Unvented Attic Detail

Usually there is a penalty with venting because venting causes greater air pulsing across the ceiling.

Unvented attic detail. Unvented attic assemblies code notes 2009 irc 2012 irc adequate attic ventilation is a long standing requirement in building codes for moisture control. The attic space becomes indirectly conditioned as a result of the air leakage heat transfer and vapor diffusion through the ceiling. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot.

How to install insulating sheathing over the roof deck with a cathedral ceiling refer to the current state and local building codes for the minimum r value of air impermeable insulation required for the roof assemblies in your climate. The irc has permitted unvented conditioned attics since 2006 with certain requirements. Air permeable insulation such as fiberglass batts dense packed cellulose or blown in fiberglass can t be used alone for an unvented assembly since these types of insulation can allow moist indoor air to reach the cold roof sheathing leading to condensation or moisture accumulation. In an unvented attic this moisture must have a way to get dried up or roof panel issues may result.

A fundamental requirement of an unvented attic assembly is the use of air impermeable insulation on the underside of the unvented roof to prevent air infiltration and exclude airborne moisture from the attic. Senior engineer manager at icynene john broniek makes the case for the unvented attic assembly and why insulating the underside of the roof makes sense. Apa osb plenum truss plenum trusses roof sheathing roof sheathing panels roofing panel unvented attics unvented roofs unventilated attics wood panels. Venting will reduce the temperature in an open attic.

The roof deck in an unvented attic must be exceptionally airtight. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. The difference in attic temperature between a vented an unvented attic with r 30 at the ceiling translates into minuscule savings. Unvented cathedralized attic detail with air impermeable insulating sheathing installed over the roof deck.

Detail of an unvented cathedralized attic showing air impermeable spray foam insulation plus batt insulation installed on the underside of the roof deck. The quest for greater energy efficiency in buildings has led to increasing interest and need for insulation improvements in hvac equipment more airtight construction new materials and assemblies. The acceptance of unvented conditioned attics continues to grow among builders and building officials. No savings have ever been measured.