Determining Roof Ventilation Requirements

The following U.S. Department of Housing and Urban Development Statutes covering the ventilation of structural space are basic guidelines to determine the proper ventilation styles and sizes for any home. These statutes appear in the latest edition of the "Minimum Property Standards" (MPS 403-3).

The correct amount of roof ventilation:

• As a general rule, one square foot of net free vent area per 300 square feet of attic floor or area to be vented is recommended.
• In the rare situation where no vapor retarder is used and proper distribution of undereave and ridge vents cannot be achieved, one square foot of net free vent area should be provided for each 150 square feet of attic floor or area to be vented.
• For a balanced system, ventilation should be equal at the undereave and ridge.
• In cases where a balanced system cannot be achieved, always provide more than 50% of the total required ventilation at the undereave and the remainder at the upper portion of the roof.

Openings: All openings greater than 1/8 inch must be screened to prevent insect penetration and louvered to protect against the entrance of rain and snow.

The Proper Amount of Foundation Ventilation:

• One square foot of net free vent area is recommended for every 1,500 square feet of floor area covered by a polyethylene vapor retarder. Ratio is 1:150 if vapor retarder is not used.
• Net free vent area is that area unobstructed by screens, louvers, or other materials.
• Heated crawlspaces and basements do not need vents.
• Earth floors should be covered with a 4 - 6 mil. polyethylene vapor retarder.
• Providing at least two crawlspace vents will allow for a flow of air in and out of the crawlspace.

Openings: All openings must be screened to prevent insect penetration and protected from the entrance of rain and snow.

Natural Ventilation

A natural ventilation system utilizes ventilators installed in openings in the attic and properly positioned to take advantage of natural air flow to draw hot summer or moist winter air out and replace it with fresh outside air.

Calculate:

Determine the Square Footage of the Attic or Area to be Vented
(To do this, multiply the width in feet by the length in feet) Choose your square footage 500 Square Feet 600 Square Feet 700 Square Feet 800 Square Feet 900 Square Feet 1000 Square Feet 1100 Square Feet 1200 Square Feet 1300 Square Feet 1400 Square Feet 1500 Square Feet 1600 Square Feet 1700 Square Feet 1800 Square Feet 1900 Square Feet 2000 Square Feet 2100 Square Feet 2200 Square Feet 2300 Square Feet 2400 Square Feet 2500 Square Feet 2600 Square Feet 2700 Square Feet 2800 Square Feet 2900 Square Feet 3000 Square Feet

Choose an Exhaust System Based on Attic Size

Needed for Exhaust:

Sq. In. NFVA at Upper Roof

Ridge Vents:

Minimum ft. of Rigid Roll

Minimum ft. of Rigid Strip

Roof Vents:

Vents Needed	Model
	VTS4-144 VTS5-144
	VTS455 VTS555
	VTS450 VTS550
	VTS452 VTS552
	VTS572
	VTS761

Net Free Vent Area Needed at Soffit (Undereave)

Needed for Intake at Soffit Sq. Inches NFVA at Soffits