By Bud Coburn
Unvented roofs operate by the principle that venting is not necessary to control moisture accumulation. The following conditions must be met in order for an unvented roofing assembly to function properly:
- The building envelope must be tight, including having adequate vapor and air barriers installed, which is generally accomplished through the use of spray-foam insulation.
- The building must be pressurized in order to counter the stack effect, which happens when hot, pressurized air in the upper part of the house tries to escape through holes in the building envelope.
Proponents argue that, when installed and implemented properly, unvented roofing assemblies offer the following advantages over vented attics:
- enhanced comfort. Wind, temperature gradients and pressure differences in a vented attic create undesirable air movement between the living space and the attic. Also, unvented attics block volatile organic compounds and other moisture-related airborne particles from migrating to the living space from the attic;
- protection against certain moisture-related problems. In vented attics in cold climates, warm air can leak from the living space and condense on the underside of the roof sheathing, while humid air can easily leak from the outdoors and condense on cold metal surfaces of ductwork and air-conditioning equipment typically located in the attic. Unvented attics do not experience such problems;
- energy conservation. An unvented attic is conditioned space and won’t be subject to the extremes of temperature common to vented attics. Heat is thus less likely to escape into an unvented attic from HVAC equipment, and if it does, it will remain within the conditioned space. Insulation around ducts and HVAC equipment becomes less critical, and the equipment is not forced to work as hard to compensate for unwanted air or heat loss. It might be possible to downsize the HVAC system if enough energy is saved in this manner. Also, cold air blowing through the eave vents in a vented attic can degrade the thermal performance of attic insulation;
- snow and ember barrier. Openings in the soffits, gables, mushroom and ridge vents easily allow snow intrusion, especially fine snowflakes, into the attic. The snow can accumulate and eventually melt, causing damage to building materials and encouraging the growth of mold. Airborne mold spores may pass through vented attics into the living space and harm susceptible individuals. Also, blowing embers from wildfires can pass through unscreened attic vents and light the house on fire. These blowing embers often fall far from the edge of the actual wildfire, which might not otherwise have reached the house; and
- expanded use and design options. Because the temperature in unvented attics is more easily controlled, they can be furnished and incorporated into the living space or used as a conditioned storage space. Also, unvented roof assembles make complicated roof geometries more viable, as they are difficult to ventilate effectively.
While unvented attics are gaining acceptance, homeowners must realize their limitations, including:
- codes. Many local building codes do not account for non-standard construction alternatives such as unvented attic assemblies. They were addressed in the 2006 International Residential Code (IRC), however, which states that they must have no vapor retarder installed between the attic and the home’s living space, and there must be air-impermeable insulation installed between the rafters;
- asphalt shingles may fail prematurely due to increased exposure to heat; and
- ice dams are more likely to form at unvented attics in cold climates.