When it comes to attic ventilation I seem to run into the same defects on a regular basis. Most homes in our service area utilize a gravity system with soffit venting along with either roof or ridge vents. Sometimes this is not possible, but this is the most common ventilation system. There are many different products available, some are excellent, but some are not. Let’s discuss the details of a well ventilated attic looks like and some common defects found in area attics.
Generally speaking the most effective attic ventilation system will have high-low venting. Most common is with soffit vents and a ridge or roof vents. The ridge vent
provides continuous venting for the length of the roof and allows air flow across all truss or rafter cavities. The low vents (generally soffit vents) should equal or exceed the ridge vent size. If the opposite were true less air would flow and may lead to moisture damage or mold growth. Sometimes due to the type of construction there isn’t a way to vent to the soffit. In these cases people generally revert to using gable vents. This is not ideal. The way the system works is that the air enters at the eave (intake) and as it increases in temperature it travels to the ridge (exhaust) vents. When the intake is high up on the wall the only air flow will be high so the lower portions of the attic don’t see any benefit. Sometimes an attic fan can increase the airflow in these situations. I should also mention when there is soffit venting along with ridge venting, gable louvers should not be installed. The exhaust will draw intake air from the high gables instead of the eave. Don’t worry, they can just be closed off in the attic with plastic or rigid insulation and don’t necessarily need to be removed.
There are now many products available to allow for low intake vents where soffit venting is not possible. These vents include: vented roof edge flashings, low profile vents and edge vents. The idea is to get as close to constant venting for the entire roof sheathing area as possible.
The most common issues I run into as far as attic ventilation are generally related to the amount of total ventilation. Current building standards call for 1/150th of the attic floor square footage as the total vent area split as evenly as possible between the low and high vents. So if your attic is 1000 square feet the total area required would be 6.67 Square feet. So your soffit vent should be a minimum of 3.33 square feet and should not be less than the high, ridge or roof venting.
Another common item is the lack of an air channel at the eave. Many times homeowners will pack insulation in the rafter cavity at the eave thinking this will increase the R value in these locations. Well, that’s probably true but at the same time if the air cannot flow from the soffit venting, your attic isn’t vented which can lead to major problems. The installation of baffles (proper vents) is recommended.
An attic with improper venting may cause the shingles to fail prematurely. Blistering can result when the roofing materials get too warm from the attic space. Another thing to consider is that if the ventilation is not sufficient extensive condensation can form during periods of cold weather which can cause mold growth and deterioration of the roof sheathing.
A even level of insulation and air sealing is also necessary to prevent moisture and condensation damage. Current building standards call for an R value between R49-R60. 14-18 inches of cellulose insulation can provide a good R value as well as air sealing. Ceiling penetrations are also currently required to be caulked to prevent air from entering the attic space. 14-16 inches of cellulose insulation generally provides good air sealing as well. Care should be taken to ensure that there is a constant layer to prevent energy loss. If warm air enters the attic during periods of cold weather, extensive condensation can be the result.
To summarize this post, attic ventilation is essential to preventing mold growth as well as preventing premature shingle failure. Homeowners can take steps to prevent energy loss but must be careful to not obstruct the attic ventilation system. If you are working to insulate your attic and are unsure, send me a few images at: firstname.lastname@example.org, I should be able to lead you in the right direction.
As always, I hope you found this post helpful, and please share it with your friends.
Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection and Residential Thermal Imaging. He can be reached by email at Aaron@Zuehlkeinspections.com or by calling the office at 608-931-7485.