Climate varies considerably Across North America. A crawlspace or attic configuration which works well in one climate may perform very poorly in another. For energy code compliance, North America is separated into climate zones, each with special recommendations for insulation levels and ventilation including configurations of crawlspaces and attics.

You can learn more about existing and proposed climate zones at the U.S. Dept. of Energy climate zone web page.

Here are examples of climate zones is use in various parts of the country.

Roofing materials: Damage and Shortened Lifespan

Roofing materials absorb solar radiation as heat. This causes two concerns…

1. Effect of heat on roofing-covering material lifespan.

Roof-covering material deterioration is largely the result of thermal expansion and contraction. The amount of expansion and contraction taking place is directly related to the differences between daily high and low roof temperatures, so keeping the roof cool helps minimize temperature swings and extend the life of the roof-covering material.

2. Heat from the roof is transferred to the living space.

Heat transferred into the home from the roof will raise cooling costs during the warm season. Heat from the roof is mostly radiated to the living space rather than transferred by conduction or convection. For this reason, ventilation has less effect on the amount of heat transferred from the roof to the living space than if conduction or convection were the main heat-transfer forces.

Ice Dams

In cold climates, keeping the attic cold can help prevent ice dams from forming on the roof. If heat from the living space rises through the roof and melts standing snow from below, when this snow re-freezes it can create ice dams which damage roofing-covering material and may cause moisture to penetrate the roof covering and cause damage from leaks and wood decay.

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CONTROLLING HEAT

Roof Color

Although there is some difference of opinion as to how much effect attic ventilation has on roof temperatures, estimates go as low as 5% depending on configuration of the venting. Roof color may have an effect of up to 25%. solar reflectance is the percentage of solar energy which is reflected by a material. Light-colored shingles reflect up to 70%, dark shingles reflect 20% or less. They absorb the rest and radiate it as heat.

Radiant Barriers

Radiant barriers are reflective, foil-like materials typically installed in attics to slow the movement of heat. These barriers are very effective, reflecting up to 97% of the radiant heat with which they come into contact. Radiant barriers are not effective at preventing heat transferred by conduction or convection. The barrier is usually aluminum and may be attached to various materials and installed or may be mixed with insulation.

Unvented, Conditioned-air Attics

Recent developments have shown that a non-vented, heated attic can outperform ventilated, unheated attics if designed and built correctly. They are allowed in the 2006 International Residential Code (IRC) and will soon be acceptable in most states.

Key to this design is the use of effective insulation installed against the underside of roof sheathing.

The design cuts heat loss through the roof and reduces the chances of wood decay as it prevents moist air which escapes into the attic from condensing on cold surfaces. In order to do this, especially in conditions which introduce moist air into the attic, it’s important to use spray foam insulation with low permeability, meaning that it’s highly resistant to allowing water vapor to pass through. Closed-cell foam is desirable in this situation.

Concerns with this system include the possibility of wood decay from moisture penetration of the roof covering material and membrane. Once trapped between foam and sheathing, it may dry slowly enough to allow wood decay fungus to become active.

Because the underside of roof sheathing will not be visible, Inspectors should consider disclaiming the condition of the roof sheathing.

TYPES of ATTIC VENTILATORS

Roof vents- Roof vents are individual ventilators which are basically covers for holes cut in the roof. Holes cut for roof vents are typically about 1 square foot.

Source: Kenton Shepard

Roof vents installed on a home

Gable vents- gable vents are installed in the gabled ends of homes. They’re much more effective if installed on the upwind and downwind sides of the home, which means to be effective, the roof’s ridge must align with the prevailing winds. This allows positive and negative air pressure to help move air through the attic space.

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Gable vents installed on a home

Continuous ridge vent- when installing a ridge vent, roof sheathing at the peak of the roof is cut back enough to leave a gap, typically 3 inches, along the peak of the roof. This gap is then covered using one of several abvailable methods designed to keep moisture and insects out of the attic. Continuous ridge vents are more effective if a baffle is installed, and if the roof’s ridge is perpendicular to the prevailing winds.

Source: Kenton Shepard

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As wind blows perpendicular to the ridge, it's deflected up and over the baffle, which creates an area of low pressure just outside the air slots which helps pull air from the attic space. This effect is lost if the ridge is parallel to prevailing winds.

Continuous soffit vent- When a continuous soffit vent is installed, a 2-inch slot on the underside of the roof overhang (the eve) is covered with flashing designed to allow air to flow but to prevent insect entry. Soffit vents create an intake for air to flow easily into the attic space, and out through roof, ridge or gable vents, improving fresh air circulation through the attic.

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Diagrams show two soffit venting scenarios

INSPECTION CONSIDERATIONS

While inspecting the attic ventilation, inspectors should watch for the following...

Roofers sometimes install roofing membranes over holes cut into sheathing for ventilation, blocking airflow. This is a common find with roof vents and continuous ridge vents.
Blown-in insulation is often blown into areas which block air flow from soffit vents. In areas where this may be a problem, baffles should be installed.

Source: Kenton Shepard

Properly installed baffles Baffles partially plugged and missing

Peak to Prairie Home Inspection Service

Attic Ventilation