Unvented Attics In Hot Humid Climates

High humidity in unvented conditioned attics.

Unvented attics in hot humid climates.

As home energy readers know venting attics in hot humid climates brings a great deal of moisture into the structure see conditioned attics save energy in hot climates he may june 97 p. Absolute humidity the air space can hold. To provide an unvented conditioned space for locating hvac equipment in the attic and or to provide a continuous thermal barrier for designs that have complex coffered ceiling planes and or numerous penetrations for lights speakers vents soffits etc which make it difficult to achieve an airtight ceiling plane. The main strategy that should be utilized when designing roof or attics to be free from moisture problems and ice dams along with control of heat gain or heat loss regardless of ventilation approach is the elimination of air movement particularly exfiltrating air in cold climates and infiltrating air in hot and hot humid climates.

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. Insulation approaches for unvented attics vary by climate. Therefore replacing the hotter attic air with cooler outside air even at a considerably higher relative humidity rh tends to dry the space minimizing the potential for condensation. In predominantly hot humid climates attics sealed to outside air exchange would correct this problem.

Article osti 1344250 title sealed attics exposed to two years of weathering in a hot and humid climate author miller william a and railkar sudhir and shiao ming c and desjarlais andre omer abstractnote field studies in a hot humid climate were conducted to investigate the thermal and hygrothermal performance of ventilated attics and non ventilated semi conditioned attics. What is less well understood is that venting causes many problems in cold dry climates as well. Due to night sky radiation the metal roof or radiant barrier temperature can be depressed below the attic air dew point temperature allowing condensation and possible water damage to ceiling materials to occur. The roof deck in an unvented attic must be exceptionally airtight.

Even when outside air is hot and humid if the attic space is air sealed from the interior it is much hotter than outside air. He says that researchers have found that in hot humid climates buildings with unvented attics are actually less likely to have condensation and mold than those with vented attics. The hotter the air the more moisture i e. If you convert your vented unconditioned attic to an unvented conditioned attic by installing open cell spray foam on the underside of your roof sheathing you may be surprised to discover.