How to determine the amount of ventilation required for your attic and a discussion of various attic vents.
Attic ventilation is found in most homes in the United States, except in areas where drywall is applied directly to roof rafters as is the case with cathedral ceilings. An average family of four generates two to four gallons of moisture per day just by cooking, breathing, bathing, cleaning and doing laundry. Additional moisture is generated from fish tanks, plants, pets, and air-drying laundry to name a few. Some of that moisture rises into the attic as a gas and condenses on the roof structure, causing mold, rot and sub-structure and shingle failure.
The five most common attic vents are discussed below.
1) Top hat, mushroom, or can vents
These vents are found in older homes and in northern climates with cooler summer temperatures.
Metal Can Vent
A few manufacturers make solar powered attic vents that can fit in the openings of these older style vents.
Solar powered Attic Vent (www.ussunlight.com )
2) Ridge vents
Ridge vents are a close second and may become more popular if the quality is maintained and they are installed properly.
They should not be installed with any other attic vents and all other vent openings other than soffit vents should be closed off. If ridge vents are added and existing can, gable, power or other vents are not closed, removed or sealed, airflow and thermal characteristics of the entire attic ventilation system can be compromised which can lead to the possibility of mold developing and reducing the life of the roof shingles.
Occasionally a builder will install gable vents with a ridge vent for aesthetic reasons. The gable vents should be closed off by nailing plywood over them so the ridge vent can function as designed.
To be effective, ridge vents must be installed with an equal or slightly greater amount of soffit ventilation. If installed correctly, they work well with most roofs. The exception is a hip roof, which has four sides to it and a limited span of horizontal ridge. Usually, there is not enough ridge length to provide adequate ventilation. In this case, can vents are a better choice for a hip roof.
Well-designed ridge vents have baffles that run the entire length of the vent, both front and back. Those baffles are critical to directing wind upward and sucking more hot air out of the attic. If older ridge vents were installed that do not contain internal baffles, there will be noticeable water marks along the length of the ridge.
Additionally, if there is not an equal or greater amount of soffit vents with a ridge vent, air rain and snow can be pulled into the attic down through the ridge vent, regardless of the vent brand.
Ridge Vent - Shingle-Over
3) Gable vents
Gable vents are louvers with a mesh screen on the attic side that are installed in the gable end walls of an attic. They are inexpensive, but the least effective of the common attic vents since they depend on wind direction to vent. They can be plastic, metal, or wood.
While gable vents do increase ventilation they are not a preferred manner of attic ventilation as they do not offer an even airflow across the attic and their ability to move large amounts of air are limited.
Gable vents can be installed with a power fan behind them similar in theory to the power roof vent. Installing a power fan will greatly improve the amount of ventilation that the gable vent can offer, but may not be as effective as a ridge vent.
Turbines are extremely effective if and when the wind is blowing; otherwise, ventilation is limited. Certain models and older turbines can be noisy.
5) Power vents
Power vents work very well, but they are electrical appliances. If and when the motor fails, it can be some time before the homeowner notices that there is a problem. Some models are not equipped to remove humidity from the attic, but they can be retrofitted by installing a combination thermostat /humidistat to turn it on when the temperature or humidity reaches certain levels.
Gable Power Vent
Regardless of the type of attic ventilation used, you should check that it is moving the required volume of air.
The common rule of thumb for attic ventilation is to have one free square foot of ventilation for every 150 square feet of attic space, 1/150. If there’s a vapor barrier beneath the insulation, it can be 1/300.
The calculation is based on a 50/50 split between the roof vents and the soffit vents. Second, you should increase the soffit venting area, but never increase the roof vents. You always need to have an equal or greater amount of soffit ventilation.
Although it’s necessary to take into account how much air flow is restricted by the screens and louvers on all the vents, most manufacturers help with this calculation by stamping on the vent the amount of ventilation it allows, technically called the “net-free vent area.
To balance a ridge vent with the correct amount of net-free area intake vents, measure the length of the ridge vent and multiply the number of feet of ridge vent by 18 and divide by 144. (18 square inches is the typical amount of net-free area for most ridge vents and there are 144 square inches in a square foot. http://www.airvent.com/homeowner/products/ridgeShingle-specs.shtml )
Example: (30 feet of ridge vent x 18 sq. in./ft)/ 144 inches/foot = 3.75 square feet net-free area of ridge vent, so you would need at least this amount of soffit vent net-free area.
The following example is provided by Air Vent Inc. (www.airvent.com ).
1. Measure the length and width and multiply to determine the square footage of attic area.
Length is 40 feet and width is 25 feet.
Calculation: 40’x25’= 1,000 square feet of attic area.
2. Determine the total net free area required.
Once attic square footage is known, divide by 150 (for the 1/150 ratio for an attic without a vapor barrier). That determines the total amount of net-free area needed to properly ventilate the attic.
Calculation: 1,000 square feet divided by 150 = 6.7 square feet of total net-free area.
3. Determine the amount of intake and exhaust (low/soffit and high/ridge) net-free area required.
For optimum performance, the attic ventilation system must be balanced with intake and exhaust vents.
Divide the net-free area by 2.
6.7 divided by 2 = 3.35 square feet of intake net-free area and 3.35 square feet of exhaust net-free area.
Hopefully this will provide you with the additional knowledge to determine if you have adequate attic ventilation to help you save on your home’s heating and cooling costs, and prevent damage to your roof system.