Ventilation alone can't take care of moisture in the attic. Keeping dampness out of the building—especially out of the basement and crawlspace—helps protect against condensation and mildew in the attic. An airtight ceiling is also important.

Installing rigid insulation in the eaves (the projecting overhang at the lower edge of a roof) reduces heat loss in the eave area. Another option is to change the framing detail to one that leaves more room between the top plate and the rafter. Cardboard or foam baffles precut to fit 16- or 24-in. on center framing can eliminate wind blowing across insulation.

Eliminate leaks that allow heated air to escape into the attic at top plates, wiring penetrations, plumbing vents, and chimney and duct chases. Recessed lights are responsible for significant heat loss; be sure to use fixtures rated for insulation contact (IC rated) and air tightness.

Heating, ventilating, and air-conditioning (HVAC) equipment and ductwork in attics will waste leaking air. If there is no alternative, all ducts should be sealed tightly and run close to the ceiling, buried in loose fill insulation to the equivalent R-value of the attic insulation.

Once you eliminate the heat loss in the attic, there is little driving force to pull air through the vents. However, code-required ventilation openings in attics and cathedral ceilings should be installed as a backup measure.

Though now valued for style, symbolism, and attractiveness, cupolas (Fig. 21-1) represented early air-conditioning. The cupola was a high point in which the hottest air in the house could collect and from which it could escape outside because hot air's natural buoyancy causes it to rise. Cooler air was in turn drawn into the house through the open windows below. This stack effect becomes most effective when there is a good source of hot air to accelerate the flow, as from an attic. When the wind was blowing briskly through the cupola, an updraft throughout the house pulled cooler air in through the windows. However, without at least a little wind, you didn't get much ventilation. Using a cupola

or ridge vents along the top of the roof will cool only the attic if there is an air and vapor barrier and blanket of insulation isolating the attic from the house below, as is customary today.

Roof windows, also called operable or venting skylights (Fig. 21-2), can create the same updraft throughout the house as an old-fashioned cupola. When shaded to keep direct sunlight out, they are one of the best natural ventilating devices available. However, their value for cooling alone does not compensate for their initial cost. Roof windows also allow moisture to escape from kitchens, baths, laundry rooms, and pool enclosures.

Roof windows are available with remote controls and rain sensors. Skylights can be prewired for sun-screening accessories, including sun-blocking shades, pleated shades, venetian blinds, or roller shades. Exterior awnings block up to 40 percent more heat than interior shades, and are available with manual and automatic controls. Energy Star® skylights use low-emissivity (low-e) glass coatings, warm edge technology that ensures that the areas around the frames don't reduce the insulating properties of the glazing, and energy-efficient blinds that improve overall energy efficiency.

Roof ventilators also increase natural ventilation. Some roof ventilators are spun by the wind, drawing air from the room below. Some rely on convective flow, while some create low-pressure areas that are then filled with interior air. Wind gravity or turbine ventilators create suction when wind blows across the top of a stack, pulling air up and out of the building. Roof ventilators require control dampers to change the size of the opening as necessary.

Doors should not be relied upon for essential building ventilation unless they are equipped with a holder set at the desired angle. An ordinary door can't control the amount of air that flows past it.

In residences, ventilation is tied to the quantity of exterior windows and the amount of natural ventilation they supply. If the bathroom does not have a window, it is required to have a fan with a duct leading directly to the exterior. A window provides not only ventilation, but also daylight and possibly a room-expanding view. A percentage of the windows in a residence must be operable for ventilation and emergency egress.

William McDonough + Partners designed the offices for Gap Inc. in San Bruno, California, in 1994 around the concept that people would rather spend their day outside. Daylight, fresh air, and views of the outdoors are celebrated throughout the two-story structure. Fresh air is available through operable windows throughout the building. A raised floor provides ventilation that puts fresh air directly at the occupant's breathing level as oxygen-depleted air and indoor air pollutants are carried upward. At night, cool night air is run across the thermal mass of the slab within the raised floor. The raised floor also eliminates the need for dropped acoustic ceilings, allowing the exposed acoustical deck to reflect lighting. Through careful use of daylighting, fresh air, and other methods, the Gap office building exceeds its goal of being 30 percent more energy efficient than is required by California law, at a cost that was expected to be repaid by energy savings within six years.

The Lewis Center for Environmental Studies at Oberlin College bases ventilation rates on carbon diox ide levels in the building. As more students enter the building, the carbon dioxide levels rise, triggering the HVAC system or automatically opening clerestory windows. This ensures that the building is not being ventilated more than it needs, thus saving heating and cooling energy.

In the past, the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) standards for building ventilation have shown a preference for mechanical ventilation systems. In response to energy conservation issues, however, these standards have been modified, and in 2002, ASHRAE is scheduled to introduce an alternative ventilation standard for naturally ventilated buildings.

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