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.

So the decision to vent an attic space is not dictated by building code. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. The net free vent area nfva shall not be less than 1 150 of the area of the space being vented. Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents.

A horizontal timber or metal resting at the peak of the roof the rafters and trusses are connected to the ridge board for a cohesive framework. It is a waterproofing layer made of regular felt stacked above the solid. This yields a total area of 1 104 square feet. Jones has an attic space that measures 23 feet by 48 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. However since the 2009 edition of the irc attics can be designed to be either vented or unvented. In other words the entire vent opening doesn t count as vented space. A composite decking made of solid materials it resembles real wood and particularly strong and stable for bearing heavy load.

Vent roofs with steeples towers or cupolas can be found in different buildings from barns to cathedrals. Jones takes this measurement and divides it by 150 for a total of 7 36 square feet of ventilation space that is needed. This means for every 150 square feet of attic floor space there should be 1 square foot of net free area. Requirements for unvented attics can be found in section r806 5 of the 2018 irc.

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. That said air resistance and interference such as vent grates reduces the area of true ventilation. It really is a design choice. The code minimum for attic ventilation in the 2012 irc section r806 roof ventilation is as follows.

Of attic floor space. Ventilation openings shall have a least dimension of 1 16 inch 1 6 mm minimum and 1 4 inch 6 4 mm maximum. The building codes and building science for. 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.