Vented Attic Cross Section

The idea of roof vents existed for hundreds of years with natural vents employed in chinese buildings during the tang dynasty natural ventilation ducts in the buddhist monks caves and wind catchers incorporated in arabian architecture.

Vented attic cross section. Of attic floor space. This yields a total area of 1 104 square feet. If there is no vapor barrier double it to one square foot of nfa for every 150 square feet of attic floor space half for intake half for exhaust. The net free vent area nfva shall not be less than 1 150 of the area of the space being vented.

Vent roofs with steeples towers or cupolas can be found in different buildings from barns to cathedrals. The building codes and building science for. Of net free area for every 150 sq. In other words the entire vent opening doesn t count as vented space.

Generally speaking you need a ratio of 1 300 where for every 300 square feet of ceiling space you need 1 square foot of attic ventilation. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents. It is a waterproofing layer made of regular felt stacked above the solid. This means for every 150 square feet of attic floor space there should be 1 square foot of net free area.

If your attic floor has a vapor barrier you will need one square foot of nfa per every 300 square feet of attic floor area half of that will be intake half for exhaust. It really is a design choice. Jones has an attic space that measures 23 feet by 48 feet. Ventilation openings shall have a least dimension of 1 16 inch 1 6 mm minimum and 1 4 inch 6 4 mm maximum.

However since the 2009 edition of the irc attics can be designed to be either vented or unvented. A composite decking made of solid materials it resembles real wood and particularly strong and stable for bearing heavy load. Requirements for unvented attics can be found in section r806 5 of the 2018 irc. That said air resistance and interference such as vent grates reduces the area of true ventilation.

Theinternational building code 2012 edition ibc 2012 requires enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing to have cross ventilation for each separate space. So the decision to vent an attic space is not dictated by building code. Jones takes this measurement and divides it by 150 for a total of 7 36 square feet of ventilation space that is needed. The code minimum for attic ventilation in the 2012 irc section r806 roof ventilation is as follows.