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How to Improve Attic Airflow

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Easy ways for the do it yourselfer to improve the airflow in your attic to reduce cooling costs, prevent ice dams, and extend the life of your roof.

Attic ventilation is found in most homes in the United States to extend the life of the roof and reduce cooling costs in the summer months. Ventilation is not usually required 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 can be generated from plants, pets, and dishwashing. 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.

Main Benefits

There are four primary benefits to attic ventilation:

1. It prevents moisture from remaining in the attic space where it can rot on your roof's framing and sheathing.

2. Can prevent ice dams in winter by keeping your roof colder.

3. Can extend the life of your shingles by keeping the roof cooler in hot weather.

4. Can reduce cooling costs in the summer, however the savings will small with a well-insulated attic and larger for attics with little insulation.

 Before you Begin

Use caution when entering your attic as there are many safety hazards to be aware of such as protuding nails, broken framing, loose sheathing used for temporary walkways, and low hanging framing that connects trusses or rafters together. If you have an older home with vermiculite insulation you should be aware that asbestos has been found in some types of vermiculite insulation.

Vermiculite is a lightweight material resembling gravel that was used as attic insulation in almost one million homes. If you have vermiculite in your attic, don't disturb it unless you have a sample checked by an accredited laboratory. Disturbing it can release the asbestos fibers, which, once airborne, can enter your lungs and eventually cause lung disease. For a list of accredited testing labs, call your local bulding or health department. For more information on vermiculite go to the Environmental Protection Agency (EPA) at www.epa.gov  or call your regional EPA office.

Signs of Poor Attic Airflow

Winter is a good time to check for proper airflow in your attic as warm, moist air will quickly condense or freeze and show you where the problem areas are located.

1. Frost forming on the underside of the roof or rafters as warm, moist air is trapped in the attic and condenses and freezes on the wood.

2. Water-stained or blackened wood is a sign of mildew or rot. This can also be found in the summer.

3. Heavily rusted nails is another sign that condensation is forming on cold metal surfaces.

4. Matted-down insulation could be a sign of roof leaks from ice damming or other roof problems.

There are five common attic vents:

1) Top hat, mushroom, or can vents

These vents are found in older homes and in northern climates with cooler summer temperatures.

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.

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.

4) Turbines

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.

Steps to Improve Airflow

Install Attic Baffles or Air Chutes

Install attic or insulation baffles, also called air chutes, in each rafter space to keep the air path clear between the rafters and the roof sheathing. Wear long sleeves, goggles, and a respirator to prevent irritation from fiberglass or other insulation. These air chutes are typically foam or plastic that keeps insulation away from the bottom of the sheathing and allows airflow from the eave vents to the gable or ridge vents.

Attic Baffle/Air Chute

Seal Gaps

Seal gaps around plumbing drain vent pipes, ductwork and electrical boxes with minimal-expanding foam or caulk. This helps keep warm, moist air out of the attic. Locate which rooms have ceiling fixtures and locate the electrical boxes in your attic. Carefully move the insulation away from the box and use a foam sealant or loose fiberglass to block any gaps between the drywall ceiling and the box.

Clean soffit vents

Soffit vents come in several different styles, but the most common have an insect screen on the inside that is protected by an aluminum louver grille. Continuous airflow can cause dirt and pollen to clog the screen and reduce the airflow. Clear your soffit vents every few years with compressed air or remove them and wash them off with a hose and soft brush. Clean the soffit vents after you install air chutes or you could dislodge the insulation and block the vent.

As stated previously, there are several different styles of soffit or eave vents and some work better than others. For any soffit vent to work, you need an appropriately sized ridge vent or gable vents. Circular soffit vents look good but are often too small to provide enough airflow especially when not removed and cleaned. Continuous vents or starter vents are a better choice except where the architecture or eave construction prohibits their installation.

Circular vents installed in exposed rafter bays

Ventilation Requirements

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)

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.

Example Calculation

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.

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Posted on Jan 17, 2011

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Daniel Snyder

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