Credit: Harry Whitver
Credit: Harry Whitver

Proper attic ventilation is one of those building practices that is often overlooked but is actually critical to overall housing performance and the durability of several roof assembly components. It will be even more so as residential energy efficiency standards and consumer expectations heighten.

Regulating the attic’s temperature, moisture content, and air pressure differences to more closely resemble conditioned living spaces reduces the likelihood of thermal loss and moisture vapor migration. The roof’s various components, from the shingles to insulation, last longer, perform better, and aren’t affected by moisture-related problems such as mold and structural degradation.

In addition, a vented attic helps a home’s heating and cooling system work less and last longer, delivers more reliable comfort at a lower cost, and mitigates indoor air quality hazards.

In fact, a properly insulated, air-sealed, and ventilated attic space should enable builders to place HVAC equipment and duct runs in a so-called “semi-conditioned” area, reducing the potential for thermal loss from those components that would otherwise erode energy efficiency.

Best Practices

The International Residential Code (IRC) provision for attic ventilation requires a ratio of one square foot of net-free (open) ventilation area for every 150 to 300 square feet of attic square footage. Climate, roof design, and finish materials, as well as the roof/attic’s construction, all factor into which set of products will comply with the code and deliver the desired effect and benefits.

That being said, building scientists and roofing experts tend to agree that, for most homes in most climates, a soffit-to-ridge ventilation design is the most effective way to help maintain desirable and code-compliant attic conditions.

That scheme is also the easiest and least expensive to deliver for new construction, and certainly not impossible or overly costly to retrofit for existing homes seeking to upgrade their energy performance and alleviate problems such as ice dams or excessive heat and moisture build-up.

The concept is simple enough: Louvered vents along the length of the soffits combine with a continuous vent along the ridge to create a natural and passive chimney effect. This configuration also supports the energy-efficient benefits of a raised-heel roof truss system (“Raise the Roof,” May 2012), which allows insulation to extend to the sidewalls for greater thermal efficiency in the attic and for the roof deck.

Easy, right? In theory, yes, but dealers who dabble in attic ventilation inventory—especially those who want to advise pro customers about their proper use and benefits for enhancing home energy efficiency and durability—should be aware of a few key details.

For instance, the building code’s ratio of one square foot of open ventilation area to 300 square feet of attic square footage assumes a vapor barrier is installed between the ceiling and the attic space to retard the migration of moisture vapor into the attic. If not, the accepted ratio drops to 1:150, requiring more net-free ventilation to presumably “dry out” the free-flowing vapor and stem moisture-related problems.

(The term “net-free” refers to the actual amount of open-air ventilation area. Airflow through a louvered vent designed to block debris and pest infiltration is perhaps half of the vent opening’s overall or gross-area dimension—a calculation that likely requires larger and/or continuous soffit vents or perhaps a higher-profile ridge vent to meet code.)

Continuous Barrier

In new construction, specifically in mixed and cold-weather climates where moisture vapor is mostly generated from cooking, bathing, and laundry, a continuous vapor barrier can be applied to the underside of the ceiling joists before the drywall is applied. (For retrofits, the process of fitting strips of vapor retarder between the ceiling joists in the attic, under the insulation, is likely ineffective. It’s easier to simply add more ventilation and provide better moisture source control.) In hot/humid climates, where moisture vapor migrates mostly from outside conditions, the vapor barrier is applied between the roof deck and the roof finish.

Airflow from the eaves to the ridge must be balanced to create the desired chimney effect and effective ventilation. If the soffit vents are not sized to the same net-free ventilation area of the ridge vent, air pressure from above will effectively block airflow from below, pressurizing the attic space and negating the cooling and moisture vapor-venting effect that the ventilation ideally provides. Oversizing the vents is risky as well as potentially problematic.

Lastly, the ridge vent opening—a 1- to-2-inch gap in the roof sheathing along either side of the ridge beam—should be held back at least 6 inches from a gable or ridge end and 12 inches on either side of a chimney so as not to compromise the structural integrity of the roof frame.

Otherwise, builders and framers should follow the instructions provided by the manufacturer with regard to caulking and sealing, fastening, and other details to ensure a quality installation and desired performance, as well as warranty coverage.

To meet all these conditions and also address today’s complex roof designs, attic and roof vent suppliers (many of them manufacturers of roof finishes) provide a wide range of integrated soffit, gable-ridge, hip-ridge, and roof-to-wall vents toward achieving a desired result.