Ventilation Terminology*

Balanced System

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Equal amounts of intake net free area ventilation at the eaves and exhaust net free area ventilation at or near the ridge.

Bernoulli Effect

A phenomenon whereby low pressure resulting from wind passing over a structure or object creates a pulling or lifting action.

CFM

Cubic feet of air moved per minute. All motorized vents have a CFM rating that defines the vent's capacity to move air. The higher the CFM number, the greater the vent's capacity.

Cold Roof

The condition in which the roof temperature is equalized from top to bottom. An equalized roof temperature can help eliminate the conditions that can lead to the formation of ice dams.

Condensation

The change of water from vapor to liquid when warm, moisture-laden air comes in contact with a cold surface.

Conduction

Flow of heat directly through a solid material; responsible for most heat loss or gain in a residence.

Convection

Transfer of heat by air currents, i.e., gravity, hot air furnace.

Deck

The surface, installed over the supporting framing members, to which the roofing is applied.

Dormer

A framed window unit projecting through the sloping plane of a roof.

Drip Edge

A corrosion-resistant, non-staining material used along the eaves and rakes to allow water run-off to drip clear of underlying construction.

Eaves

The horizontal, lower edge of a sloped roof which typically overhangs the walls.

Exhaust Vent

An outlet or opening installed high on the roof near the ridge or in the gable for the purpose of ventiliating the underside of the roof deck.

External Wind Baffle

The built-in wing or lip on a ridge vent that deflects wind up and over the vent creating the Bernoulli Effect that enhances airflow performance by pulling or lifting the air out of the attic. It also deflects weather elements over the vent away from the attic.

Flashing

Pieces of metal or roll roofing used to prevent seepage of water into a building around any intersection or projection in a roof, such as vent pipes, chimneys, adjoining walls, dormers and valleys. Galvanized metal flashing should minimum 26-gauge.

Gable

The upper portion of a sidewall that comes to a triangular point at the ridge of a sloping roof.

Gable Roof

A type of roof containing sloping planes of the same pitch on each side of the ridge. Contains a gable at each end.

Gambrel Roof

A type of roof containing two sloping planes of different pitch on each side of the ridge. The lower plane has a steeper slope than the upper. Contains a gable at each end.

Gutter

A shallow channel or conduit of metal or wood set below and along the eaves of a house to catch and carry off rainwater from the roof.

Hip

The inclined external angle formed by the intersection of two sloping roof planes. Runs from the ridge to the eaves.

Hip Roof

A type of roof containing sloping planes of the same pitch on each of four sides. Contains no gables.

Ice Dam

A collection of melted snow that refreezes, typically at the projecting eave of a sloping roof. The ice dam causes the water from melting snow to back up under roof shingles.

Intake Vent

An inlet or opening installed in the soffit or undereave area for the purpose of ventilating the underside of the roof deck.

Internal Weather Filter

An untreated, unwoven fiberglass material inside ShingleVent II and FilterVent ridge vents that provides extra weather protection from wind-driven rain, snow and dust.

Mansard Roof

A type of roof containing two sloping planes of different pitch on each of four sides. The lower plane has a much steeper pitch than the upper, often approaching vertical. Contains no gables.

Net Free Area

The total unobstructed area through which air can enter or exhaust a non-powered vent; generally measured in square inches. All nonpowered vents have a Net Free Area rating.

Overhang

The portion of the roof structure that extends beyond the exterior walls of a building.

Pitch

The degree of roof incline expressed as the ratio of the rise, in feet, to the span, in feet.

R-Value

Thermal resistance, a measure of a material’s or a construction’s ability to retard heat flow. R-Values in a series of materials can be added to determine a construction's total thermal resistance.

Radiant Heat

Heat transferred from one body to another which are not in contact (i.e., from the sun to a roof).

Rafter

One of a series of structural members of a roof, designed to support roof loads. The rafters of a flat roof are sometimes called roof joists.

Rake

The inclined overhang of a gable roof.

Ridge

The horizontal line at the junction of the top edges of two sloping roof surfaces. The rafters of both slopes are nailed to a board at the ridge.

Sheathing

Exterior grade boards used as a roof deck material.

Shed Roof

A roof containing only one sloping plane. Has no hips, ridges, valleys or gables.

Slope

The degree of roof incline expressed as the ratio of the rise, in inches, to the run, in feet.

Soffit

The finished underside of the eaves.

Square

A unit of roof measurement covering a 10 ft. by 10 ft. roof area, or 100 square feet of roof area.

Thermal Effect

The inherent property of warm air to rise, also known as thermal buoyancy.

Vapor Diffusion

The process in which water vapor naturally travels from high-humidity conditions to low-humidity conditions; for example, from the living space into the attic.

Vapor Retarder

Any material used to prevent the passage of water vapor. Applied to insulation or other surfaces, it retards vapor travel to regions of low temperature where it may condense. A material is considered a vapor retarder if it has a perm rating of 1 or less (the lower perm, the better the vapor retarder). Examples: Kraft facing on insulation, foil facing on insulation.

Vent

Any device installed in a roof, gable or soffit for the purpose of ventilating the underside of the roof deck. Any outlet for air that protrudes through the roof deck such as a pipe or stack.

Waterproofing Shingle Underlayment

A special self-adhering waterproofing shingle underlayment designed to protect against water infiltration due to ice dams or wind-driven rain.

 

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*Ventilation Terminology from Air Vent. Visit www.airvent.com to learn more about ventilation products.