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Code Cycle: 2016 & 2019 Standards
Attic Ventilation (Options B and C)
Proper attic ventilation occurs at two points at the roof: the soffit (or eave) vents and the ridge (or eyebrow) vents. Ridge or eyebrow venting must be maintained, as shown in Figure 3-26.
When installing insulation below the roof deck, vent baffles and insulation barriers should be used to maintain proper ventilation space. Proper airflow through the space helps remove moisture and prevents any associated issues.
Where ceiling insulation is installed next to eave or soffit vents, a rigid baffle should be installed at the top plate to direct ventilation air up and over the ceiling insulation. (See Figure 3-27.) The baffle should extend beyond the height of the ceiling insulation and should have sufficient clearance between the baffle and roof deck at the top. There are several acceptable methods for maintaining ventilation air,
including preformed baffles made of either cardboard or plastic. In some cases, plywood or rigid foam baffles are used.
The California Building Code (CBC) requires a minimum vent area to be provided in roofs with attics, including enclosed rafter roofs creating cathedral or vaulted ceilings. Check with the local building jurisdiction to determine which of the two CBC ventilation requirements are to be followed:
1. CBC, Title 24, Part 2, Vol. 1, Section 1203.2 requires that the net-free ventilating area shall not be less than 1/300 of the area of the space ventilated.
2. CBC, Title 24, Part 2.5, Section R806.2 requires that the net-free ventilating area shall not
be less than 1/150 of the area of the space ventilated. This ratio may be reduced to 1/300 if
a ceiling vapor retarder is installed in Climate Zones 14 and 16.
If meeting Option 1 above, a minimum of 40 percent and not more than 50 percent of the
vents must be located in the upper portion of the space being ventilated at least 3 feet
above the eave or cornice vents.
Insulation shall not block the free flow of air, and a minimum 1-inch air space shall be
provided between the insulation and the roof sheathing and at the location of the vent.
Ventilated openings are covered with corrosion-resistant wire cloth screening or similar
mesh material. When part of the vent area is blocked by meshes or louvers, the resulting
“net-free area” of the vent must be considered to determine if ventilation requirements are
Many jurisdictions in California are covered by Wildland Urban Interface (WUI) regulations
where specific measures for construction materials must be used to improve fire resistance
for the building. These regulations require special vents that are expressly tested to resist
the intrusion of flame and burning embers. Check with the building department to ensure
compliance with local codes.
RCM 3.6.1 Unvented Attics
Attic ventilation is the traditional way of controlling temperature and moisture in an attic. In
an unvented attic assembly, insulation is applied directly at the roof line of the building,
either above or below the structural roof rafter. The roof system becomes part of the
insulated building enclosure. For this case, the thermal boundary of the building results in
an unvented attic space between the ceiling gypsum board (gypboard) and the insulated
roof above (Figure 3-35).
In addition to meeting the mandatory minimum insulation requirements, the provisions of CBC, Title 24, Part 2.5, Section R806.5 must be met. Check with the local building jurisdiction to determine its specific requirements for unvented attic conditions. Combining this strategy with the additional design improvement of low air leakage for the rest of the building would achieve energy savings and compliance energy credit. Furthermore, this design eliminates the need to seal or limit penetrations at the ceiling level, such as recessed cans, because the air and thermal boundary is now located at the roof deck.