The two fundamental benefits of an effective attic ventilation system are:

1. A cooler attic in summer.
2. A dryer attic in the winter.

Both benefits result in energy savings. Greater homeowner comfort; and higher structural integrity of the dwelling.

Summer sunshine causes a buildup of heat in the attic space, created by radiant heat from the sun increasing the roof temperature so that it re-radiates heat into the attic. During the night, the cooler outside air and absence of sunshine permits the attic to re-radiate or conduct stored heat to the atmosphere. However, an unventilated – or poorly ventilated attic is not able to loose all of its stored heat in this manner. This results in heat build-up over a period of days, with ever increasing attic temperatures

It there is no attic ventilation, a 90° day with full sunshine can heat the roof decking to 170° or more. Heat radiating down to the attic floor (or ceiling insulation) can raise its temperature to as much as 140° . (See Figure 1)

Homes today are being built with heavier insulation than ever before. This increases the need for effective attic insulation. Without it, today’s heavier insulation absorb and holds more of the heat build-up in the attic during the day, making it less likely that all of the heat of the attic will be removed into the nighttime air. Residual stored heat can build up over a period of hot days, maintaining the insulation at a higher temperature and increasing the heat radiation down to the rooms below.

Over-heated ceiling insulation conducts heat through to the ceiling, and this heat is then radiated downward to persons and objects to the rooms below. (Figure 2) There are two consequences:

The solution to the problem is attic ventilation. A sufficient volume of ventilation air must be moved through the attic space – under varying conditions of wind forces and direction. Also, the ventilation air must move in a uniform pattern along the entire underside of the roof, avoiding isolated non-ventilated "hot spot" areas.

1. Does not itself use electric power and thus nullify energy savings.

2. Completely ventilates and cools the underside of the roof decking, which is the source of heat radiation downwards to the attic floor or top of the ceiling insulation.

Winter conditions bring a different type of problem. During the cold months of the year, the air inside the home is warmer and carries more water vapor than the colder, dryer air in the attic. Cooking, laundry, showers, humidifiers and other activities using water contribute to this condition. There is a strong natural force, termed "vapor pressure" that causes water vapor to migrate from high-humidity air or materials to low-humidity air. This migration of water vapor passes through ceilings, insulation and wood – and even successfully circumvents a vapor barrier. Its moves into the attic space where it can readily condense into liquid water on the cooler structural members – rafters, trusses, and especially the cold roof sheathing. (See Figure 3)

Another factor increasing the likelihood of a moisture condensation problem in many homes today is the use of an electrical heating system, more prevalent with the nationwide shortage of gas and oil. Because of the higher cost of heating with electricity, these homes are being built "tighter" and with greater insulation. This helps retain moisture in the home, which is transferred to the attic by the strong driving force – vapor pressure. This moisture problem is heightened by the fact that, in an electrically heated home, outside air is not drawn into the home to replace furnace combustion air. A minimum of dry outside air enters the home; air within the home acquires a higher moisture content and becomes a more likely source of problems.

The water vapor moving into the cold attic space in winter can be present in such a volume that it condenses on cold structural parts and drips down to soak ceiling insulation, compressing its volume and reducing its insulating effectiveness. As a consequence, heat loss through the ceiling increases and more energy is required to maintain the residence at a comfortable temperature.

Figure 4 shows how condensing moisture within an attic or ceiling space can dampen and compress insulation. Even small amounts of such condensation can have a substantial effect on R-Value.

Research has shown that, as moisture is added to fiberglass insulation, the R-Value is reduced as shown below:

How much attic ventilation is required to provide proper temperature and moisture control? A number of studies sponsored by federal energy funds are under way to look at ventilation rates and methods. However, several studies previously made can help with this decision. The maximum ventilation rate is required to remove heat during the summer cooling months. Attics can reach temperatures of 150 to 160 degrees F during a summer day, although outside air temperatures are only 95 to 97 degrees F. The cooling load for a home air conditioner depends on the difference in temperature between the inside and outside air, and reduction of attic temperatures from 155 degrees to 105 degrees F will result in a significant reduction in cooling load. In a home with poor ceiling insulation, heat movement through ceilings may account for 30 percent or more of the total cooling cost. With a well-insulated ceiling, this source of heat may account for only 12 to 15 percent of the total cooling cost. Thus, high attic ventilation rates are most important for poorly insulated ceilings. A poorly insulated ceiling is one whose R rating is less than 14 or one with fewer than 4 inches of fiberglass, rockwool or cellulose insulation.

