This content has been created for those in the insurance & construction industry.
(A heating climate is where you spend more money on heating your home in the winter than money spent cooling you home in the summer.)
VENTILATION MINIMUM REQUIREMENTS – The industry recognized rule-of-thumb for attic ventilation minimum requirements is to provide one square foot (SF) of Net Free Area (NF) ventilation for every 300 SF of floor space. This assumes there to be a vapor barrier, properly installed, below the attic insulation. In the event there is not a properly installed vapor barrier, the minimum requirement is to provide one SF of ventilation for every 150 SF of Floor space. The goal of proper attic ventilation is to exhaust moisture from the attic, and keep the air in the attic space as close as possible to the exterior air.
These ventilation requirements (i.e. 1SF per 300SF) are a total of both the intake and exhaust ventilation requirements. In other words, half of the ventilation should be intake, and the other half should be exhaust. The ratio of intake vs. exhaust should be relatively even. If you get this ratio too far out of whack, there can be improper ventilation as well as rapidly increasing the amount of snow/rain to enter through the exhaust vents.
HIP ROOFS – Most of the time when a home has a hip roof, there is not enough ridge length to accommodate enough air space for exhaust ventilation. This is particularly true if there is not a vapor barrier properly placed below the attic insulation. Often, the only practical solution for exhaust venting in a hip roof is to install “Turbine” vents or powered exhaust vents.
POWERED EXHAUST VENTS – As the goal of exhaust venting is to remove excessive moisture from an attic, it is imperative that the powered exhaust vent is humidistatically controlled as well as thermostatically controlled. Many times we see attics with powered exhaust vents that are only thermostatically controlled, and they have a little effect for attic ventilation in the winter months when moisture control is the most crucial.
MIXED EXHAUST VENTING – An attic ventilation system cannot have mixed exhaust venting, such as using a ridge vent in conjunction with a gable end vent, or using “mushroom” exhaust vents with a powered exhaust fan. As air will intake and exhaust wherever it is easiest, mixed exhaust venting will often allow an exhaust vent to become an intake vent, making the ventilation system ineffective.
MIXED EXHAUST VENTING – An attic ventilation system cannot have mixed exhaust venting, such as using a ridge vent in conjunction with a gable end vent, or using “mushroom” exhaust vents with powered exhaust fan. As air will intake and exhaust wherever it is easiest, mixed exhaust venting will often allow an exhaust vent to become an intake vent, making the ventilation system ineffective.
IMPROPER PLACEMENT OF EXHAUST VENTS – Exhaust vents should be placed at the same height on the roof, at or near the peak. They should also be placed (as in when “mushroom” vents are used) on the same roof slope. If they are placed on opposite roof slopes, wind direction and velocity may cause the exhaust vents to become intake vents, making the ventilation system ineffective.
PLUGGED INTAKE VENT OPENINGS – It is imperative that the intake vent openings remain unobstructed by insulation or other materials. This is usually resolved by placing “baffle” vents at the base of the roof cavity keeping the opening clear of obstructions. The typical “baffle” vent will provide 2″ of clearance between the bottom of the roof sheathing and the top of the insulation material. If the intake vent openings are obstructed, it will make the ventilation system ineffective.
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