Unvented roof assemblies

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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;Image may be NSFW.
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• 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.