Unvented Attic Insulation Requirements

The roof deck in an unvented attic must be exceptionally airtight.

Unvented attic insulation requirements.

These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. Unvented roof assemblies should be insulated either with air impermeable insulation rigid foam or spray polyurethane foam or with a combination of foam and air permeable insulation. Install insulation along the underside of the roof deck of an unvented attic rather than on the ceiling deck of a vented attic for either of two reasons.

In climate zones 1 2 and 3 when air permeable insulation is installed in unvented attics it shall meet the following requirements. The acceptance of unvented conditioned attics continues to grow among builders and building officials. 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. 1 an approved vapor diffusion port shall be installed not more than 12 inches 305 mm from the highest point of the roof measured vertically from the highest point of the roof to the lower edge of the port.

Insulation requirements for roofs are therefore spread across ibc chapters 12 13 and 15. 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 attic space becomes indirectly conditioned as a result of the air leakage heat transfer and vapor diffusion through the ceiling. If you don t want to use sips or nailbase there are four basic approaches.

Code requirements vary across the country. The irc has permitted unvented conditioned attics since 2006 with certain requirements. Further the relationship of unvented attic spaces to maintaining the interior environment through use of insulation is also discussed in chapter 12 a section seldom associated with roofing materials. An unvented attic air sealed with spray foam insulation limits outdoor air entry and as a result indoor combustion air should not be considered available for fuel fired combustion appliances located within the attic space.

The overall insulation level in the roof assembly must meet the baseline requirements for energy efficiency as given in the irc section n1102 or iecc section 402.