Calculating Attic Passive Ventilation

Calculating Attic Passive Ventilation

Building codes require that vented roofs have a specified amount of ventilation area. This is usually expressed as a ratio. For example the International Residential Code (2015 IRC R806.1) requires that the total net free ventilating area should not be less than 1/300 of the area of the attic space to be ventilated. “Net free ventilation area” refers to the clear open area of a vent, taking into account the restrictions of the vent screening itself and the dimensions of the vent slots. Most screened vents have a 60% free area.  Ventilation openings should be located both high and low in the roof assembly. Lower ventilation openings should be provided by soffit or eave vents and there should be a lower vent in each rafter bay of the roof assembly. A minimum 1-inch of space should be maintained between the insulation and the roof sheathing and there should also be a barrier between the insulation and any roof vent openings to allow for free air movement. Baffles may be used to provide both functions.

The area of the attic space to be ventilated is the area of the flat floor or ceiling that separates the attic space from conditioned space. Therefore, the minimum required total net free vent area for an attic space is equal to the area of the flat floor or ceiling beneath the roof assembly, divided by 300.

No more than 50% (and no less than 40%) of the required ventilation area should be located high on the roof. These upper ventilation openings should be located at least 3 feet above the soffit or eave vent openings and no more than 3 feet below the highest point of the roof. In sloped ceiling situations, such as where the ceiling is applied directly to the underside of roof rafters or where the roof space is within scissor trusses, the area of the attic/roof space is equal to the sloped ceiling area.

Homes without roof overhangs or other accommodation for ventilation openings at the soffit or eave should have a net free ventilating area of not less than 1/150 of the area of the attic/roof space. Refer to the example for 1:150 attic ventilation below.

Example for 1:300 high/low attic ventilation

Consider a house that is 30 ft wide by 42 ft long. The top floor ceiling is flat in this example so the area of the attic space is equal to the area of the foot print of the house.

The area of the ventilated attic space is therefore:

30 ft × 42 ft = 1,260 ft²

The required total net free ventilation area is:

1,260 ft²/ 300 = 4.2 ft²

Converting feet squared to inches squared, the required total net free ventilation area yields:

4.2 ft² × 144 in²/ft² = 605 in² (Note that the required area is rounded up to the nearest inch.)

Approximately 50% of the total net free ventilating area should be located in the upper portion of the roof, at least 3 ft above the soffit or eave vents. 50% is expressed as 0.5 in decimal form. The required net free ventilating area of the upper roof vents should be:

0.5 × 605 in² = 303 in²

If screened vents with a net free area of 60% will be used, then the total vent area required for the upper portion of the roof will be:

303 in² / 0.6 = 505 in²

The remainder of the required net free ventilating area should be provided by soffit or eave vents so that the total required net free ventilating area is achieved.

Example for 1:150 attic ventilation

Consider the same 30 ft by 42 ft house only this time without soffit vents, eyebrow vents, or low gable vents. If it is not possible to provide ventilation openings at the bottom of the roof assembly, then the required minimum net free ventilation area is 1/150 of the area of the attic space to be ventilated.

The area of the ventilated attic space is:

30 ft × 42 ft = 1,260 ft²

The required total net free ventilation area is:

1,260 ft²/ 150 = 8.4 ft²

Converting ft² to in², the required total net free ventilation area yields:

8.4 ft² × 144 in²/ft² = 1,210 in² (Note that the required ventilation area is rounded up to the nearest inch.)

If screened vents with a net free area of 60% will be used, then the total vent area required will be:

1,210 in² / 0.6 = 2,017 in²