Estimating Ventilation Requirements For Attic

According to the national roofing contractors association nrca a power vent with an airflow rate of 1 0 cubic foot per minute per square foot of attic space measured at the attic floor is generally considered to be equivalent to a 1 150 ventilation ratio.

Estimating ventilation requirements for attic.

As an example in an attic that is 30 feet by 20 feet you would have 600 square feet of floor area. The calculations before indicated that we needed 2 square feet of ridge or upper roof ventilation. A normal metal pot vent produces only 50 squares inches of free net area. According to their requirements you should have 1 square foot of attic ventilation per 300 sq ft of attic floor space.

Two square feet equals 288 square inches. There should also be a 50 50 balance of attic venting and roof venting. Hence you would need two square feet 600 300 of attic ventilation. So we would need at least six metal vents in addition to the lower soffit ventilation to meet minimum ventilation requirements.

This shortcut conveniently calculates the 2015 international residential building code minimum irc section r806 roof ventilation 1 which states in part 1 square foot of net free area for every 150 square feet of attic floor space with the attic defined as length x width floor of the attic. The federal housing authority fha and the urban development hud have established minimum requirements for attic ventilation. Ventilation of attic spaces is required by most building codes as well as by roofing material manufacturers and the national roofing contractors association nrca. Before we explore how to ventilate an attic space it is prudent to understand how much ventilation is needed.

Regarding attic ventilation nrca recommends designers provide at least 1 square foot of nfva for every 150 square feet of attic space 1 150 ventilation ratio measured at the attic floor level ceiling. Enter square footage calculate or enter the square footage of the attic or area to be vented. According to the fha federal housing administration us for every 300 square feet of attic floor area you should have at minimum one square foot ventilation. For large volume attics such as where roof slopes are greater than 8 12 designers also should consider increasing.

Most building codes require a ratio of 1 150 ventilation space to attic floor space. Of vent for every 300 sq. Measure the length and width of the attic floor space to be ventilated. Ventilation can be accomplished using various products and techniques.