May, 2010
Inspection News and Views from the American Society of Home Inspectors

Attic Ventilation


Insulation and attic ventilation are listed together in the ASHI Standards of Practice. Although there is a hot-roof theory, which recommends sealing an attic airtight, overwhelmingly home inspectors satisfy the ventilation requirement of the Standards by inspecting, evaluating and reporting on the basic methods of ventilating attics. In order to do so, they become familiar with soffit and roof vents and how to calculate attic-venting requirements.

The benefits of an appropriate combination of attic ventilation; exhaust fan venting to the exterior and insulation are widely accepted. Benefits include lowering heating and cooling costs; preventing premature failure of the roof and deterioration of floorboards, and lessening susceptibility to ice dams in cold climates and mold in all climates.

In the attic, insulation and venting deserve equal attention. In general, insulation should seal all gaps between and around chimneys, chases, plumbing vents and recessed lights that can be covered with insulation. Lights that can be covered have IC stamped inside. IC stands for “insulation contact.” Those without the IC stamp should have a 3-foot-square wall built around them, creating a box that can be covered with insulation.

For more information about attic insulation, there is a map at showing the recommended amounts by geographical area, and I’ve posted an article about adding insulation on my Web site. See “Packing It In” on

Inspecting attic ventilation

Soffit vents and five common attic vents are the basic methods of attic ventilation encountered or recommended by home inspectors.

Soffit vents
Most people know hot air rises and cold air falls. Soffit vents are the air intake for moving what otherwise could be stagnant or slow-moving hot air through the attic and out through the roof vents.

If a house has eaves (commonly referred to as overhangs), they should be vented.

The best vents are continuous, running the full length of the eave.

Next are rectangular vents, followed by small 3/4-inch to 3-inch holes spaced along the entire length.

Aluminum and vinyl soffit trim usually has small holes or slots, that provide airflow.

All soffit vents have screening or louvers to keep out birds and insects. However, the screening also restricts the airflow, though not as much as those small openings in the aluminum and vinyl soffit trim.

As a general rule of thumb, you almost never can have enough soffit vents.

Common issues with soffit ventilation include the following:

• None or inadequate vents

• Lack of vents behind aluminum soffit trim (Vents were not cut out; do not exist.)

• Insulation stuffed into the eaves or blown over the vents

• Over-painted vents, which restricts air-flow through the louvers and screens

• Vents obstructed by dust or dirt

• No baffles in the attic

Baffles can be inexpensive cardboard or Styrofoam chutes. Their purpose is to keep the soffit ventilation clear and unobstructed. Baffles are installed between the roof trusses or rafters and are placed over the vents, where the roof slopes down and attaches to the top of the exterior wall. Insulation is placed against the baffle to keep it in place. Ideally, you should see some light from the vent if you look down the baffle between the underside of the roof boards and the baffle.

If a house has few or no eaves, venting is still possible. At least two companies make venting systems for this type of house: Smart Vent® by DCI (800-622-4455, and The Inhaler® from Great Northern Building Products (800-258-6245, Both systems are installed under the shingles along the edge of the roof behind the gutters. They are expensive, but a lot less costly than premature roof failure, ice-dam damage and mold.

Attic vent types

The five common types of vents for attics are described below, with special attention given to ridge vents because this method of venting is highly effective when done right, but it causes problems when done incorrectly.

1) Top hat or can vents are most common in northern states and the Midwest.

2) Ridge vents
are a close second and may become more popular if the best-designed types are used properly.

They should not be installed with any other attic vents and all other vent openings (except soffits) should be closed off. If ridge vents are added and old can, gable, power or other vents are not closed, removed or sealed, the thermal dynamics of the entire attic ventilation system is thrown off, presenting the possibility of mold developing and reducing the life of the roof shingles.

Sometimes, 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 (unlike a gable roof, with just two sides). Usually, there is not enough ridge length to provide adequate ventilation. 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/moist air out of the attic.

I have inspected numerous houses in my area that have ridge vents without baffles. It is obvious rain and snow blows into the attic through those vents, causing problems.

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
are louvers with screening that are installed in the gable walls of the attic. They are inexpensive, but the least effective of the common attic vents since they depend on wind direction to vent.

