September, 2001

Inspection News and Views from the American Society of Home Inspectors

An Attic Ventilation Checklist for Home Inspectors


Some home buyers may wonder why home inspectors pay such close attention to the attic, but others know attic ventilation is critical to the performance of the roof and attic structure. A properly ventilated attic will help building materials last longer and help protect the home from costly damage. Here are some key points of ventilation for home inspectors to consider when inspecting attics.   

Why ventilate?
Three important reasons for ventilating an attic follow.

1. Ventilation helps prevent costly and damaging heat buildup in the summer, thereby improving the comfort level inside the house, keeping air conditioning costs to a minimum, and helping roofing materials to last longer.

2. Ventilation helps prevent moisture buildup in the winter, which means less likelihood of mold, mildew, wood rot and poor indoor air quality. It also helps to fight the formation of ice dams.

3. Most shingle manufacturers require proper attic ventilation to validate the full terms of the shingle warranty.

For all homes
For both new and older homes, watch for these are three critical attic ventilation issues.  

• Is the system balanced? Check to see that there is a balance between exhaust vents (at or near the ridge of the roof) and intake vents (in the soffit or undereave). Pay careful attention to the number and location of the intake vents. Most of the problems I see are the result of insufficient intake vents. Continuous soffit vents provide optimum intake ventilation, and I recommend using them whenever possible. Undereave vents also work well, if the correct number is installed. See Figures 1 and 2 below.

1-SVIIUNIN-duotone.gifFigure.1: This ridge vent (ShingleVent® II, by Air Vent) provides 18 square inches of Net Free Area (NFA) per linear foot. Using the “Doing the math” example, it would take 33 feet of this ridge vent to provide the required amount of exhaust NFA based on 1/150.


Figure. 2: A continuous soffit vent, like the one pictured here, provides optimum intake ventilation.

• Are the holes cut to size? Make sure the hole cut for  exhaust and intake vents is sized correctly. A hole the size of a dollar bill for a 16" x 8" intake vent isn’t effective. And make sure the holes are actually cut, especially in the soffit. I’ve seen many vents in place without a hole cut or with felt covering over the hole in the roof deck.

• Are insulation baffles being used? Check inside the attic to see that insulation baffles are in place to prevent the insulation from blocking the soffit. Blocked soffits make the intake vents useless and can cause serious weather infiltration problems through the exhaust vents that are “starving for intake air.”

Trouble spots to watch for
There are specific trouble spots to watch for when inspecting an older home.

• Moisture damage. Mold stains on wood, compacted insulation from moisture droplets and rusty nails indicate that there may be a lack of ventilation. Solutions include upgrading the type of
system or making sure the system has been properly sized for the attic space.

• Debris. Unfortunately, intake vents fall victim to blockages caused by dirt, debris and paint. Make sure the intake vents have not been painted closed. Are leaves and dust preventing the air from freely flowing through the intake vents? Inside the attic has debris or blown-in insulation made its way behind the insulation baffle?

• Different exhaust vents on the same roof. Another common mistake I see is mixing two different types of exhaust vents on the same roof (ridge vents with power vents; ridge vents with gable louvers, etc.). When this is done, one of the exhaust vents could become an intake vent because air follows the path of least resistance. Though the primary exhaust vent should get its intake from the soffit, it could pull air from the secondary exhaust vent – opening the door to weather infiltration and unbalanced ventilation along the underside of the roof deck. See Figure 3 below.

Fig. 3: Pictured in three sizes, the rectangular undereave vent is one type of intake vent. Too often this type of vent is painted closed or placed over a hole that is not cut properly for the size of the vent, compromising its effectiveness.

• Cathedral ceiling applications. One of the most difficult roofs to inspect is a cathedral ceiling. The lack of an attic makes it impossible to enter, but the same problems may be occurring as in any other roof system. Here are a couple of things to try in order to help determine if there is a problem in the roof system. Look carefully in the area of any ceiling penetrations (recessed lights, etc.) for signs of moisture, paint chipping, discoloration, or warping. While on the roof pull a nail or two out from the shingles and check it for rust. If installed correctly, the nails should penetrate into the cathedral cavity and will show rust if moisture is present. Be sure to replace any nails that are removed to prevent the shingles from blowing off.  

Doing the math
Here’s a shortcut to determining whether or not an attic ventilation system meets code:

• (To meet 1/300 code requirements) Attic square footage divided by 4 = amount of intake Net Free Area (in2) and the amount of exhaust Net Free Area (in2) needed. For example: 1200 square feet divided by 4 = 300 in2 NFA intake and 300 in2 NFA exhaust.

• (To meet 1/150 code requirements) Attic square footage divided by 2 = amount of intake Net Free Area (in2) and the amount of exhaust NFA (in2) needed. For example: 1200 square feet divided by 2 = 600 in2 NFA intake and 600 in2 NFA exhaust.

For more information, refer to the 2000 International Building Code. In short, 1/300 ventilation is required if a vapor retarder is being used or the system is balanced – one-half the vents high on the roof for exhaust and one-half the vents low in the soffit area for intake. When there is no vapor retarder in use, 1/150 ventilation is required. Again, the system must be balanced – one-half the vents high on the roof for exhaust and one-half the vents low in the soffit area for intake.

I hope the tips and guidelines I’ve presented help you with future inspections.