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Message: <blockquote data-quote="Obi-Wan" data-source="post: 266886" data-attributes="member: 709">You will need more than a couple vents on the eaveGuidelines for attic ventilationNew building code requirements may limit roof system designsby Mark S. GrahamProper attic ventilation can be an important performance consideration when designing and installing steep-slope roof systems.Typically, building codes include minimumrequirements applicable to attic ventilation.The roofing industry also has practical guidelines for attic ventilation.Code requirementsThe International Building Code,® 2012Edition (IBC 2012) requires enclosed atticsand enclosed rafter spaces formed whereceilings are applied directly to the undersideof roof framing to have cross-ventilation foreach separate space. The net free vent area(NFVA) shall not be less than 1:150 of thearea of the space being vented. Blocking andbridging must be arranged so they do notinterfere with air movement; air space noless than 1 inch must be provided betweenany insulation or obstruction and the roofsheathing. Ventilation openings must be protected to prevent rain or snow infiltration.An exception to IBC 2012’s ventilationrequirement permits the NFVA to be reducedto no less than 1:300 as long as 50 to 80percent of the ventilating area is provided ator near the upper portion of the space beingvented or where a vapor retarder is providedon the warm-in-winter side of the ventilationspace.The International Residential Code,® 2012Edition (IRC 2012) contains requirementssimilar to IBC 2012 except IRC 2012 limitsthe net free ventilation reduction from 1:150to 1:300 when at least 40 percent but nomore than 50 percent of the required ventilation area is provided at or near the upperportion of the space being ventilated.Also, when using a vapor retarder in an atticspace, IRC 2012 limits the net free ventilationreduction from 1:150 to 1:300 for buildingsin Climate Zones 6, 7 and 8.NRCA guidelinesRegarding attic ventilation,NRCA recommends designersprovide at least 1 square foot ofNFVA for every 150 square feetof attic space (1:150 ventilationratio) measured at the attic floorlevel (ceiling). For large-volumeattics, such as where roof slopesare greater than 8:12, designersalso should consider increasingthe amount of attic ventilationto account for the additionalvolume of attic space.Furthermore, NRCA also recommends the amount of ventilation in static ventilation systemsbe balanced, as shown in thefigure, between the soffits or eaves and theupper portion of the space being ventilated.In a balanced ventilation configuration,ambient outside air enters into the attic spacevia soffit or eave vents; this air passes throughthe attic space where it displaces warm,moisture-laden air, which, in turn, exits theattic via vents at or near the top of the spacebeing vented. This configuration relies onconvection—a mode of heat transfer thatcauses warm air and water vapor to rise.NRCA considers use of the balanced ventilation approach to be an important designconsideration for proper attic ventilation performance. It needs to be recognized it is notreadily possible to vent more warm, moist airout of an attic than the amount of new airallowed into the attic, such as with soffit oreave vents.In unbalanced attic ventilation situationswhere the NFVA at or near the top of thespace being vented greatly exceeds that of thesoffit or eave vents (such as is permitted inIBC 2012), it is possible for a slight negativepressure to be created in the attic space. Thishas the potential to allow conditioned airfrom the occupied areas below the attic to bedrawn into the attic. Unsealed openings inthe ceiling plain, such as ceiling recessed lightfixtures, exhaust fan openings and plumbingvent stacks, are common areas of conditionedair loss into attics.Additional information about attic ventilation is provided in the Condensation andAir Leakage Control Section of The NRCARoofing Manual: Architectural Metal Flashing,Condensation and Air Leakage Control, andReroofing—2014. 123MARK S. GRAHAM is NRCA’s associate executive director of technical services.</blockquote>

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