Your home’s roof vent provides a weatherproof air outlet; it helps cool the attic and to cool the whole house off at night. It also removes excess humidity, moisture and odors from your home environment. With modern home’s emphasis on energy efficiency, construction is tighter, walls, windows and doors breathe less air in and out of the house, and adequate ventilation has become more important.
The point to remember is that a balance between the amount of air being drawn in to a house and the amount of air being exhausted needs to be maintained. If a home’s ventilation is not properly designed, bathroom exhaust fans, clothes dryers and vent fans may create a negative air pressure, drawing in air through chimneys, exhaust flues and framing gaps; this in turn can cause combustion appliances such as wood burning fireplaces, gas stoves and furnaces to backdraft, spreading carbon monoxide into the home, a dangerous condition.
How does Roof Ventilation Work?
Ventilation can be defined as the process of moving volumes of air from one area to another (as opposed to Circulation, the process of moving air around in one confined space). The air movement that happens during ventilation can be achieved by either mechanical methods (using exhaust fans) or passive, natural methods (vent openings). Passive ventilation relies on inductive action and thermal effects.
Inductive ventilation is due to the action of wind force. It works on same the principle as the lift action created by a wing of on aircraft, except the effect here is side to side, rather than up and down. Picture a stream of wind blowing sideways at a constant velocity. When an object is placed in this airstream, the wind must flow around it, creating a vacuum on the downstream side of the object.
A simple ventilator could then be constructed by creating an opening on the downstream side of the object, and placing the object on top of a flue pipe attached to the roof of a home. The vacuum will induce an upward flow of air from the interior of the home into the airstream.
Thermal ventilation works through the difference in temperature between the indoor and outdoor air. When the outdoor air temperature is lower than the indoor temperature, warm indoor air, being lighter in weight, will rise up and out through any ventilation, while cooler, heavier air will flow in. Thermal effects and wind inductive effects combine to produce passive ventilation.
Mechanical ventilation is required when the air volume necessary to ventilate a space cannot be moved through passive means. This involves the use of electrical fans and air ducting to increase the air flow. The volume of air needed to properly ventilate a space is established by dividing the actual volume of the space by the rate of change, measured in minutes. Local regulations and building codes generally will govern ventilation requirements for given space usages and number of occupants.
Different Types of Roof Vents
There are many different designs and sizes of roof vents, but most can be classified into one of the following five types: Revolving, Stationary Head, Ridge, Turbine, or Siphonage. Bases of ventilators should be shaped like a tapered cone in order to provide a bell nozzle effect, ensuring the highest air flow compared to a square entrance.
Remember that all ventilator types will be much less effective if installed on a roof in an area where no wind passes, such as between two taller buildings, or a depression in the roof.
Stationary head ventilators are the simplest and most common type; they are considered the most efficient. Their advantages over other types include that fact they have no moving parts to maintain, thus no upkeep is required. They are also very quiet and have a higher exhaust capacity in all wind conditions.
Revolving ventilators have a top portion that rotates on a pivot such that their open end will face opposite the direction of the wind; this is accomplished by an integral wind vane. The inductive force generated by this type is not as efficient as the stationary head type. Another disadvantage is that the revolving head has low ventilating capacity when there is no wind, unlike a stationary head ventilator. Maintenance is required in order to prevent the revolving portion from becoming stuck in position since, if it does not revolve, it will not follow the wind and have no inductive power. It will also become noisy when changing direction, creaking and rattling.
Turbine ventilators have a head that features series of rotating blades shaped like an onion dome. As the wind rotates the head, the blades draw air up from the flue shaft and expel the air through the openings between the blades. As in the revolving ventilator, the design has a big disadvantage when there is no wind; not only is ventilation reduced, but rain and snow can enter the ventilator and hence the home.) Also, if upkeep is not done and the vane bearings do not turn freely, the ventilation capacity is reduced. Upkeep includes cleaning, oiling and replacing the bearings. This type also has the tendency to become noisy as the blades age and go out of balance.
Ridge roof vents are box-like enclosures that run along the entire top ridge of the roof to be ventilated. They employ a valve in the roof which exhausts warm air rising. It relies mostly on thermal effects, as the wind can only create inductive forces if it is blowing directly across the ridge vent.
Spihonage ventilators have no moving parts and rely on wind inductive force. The induction is created by a duct concentric to the shaft of the ventilator which causes wind to flow upwards. The duct head is designed to exhaust both the indoor air and the siphon airstream creating the induction, and this makes for a somewhat less efficient airflow.
Another type of ventilator is the fan ventilator, which has a motorized fan which boosts ventilating capacity greatly. It also greatly reduces any dependence on wind, since it does not rely on passive inductive ventilation. This allows more leeway in placement so that ventilation can be provided directly over the area requiring ventilation.
Photo by noraxx, Creative Commons Attribution License