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Technical Details: Toys in the Attic: The IBC Ventilation Code

April 24, 2009
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The International Building Code (IBC) requires that ventilation be provided in all buildings. Building ventilation can be provided through natural or mechanical means. Mechanical ventilation is typically required in under-floor areas, bathrooms and some attic spaces.



The International Building Code (IBC) requires that ventilation be provided in all buildings. Building ventilation can be provided through natural or mechanical means. Mechanical ventilation is typically required in under-floor areas, bathrooms and some attic spaces. Natural ventilation is provided through windows, doors, louvers or other building openings in which the building occupants control the openings. For the roof component, the definition of ventilation is as follows: “The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, attics, cathedral ceilings or other enclosed spaces over which a roof assembly is installed.”

Chapter 15 (Roof Assemblies and Rooftop Structures) of the IBC identifies roof ventilation under Section 1503.5, Roof Ventilation. This section states that intake and exhaust vents shall be completed in accordance with Section 1203.2 of the IBC. Section 1203, Attic Spaces, is defined in Chapter 12 (Interior Environment). This section specifically applies to enclosed attic and rafter spaces that are directly applied to the underside of roof framing members. The section states that cross-ventilation is required for each separate space that is protected against moisture intrusion from snow or ice. Any blocking or bridging applied in the rafters must be arranged so that it does not interfere with the movement of the air.

This diagram details a natural attic ventilation system with intake vents at the soffits and exhaust vents at the ridge, preventing any dead spots. (Graphic courtesy of Air Vent Inc.)

Code Requirements

Code-specific requirements of ventilation are as follows:

• A minimum of 1 inch (25 mm) of airspace shall be provided between the insulation and the roof sheathing.

• The ventilating area shall not be less than 1/150 of the area of the total space.

• 50 percent of the ventilation shall be provided by ventilators located in the upper portion of the ventilated space.

• The upper vents shall be a minimum of 3 feet (914 mm) above the eave or cornice vents.

• The remaining 50 percent of ventilation shall be provided by eave or cornice vents.

The only exception to this section is that the minimum ventilating area may be 1/300 if a vapor retarder meeting ASTM E 96 is applied at the warm side of the attic insulation. The exception still requires that 50 percent of the ventilation be supplied by upper vents and the remaining 50 percent supplied by eave or cornice vents.

Openings in the Attic

The code is also very specific regarding coverings for attic openings. Exterior openings in buildings intended for human occupancy require coverings manufactured with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl or similar materials to prevent the entry of birds, rodents (bats, squirrels, snakes, mice, etc.) or other creatures. The openings shall be a minimum of 1/8 inch (3.2 mm) and not exceed 1/4 inch (6.4 mm).

Roofing contractors should make certain that they are in compliance with the IBC during installation on all steep-slope applications. The contractor should also make certain they are in compliance with state and local building codes. There are several fine ventilation products available in the U.S. market that meet IBC requirements. Ventilation installation should always be completed in compliance with the manufacturer’s latest printed specifications and requirements.

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