It s one of the more confusing aspects of home building and.
Fine homebuilding attic ventilation.
Attic fans are intended to cool hot attics by drawing in cooler outside air from attic vents soffit and gable and pushing hot air to the outside.
In mixed climates ventilation serves either role depending on the season.
Roofs need a driving force to move air in and out and intake and exhaust vent openings large enough to facilitate this movement.
Absent that try to provide at least a net free vent area of all ventilation openings of 1 150th of the horizontal area of the attic with reasonable cross flow to eliminate stagnant pockets of air.
But building scientist joe lstiburek takes issue with that opting instead for more ventilation at the eaves say a 60 40 split with the eaves getting the greater proportion.
As the warm air escapes through vents high on the roof cooler air enters the lower vents.
The baffles will create vent spaces and prevent windwash energy robbing convective short circuiting air moving through the insulation 1 1 4 in.
In a situation where mechanical systems or ductwork has to be in the attic space or when there are lots of penetrations in the ceiling below the attic it s best to bring the entire attic area inside the thermal envelope.
If an attic measures 900 sq.
Of attic floor area with between 40 1 2 sq.
Ft of ventilation area at the ridge and the remainder at the soffits.
Attic ventilation works on the principle that heated air naturally rises primarily utilizing two types of vents.
However if your attic has blocked soffit vents and is not well sealed from the rest of the house attic fans will suck cool conditioned air up out of the house and into the attic.
Of attic space equally divided between eave and ridge vents.
Of vents for each 300 sq.
Ft code requires that there be a minimum of 3 sq.
If you ve got a roof over your latest building project chances are you ve also given some thought to roof venting.
Codes require a net free ventilation area nfva of 1 sq.
Intake vents located at the lowest part of the roof under the eaves allow cool.
In a hot climate the primary purpose of ventilation is to expel solar heated hot air from the attic or roof to reduce the building s cooling load and to relieve the strain on air conditioning systems.
Ft and 50 1 5 sq.
Vent the roof deck.
This way it s not as big a deal if the ceiling leaks air or if the ducts are leaky and uninsulated.
In an attic the primary driving force is the buoyancy of warm air rising in the vault of the roof.
15 articles and videos from the fhb archive that will take you through every step of the roof venting process.