4) Turbines
are extremely effective if and when the wind is blowing; otherwise, ventilation is limited. Some brands and older turbines can be noisy.

5) Power vents
work great, but they are mechanical devices and when the motor fails, it is out of sight, out of mind. Also, some types are not equipped to remove humidity from the attic, but this can be corrected by installing a combination thermostat /humidistat to turn it on when the temperature or  humidity reaches certain levels.

How Much Ventilation is Enough?

Regardless of the method used, there is always a question about whether or not it’s doing the job.

The dominant theory is 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.

Wouldn’t it seem you could cut one-square-foot holes in the roof until you get to 1/150? Of course, it doesn’t work that way.

First, the total equation is based on a 50/50 split between the roof vents and the soffit vents. Second, you can and should over-vent the soffit area, but never over-vent the roof vents without an equal number or greater amount of soffit ventilation.

How do you balance a ridge vent with the correct amount of net-free area intake vents? A general rule of thumb is to measure the length of the ridge vent and multiply by 18. Install the same or greater amount of net-free venting in the soffits.

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.” See the example provided by Air Vent, Inc. for more help with the calculations.

The Hot Roof Theory

Although seldom encountered, there is a theory that it is better to seal an attic than vent it.

For an unvented attic, spray foam insulation, which can be closed- or open-cell, is sprayed onto the entire underside of all the roof boards and rafters. It is sprayed down to the eaves, blocking off any opening where air can get into the attic from the exterior.

Open-cell has an insulation R-value of about R 3.4 per inch. While it is a good insulation, it is not a vapor retarder, which might be important in an attic.

Closed-cell foam provides a higher R-value of approximately 6.6 per inch and also is less permeable than open-cell foam, which is better for colder climates.

Both types of foam are two-part compounds mixed by the installer as it is being sprayed in place. Both begin to expand immediately. That’s a big plus when you’re trying to fill every void.

The foam is perfect for walls, crawlspaces, cathedral ceilings and attics that have furnaces in them.

Most builders, architects and inspectors in the United States subscribe to the cold roof theory rather than the hot roof theory. That’s why we insulate the interior ceilings and walls to keep conditioned air in our living space and ventilate the attic.

Like all projects, if not done properly, there will be problems down the road with the hot roof application. If the foam is not 100 percent complete, even small air leaks can cause extensive rot to the roof boards.

Finally, Dr. Joe Lstiburek, an internationally renowned building and construction expert, stated in a recent ASHI seminar that a totally insulated and sealed attic reduces approximately 10 percent of shingle life.

While current building codes may allow unvented attics, a homeowner should always check with the roof shingle manufacturer before going this route. Some shingle manufacturers may void their warranty.

Why Vent?

If clients question the importance of ventilation, we can remind them the average family of four generates two to four gallons of moisture per day just by cooking, breathing, bathing, cleaning and doing laundry. That doesn’t include fish tanks, plants, pets, air-drying laundry and so forth. Some of that moisture rises into the attic as a gas and condenses on the roof boards, causing mold, rot and sub-structure and shingle failure.

It was with good reason ASHI included ventilation in the ASHI Standards of Practice and Code of Ethics and specifically mentioned attics.


How to calculate attic ventilation requirements

According to “Principles of Attic Ventilation” from Air Vent Inc. (800-AIR-VENT,, all you need are a tape measure, paper and pencil, and it will be as easy as 1,2,3.

1. Determine the square footage of attic area to be ventilated. To do that, just multiply the length of the attic (in feet) by its width.

Example: For this and the following calculations, we’ll assume the home has a 40’ by 25’ attic area.

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). That determines the total amount of net-free area needed to properly ventilate the attic.

Calculation: 1,000 square feet divided by 150 = 6.6 square feet of total net-free area.

3. Determine the amount of intake and exhaust (low and high) net-free area required.

For optimum performance, the attic ventilation system must be balanced with intake and exhaust vents.

This is a simple calculation: just divide the answer from step 2 by 2.

Calculation: 6.6 divided by 2 = 3.3 square feet of intake net-free area and 3.3 square feet of exhaust net-free area.


Check out the following ASHI Reporter articles on attic ventilation:

An Attic Ventilation Checklist for Home Inspectors by Dustin Ciepliski from the September 2001 issue.

Check the Intake Ventilation by Paul Scelsi from July 2003.