Why Should You Ventilate?

Attic ventilation serves two main functions: to lower attic temperatures and remove excess moisture from the attic.

Lowering the temperature in the attic can help reduce air-conditioning energy costs and roof deck temperature, optimize the service life of a roof covering, and minimize ice damming. Removing excess moisture reduces the possibility for mold and mildew growth and minimizing the potential for wood rot.

To focus on issues of heat buildup and excess moisture in attics with poor ventilation. Marco is leveraging its considerable manufacturing, engineering, and design capabilities to address the many costly issues plaguing the ventilation of shingle roofs. The realities of an aging housing stock combined with newer highly insulated and sealed homes means the challenges have never been greater. These issues have not been adequately addressed by an industry noted for low innovation.

Extreme to complete granule loss, blistering on the shingle, shingles curling up on the end, brittle shingles, premature dry-out, cracking and fracturing throughout the shingle.

Condensation causing the deck to swell and waviness and buckling the deck and shingles. Water can rot the roof deck, destroying its ability to carry weight loads and its nail-holding capability. Condensation can drip onto the insulation, reducing its effectiveness and possibly seeping through to the ceiling below. The impact is that damp insulation causes wood rot, mold, and mildew, leading to poor indoor air quality.

Ice dams are barriers to water runoff on the roof. They usually form at the roof edge, just above the gutter, in cold, snowy climates. They form when snow melts on warmer areas of the roof, due to inadequate ventilation, inadequate insulation, and inadequate sealing from conditioned spaces below. So, provide adequate and balanced attic ventilation, properly insulate the attic and ensure that the ceiling below the attic is sealed.

The total unobstructed area through which air can enter or exhaust a non-powered vent; typically measured in square inches. This is shown as the NFA for a box or slant back vent or the rating is square feet of NFA per lineal foot in the case of rolled vents. The NFA required by code is determined by the total square footage of the floor space of the attic to be ventilated. The bigger the attic floor space the more vents that are required.

Use our online vent calculator here. It’s all based on the size and square footage of the attic. Building Code IRC 2021 requires; 1 square foot of Net Free Area for every 150 square feet of attic floor space (1/150). Attic floor is defined as length x width floor of the attic. In certain Climate Zones 6-8 (more northerly regions of USA) the use of a vapor retarder reduces the requirement to 1/300.

First divide square feet of attic floor space by 300 providing the total NFA between intake and exhaust required. The total NFA as calculated is then further divided in half each for determining exhaust required and intake required. That total is then divided by the NFA rating of each vent to determine the number of vents required.