Attic Ventilation

Attic Ventilation Requirements

1. Provide ventilation to attic spaces, unless you design and build the attic as an unventilated, conditioned attic assembly. Provide ventilation when ceilings are applied directly to roof rafters, such as cathedral and vaulted ceilings, and in attics over unconditioned spaces, such as garage attics. Provide ventilation to each individual attic and ceiling space, unless there is eff ective means for air to move between spaces. The building official may waive att ic ventilation requirements.

2. Protect ventilation openings from entry of rain or snow.

3. Make attic ventilation openings at least () 1/16 inch and not more than () ¼ inch.

4. Cover attic ventilation openings that are larger than ¼ inch with corrosion-resistant wire mesh with openings at least () inch and not more than () ¼ inch.

5. Provide at least () 1 inch clearance between insulation and roof sheathing. Do not cover ventilation openings with insulation.

Attic Ventilation Openings Area

1. Provide a net free ventilation opening area of at least () 1 square foot for every 150 square feet of ventilated area, unless one of the reduction exceptions applies. The ventilated area is usually defined as the fl oor area under the attic. The net free ventilation opening area is the total opening area minus the area of any screens and/or louvers that protect the opening.

2. You may reduce the net free ventilation opening area to at least () 1 square foot for every 300 square feet of ventilated area: (a) if at least () 40 percent but not more than () 50 percent of the ventilation openings are located not more than () 3 feet below the highest point in the attic and the balance of the ventilation openings are eave ventilation openings, or (b) by installing a Class I or Class II vapor retarder on the warm side of the ceiling in climate zones 6, 7, and 8.

3. Install mechanical attic ventilators (typically roof-mounted or gable mounted fans) according to manufacturer’s installation instructions including instructions for required   flashing.

Conditioned Attic Assemblies

1. Refer to the IRC and to your local building official to determine your climate zone before installing a conditioned attic assembly. You are not required to make attics conditioned.

2. Comply with all of the following when installing a conditioned attic assembly:

(A) Build and seal the entire attic area so it is completely within the building thermal envelope (see Chapter 11 for information about the building thermal envelope), and

(B) Do not install a Class I vapor retarder on the attic fl oor and do not install a Class I vapor retarder on the ceiling (drywall) side of an unventilated rafter assembly (drywall ceiling applied directly to the rafters and sometimes called a cathedral ceiling); and

(C) Provide at least () a ¼ inch a continuous ventilated air space between the exterior roof sheathing and the roofing underlayment and wood shingles and shakes roof covering materials, and

(D) Use insulation material that is a Class II vapor retarder or install a Class III vapor retarder in direct contact with the underside (conditioned side) of insulation in climate zones 5, 6, 7, and 8 (this provision applies only to air impermeable insulation), and

(E) Install insulation in the rafter and roof sheathing area by one of the following methods:

(a) Install air impermeable insulation directly in contact with the inside (unconditioned side) of the roof sheathing, or

(b) Install air permeable insulation directly in contact with the inside (attic side) of the roof sheathing and install rigid board or sheet insulation directly in contact with the outside (exterior side) of the roof sheathing according to Table R806, or

(c) Install rigid board or sheet insulation directly in contact with the inside (attic side) of the roof sheathing according to Table R806 and install air permeable insulation directly in contact with the rigid board or sheet insulation.

3. Seal the perimeter of each sheet of air impermeable rigid insulation board to for a continuous air and vapor sealed layer.

Attic Ventilation Discussion

Screens and louvers will reduce the ventilation opening area. Use manufacturer’s specifications to determine the net free ventilation opening area of commercially available ventilation opening covers. If manufacturer’s specifications are not available, a safe assumption is that metal louvers provide a net free area of 75 percent of the gross louver area and that wood louvers provide a net free area of 25 percent of the gross louver area.

Attic Ventilation Best Practice

Use cardboard baffles or similar materials to keep insulation from blocking soffi t vent openings, and to keep it from blocking the space between insulation and roof sheathing in cathedral and vaulted ceilings. Use baffles or other means to keep att ic ventilation air currents from disturbing loose fill insulation. Air currents can blow loose fill insulation out of position, reducing or even eliminating insulation coverage over an area. Loose fill fiberglass insulation is particularly vulnerable to being disturbed by air currents. Instruct painters to mask or otherwise protect soffit vent openings. It is common that, over time, these openings become restricted by paint buildup. Consider making the attic a conditioned assembly. Conditioned attics help HVAC appliances and ducts located in the attic function more effi ciently because energy is retained inside conditioned space instead of being lost to the outdoors. Recent building science research indicates that ventilated attics in warm humid climates may do more harm than good. This is because water may condense out of the humid air near where cool conditioned air leaks into the attic from openings such as recessed ceiling lights. In some cases, this water can damage the building and provide moisture for fungal growth.

Attic Ventilation Examples

Given a 2,100 square feet single story home with a 600 square feet attached garage, the required total net free ventilation area is at least () 18 square feet at the 1/150 ventilation ratio or at least () 9 square feet at the more commonly used 1/300 ventilation ratio. For best results, the ventilation openings should be evenly distributed between the home and garage attics. If the garage and home attics are separated so that air flow between the two is limited, the ventilation openings must be distributed between the two attics with at least () 7 square feet in the main home attic and at least () 2 square feet in the garage attic. Given a 1,800 square feet two story home with a 300 square feet    attached garage and given that the first and second floors each have an area of 900 square feet, the total net free ventilation area is at least 4 square feet at the 1/300 ventilation ratio. Because the home and garage attics are almost certainly separated so that no air flows between them, the ventilation openings must be distributed between the two attics with at least 3 square feet in the main home att ic and at least 1 square foot in the garage attic.