July, 2003
Inspection News and Views from the American Society of Home Inspectors

Check the Intake Ventilation


If you’re not convinced how important intake venting is to an attic ventilation system consider this: It’s the single biggest reason for most of the attic ventilation callbacks we see and hear about. Not the exhaust vents. The intake vents.


Soffit with intake ventilation

That’s because for an attic ventilation system to perform properly it needs a balance of both intake and exhaust. Unfortunately, too many houses have plenty of exhaust but not enough intake ventilation. This is confirmed by roofing contractors nationwide who attend our Attic Ventilation: Ask the Expert™ seminars. The potential problems vary with the type of exhaust vent. With an externally baffled ridge vent, insufficient intake can cause the ridge vent to pull intake air from itself.

That means it could pull in weather. With a power attic ventilator, improper intake could cause premature motor burnout and could force the power vent to pull its source of intake air from the living space in the home.

Tips for inspecting

Here are some tips for inspecting for intake ventilation.

Be sure the amount of intake matches or exceeds the amount of exhaust. At least half, if not more, of the total required net free area needed for proper ventilation should be in the intake area. If the house calls for a total of 1,500 square inches of net free area, half of that – or 750 square inches – must be intake ventilation.

When there’s a soffit, it should have intake vents. If a house has a soffit, look there for the intake vents, either continuous soffit vents or rectangular undereave vents. The soffit provides the most protection against possible weather infiltration. If the house doesn’t have a soffit, intake ventilation can be provided by a vented drip edge.

AirVent2.gifCheck the position of attic insulation. To work properly, intake vents need an unobstructed airflow path to feed the exhaust vents with cool, dry air. If the attic insulation is over the soffit, the intake vents can’t work. Be sure the attic insulation is pulled back to create that clear airflow path.

Evaluate insulation baffles. Insulation baffles can be installed in every rafter bay to ensure attic insulation doesn’t block airflow in the soffit area. But if a baffle extends further out into the soffit than where the vent is located, the insulation will push up against the baffle and block the vent.

Intake vents belong far out on the rafter tail. Intake vents in this position provide an optimum airflow path from intake to exhaust. Furthermore, intake vents in this position are not likely to be blocked by attic insulation.

Check for the right sized hole. The hole cut in the plywood should be properly sized for the intake vent to maximize the net free area specified for the particular intake vent. For example, two 4" round holes cut for a 16" x 8" undereave vent reduces the vent’s net free area from 56 square inches to 25.

Look for debris and other blockages. Over time intake vents can become clogged or blocked by dust, dirt and other debris. Or the homeowner may have painted them. Periodic inspection of the intake vents is important.


Evidence of moisture in the attic? Check the basement

When I find evidence of condensation or mold growth in an attic, I look for moisture in the crawl space or basement. A wet crawl space or basement can be an overwhelming source of moisture. Often drying this space can be as beneficial to the attic as adding ventilation. The crawl can generate gallons of water a day, while a family contributes a few quarts. The best approach may be to do both, increase ventilation and dry the space below to cut off the moisture at its source.

Barry Irby,
ASHI Technical Committee,
Home Reporters, Inc.
Chester, VA


Intake with Little or No Soffit

Sometimes providing proper intake is a lot easier said than done. Especially when we’re talking about houses with little or no soffit. Vented drip edge products, like Air Vent’s new Pro Flow™, are specifically designed for such applications. They combine louvers with a drip edge to allow for intake airflow.

One of the many good features of a properly designed vented drip edge product is it balances with most ridge vents because it provides exactly half of the net free area (9 square inches, typically) of common ridge vents on the market. For a balanced vented attic system, vented drip edge is installed on both sides of the house.

Here are a few things to keep in mind when inspecting vented drip edge products.
The louvers on the vent should always be parallel to the ground so that the wind cannot blow rain and snow directly into the openings.

The fascia should be installed so the top is above the louvers.

There should be a 1-1 1/2" space between the louvers on the vent and the top of the gutter. If the vent is installed tightly above the top of the gutter, the vent louvers can become blocked as soon as the gutter fills with debris or snow. Should this happen, the vented drip edge will temporarily no longer function as an intake vent until the blockage is clear.


Paul Scelsi presents Air Vent’s Attic Ventilation: Ask the Expert™ seminars to the industry nationwide January through April. For more information about the seminars or to request a copy of Air Vent’s latest Ventilation Views covering intake ventilation, call 1-800-AIR-VENT.