Unvented Attic Problems

In warmer climates unvented attics can prevent the condensation that occurs on ductwork in some vented attic spaces.

Unvented attic problems. It is a common misconception that one of the unvented attic problems is that the roof needs to breathe or that the insulation will make the roof hot. The intent of this brief is to provide code related information about controlling moisture in unvented attics by installing a vapor diffusion port vent that would convey water vapor from an unvented attic to the outside when air permeable insulation materials are installed and can be verified as being in compliance with the related codes and standards for residential construction. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs. 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.

However they require different approaches in different climate locations. In high wind regions particularly in coastal areas wind driven rain is a problem with vented roof assemblies. Unvented attic roof designs have the advantage of providing conditioned spaces for ductwork and air handlers. Poor insulation is usually the culprit although if you enter the attic on sunny winter day your attic space can be warmed by the sun more than your furnace.

Should you opt to run ductwork or place mechanical systems in the attic an unvented roof is best for avoiding thermal penalties as the ductwork will be located in a conditioned space allowing the system to recover thermal losses through the ducts. In theory if you install a small exhaust fan in the gable wall of an unvented attic along with a passive duct or grille that allows conditioned air from the floor below to enter the attic as makeup air the humid air in the attic will be exhausted and will be replaced with less humid air from the home. Unvented attics are also said to perform better in areas where high wind events and storms are common. Moisture resistance the attic will stay dry avoiding problems with mold and wood rot and thus can serve as living or storage space.

Unless your roofing system has insulation on the roofing deck and is designed without ventilation your furnace should not be heating your attic. In reality if spray foam insulation is used on the roof deck it could raise the temperature of the shingles a few degrees but not enough to cause any damage. Generally it has been found by researchers that the net impact of eliminating venting is to raise shingle temperatures only by five to eight degrees fahrenheit. An unvented attic is conditioned space and won t be subject to the extremes of temperature common to vented attics.

Joseph lstiburek from building science corporation. Among the significant issues that get raised pertaining to unvented attics is concern over asphalt shingle temperature durability in hot climates.