Attic temperature depends on the amount of solar radiation, construction details and the rate of ventilation. Calculations indicate that on a July day in Texas, a ventilation rate of one air change per minute for a typical attic using 95-degree F air will lower the peak attic temperature to about 101 degrees F. Providing half air change per minute will lower the temperature to about 106 degrees F. Thus, the first half change per minute is most effective and a doubling of this rate only achieves about 5 degrees F additional cooling. Studies indicate that further increases in ventilation are not effective in significantly reducing attic temperatures.

Winter attic ventilation must be sufficient to remove moisture vapor moving from the living space to the attic. In general, ventilation adequate for summer cooling is more than adequate for winter ventilation. Winter rates need not be more than about a tenth of the summer rate.

Calculate the required summer ventilation rate by determining the volume of attic space and dividing by 2. This will be the cfm (cubic feet per minute) of ventilation air needed. The volume is determined approximately for a rectangular house by multiplying the height from the ceiling to the peak/ridge (H) times the width of the house (W) times the length (L) and dividing by 2 -- ( H x W x L / 2 ). For a gable roof, this will be reasonably accurate. For a hip roof house, the volume will be overestimated but adequate.

A common method of ventilating attic space is with soffit vents, usually but not always in combination with other vents in the roof or gable ends. The soffit is an excellent location for venting, particularly for intake venting, because it is less exposed to rain and snow. Also, the wind is parallel to the vent regardless of direction. Thus a soffit-vent-only system is always in balance.

The problem with soffit-vent-only system is that ventilation is confined to the attic floor. There is no air movement into the upper part of the attic, consequently, very little moisture removal. There is no air movement due to thermal effect. There is virtually no air movement across the underside of the roof decking. (See Figure 18)

Roof vents, while they can be installed near the ridge, and thus providing an escape route for over-heated air, having a number of disadvantages. If used with no other type of venting, the pattern of air circulation is very small, confined to the space immediately surrounding the vent. (See Figure 19) Air will enter through some vents and exhaust through others, depending on wind pressure areas on the roof. Weather infiltration can be a problem under these circumstances.

The usual installation combines roof vents with soffit vents. The air flow pattern is not uniform (See Figure 20) with large portions of the roof decking unventilated. Also, since roof vents provide only 40 to 80 square inches of net free area apiece, it is virtually impossible to have enough roof vents to balance the soffit vents. Air usually enters one soffit and flows out the other soffit, with some finding its way up to the roof vents.

A roof vent with a turbine wheel mounted on top of it continues to act as a roof vent, with the same pattern of airflow as shown in Figures 19 and 20. At low wind speeds, when the movement of ventilation air into and out of the attic is most needed, the turbine vent is not better or worse than an ordinary roof vent. It has been shown in tests that air may actually enter through the turbine vent, depending on wind pressure conditions. Air, rain, and snow may enter; even with the turbine wheel turning, just as it would with a regular roof louver.

The gable or end wall louver, whether triangular or rectangular in shape, offers a ventilation air pattern as limited as the roof vents. When there are louvers in each gable – and no soffit venting – the ventilated area depends upon the wind direction. Figure 21 shows what happens when the wind is perpendicular to the ridge. Each louver acts as both intake and exhaust, and only the ends of the attic receive any air circulation.

The soffit vents added, the airflow has changed in that there is movement across the attic floor – similar to the pattern of soffit vents used alone. As Figures 23 and 24 indicate, gable and louvered soffit vents largely operated independently of each other instead of combining into an effective system.

Vents in the roof or gabled ends employing a powered fan to move air out of the attic are of questionable value. In the first place, they cannot save energy! The power required to operate the fan exceeds the power saved in air conditioning the residence, compared to any fixed louver ventilation system meeting FHA Minimum Property Standards. [FHA Standards are "minimum" at best!] Depending on air location, air moves in the direct path or shaft from the intake vent – such as in the soffits – to the fan; creating a narrow, restricted pattern. (See Figures 25 & 26)

If the power fan is controlled by a thermostat switch, there is no fan operation during the winter months, when air circulation over the roof decking is needed in order to prevent moisture condensation. Thus, a humidistat must be added to the installation in order to effect winter ventilation, which in any event occurs in an ineffective pattern. The need for electrical installation adds to the high initial cost of a powered vent, with maintenance and repairs costs also a factor.

There is one attic ventilation system that effectively uses the two natural forces of wind pressure and thermal effect to continuously and uniformly ventilated the entire underside of the roof decking. It is the combination of continuous ridge vent and equal net free area of soffit venting, half of each on each side of the structure.