Vented Attic Detailed Cross Section

Section r806 4 of the 2009 international residential code irc and section r806 5 of the 2012 irc have requirements for unvented conditioned attic assemblies.

Vented attic detailed cross section.

Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing members shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain and snow. If there is no vapor barrier double it to one square foot of nfa for every 150 square feet of attic floor space half for intake half for exhaust. Intake vents located at the lowest part of the roof under the eaves allow cool. 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.

The idea of roof vents existed for hundreds of years with natural vents employed in chinese buildings during the tang dynasty natural ventilation ducts in the buddhist monks caves and wind catchers incorporated in arabian architecture. If your attic floor has a vapor barrier you will need one square foot of nfa per every 300 square feet of attic floor area half of that will be intake half for exhaust. The building thermal envelope must meet the requirements of table r402 1 2 n1102 1 2 based on the climate zone specified in chapter 3 and the building assemblies associated with the unvented attic assemblies that are considered part of the building thermal envelope. Theinternational building code 2012 edition ibc 2012 requires enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing to have cross ventilation for each separate space.

For decades designers of attics and crawl spaces have used cross ventilation to minimize the potential for moisture accumulation and condensation. The net free vent area nfva shall not be less than 1 150 of the area of the space being vented. 2015 iecc irc section r402 1 3 n1102 1 3 or 2012 iecc irc. This section indicates that the prescriptive requirement for r 38 ceiling insulations is deemed to be met by r 30 insulation when the r 30 insulation extends over the wall top plate at eaves and when the insulation is at full loft and uncompressed over the wall top plate at eaves.

However spurred by recent claims of energy savings and moisture control unvented attics have become popular in both residential and commercial applications. 2015 iecc irc section r402 1 2 n1102 1 2 insulation criteria. Blocking and bridging shall be arranged so as not to interfere with the movement of air. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents.

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