Attic Ventilation Systems

Continuous Ridge Vent Attic Ventilation

One of the most efficient attic ventilation methods is the continuous soffit vent combined with the continuous ridge vent.

To form the ridge vent, a slot is cut out of the roof sheathing along the ridge. A fibrous material is installed over the top of the slot and held in place by a cap. The fibrous material will keep out insects while still allowing air to flow. The cap holds the fiber in place and diverts air.

Different manufacturers produce ridge vents in different attic ventilation configurations.

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Occasionally, you’ll find a ridge vent with nothing installed to keep insects out. This is a defective attic ventilation installation.

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This grey fabric used in this attic ventilation ridge vent is one type of barrier, but you will see other types.

This is the vent for the bathroom exhaust fan improperly terminated inside the attic. Bathroom and laundry vents should terminate at the outside — not just beneath the ridge vent…

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…and not under the insulation.

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Although thermal buoyancy — which is the tendency for hot air to rise – will evacuate hot air from an attic space or rafter bay, attic ventilation is much more effective if the air is actively pulled out of the upper vents by other forces, such as air-pressure differentials.

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When ridge vents have baffles, the baffles divert wind blowing across the roof upward. This creates an area of low pressure just above the opening, producing an air-pressure differential between the attic space and the exterior, which helps pull air out of the attic.

This effect is lost if the prevailing winds blow parallel to the ridge.

You may not know the direction from which prevailing winds blow. A home may have a well-designed attic ventilation system which is poorly oriented to the wind. In this case, it may be out-performed by a more poorly designed system which is better oriented to the wind.

You don’t have to know exactly how well the attic ventilation system performs, but knowing how systems are designed to perform may help if you find a problem that may be related to poor roof or attic ventilation.

You’ll find different types of continuous ridge vents installed. Whatever the type, you should check to make sure that the cap is securely fastened, since longer nails are required because of the additional thickness.

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This ridge vent was improvised using sections of soffit vent. Obviously, the fasteners did not hold well enough.

Adequate Attic Ventilation

Most home inspection Standards of Practice require comments on the adequacy of attic ventilation.

The most commonly accepted formula for determining adequate attic ventilation is a minimum of 12 square inches of net free area for every 150 square feet of attic space. Twelve square inches might be an opening of 3 inches by 4 inches.

“Net free” means that louvers or other devices that reduce the actual size of the opening cannot encroach upon the opening.

This minimum can be reduced if a low-perm vapor barrier is installed on the warm side of the attic floor, which would be beneath the insulation. If this is the case, the formula changes to 12 square inches of net free area for every 300 square feet of attic space. So, with no air barrier, it’s 150, and with an air barrier, it’s 300.

An example of a commonly installed, low-perm vapor barrier is plastic attached to the underside of the ceiling joists before the ceiling was drywalled.

Attic Ventilation Radiant Barriers

Radiant barriers are materials that are thermally reflective. They reflect heat. They often look like foil bonded to cardboard. Here’s an example. You may see sheets stapled to roof framing in the attic.

Stapling sheets to the bottoms of rafters will trap hot air in the rafter bays.

Installing them beneath solid roof sheathing is also unacceptable. This roof originally had wood shakes installed. When they were replaced with asphalt composition shingles, a radiant barrier was installed on top of the spaced sheathing before a solid roof deck was installed as a substrate for the shingles.

Heat radiated downward by the roofing materials will be trapped and increase the temperature on the roof. This condition can contribute to premature failure of roofing materials, especially asphalt underlayment and asphalt shingles. It will also void the shingle manufacturer’s warranty. If the radiant barrier installation instructions conflict with the shingle manufacturer’s instructions, the shingle manufacturer’s instructions should take precedence.

A better installation and attic ventilation would be to fasten radiant sheets to the attic floor. The amount of heat radiated to the living space will be reduced, but the attic ventilation system will still be able to evacuate hot attic air to the outside.

Generally, asphalt shingle manufacturers require that the underside of the roof sheathing be ventilated for proper attic ventilation for the warranty to be effective.

Passive Attic Ventilation Systems

Attic Ventilation Passive Systems

Passive attic ventilation requires no electricity, but it’s effectiveness often depends on it’s being properly designed for a specific home site.

A well-designed passive attic ventilation system has an intake device located low in the roof, with an exhaust installed near the ridge.

In a balanced system, the net opening of the intake will be approximately the same as the net opening of the exhaust.

Soffit Attic Ventilation

soffit vent Passive Attic Ventilation Systems

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If the system is out of balance, it’s better to have more attic ventilation lower in the roof by way of larger intakes.

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You may see individual vents each serving a rafter bay, or continuous vents, as you see here.

Make sure that the soffit vents are not blocked by insulation. This is especially common in attics with blown-in insulation.

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If you find blocked vents, be sure to make note and correct.

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If the insulation is not held back, you should see baffles installed to create an air space.

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It’s important that baffles extend through to the exterior so that they actually allow air flow into the attic. Once the siding and soffits are complete, you won’t be able to confirm this visually. Unless you can actually feel air flowing from the baffles, you will not be able to confirm functional air flow, and you should disclaim it.

Gable Attic Ventilation

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Gable vents are installed in the gabled ends at opposite ends of the attic. They’re most effective when the vents align with the prevailing winds. This allows high air pressure on the upwind side of the home to push air into the attic, and low pressure on the downwind side to pull air out.

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When prevailing winds blow perpendicular to the vents, the gable vents act as both intake and exhaust. Less air exchange takes place and attic ventilation is not uniform, so it’s less effective.

Roof Attic Ventilation

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Roof vents, sometimes called “turtle” vents, should be installed near the roof peak. This illustration shows cool air entering the soffit vents.

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It rises along the underside of the roof sheathing and absorbs heat before exiting the attic through the roof vent.

Here, you see it before and after installation.

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Sometimes, the roofers forget to cut the underlayment. Roof vents do not work unless the underlayment is cut out.

Wind Turbine Attic Ventilation

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Occasionally, you’ll see a turbine vent. Turbines use a series of specially-shaped vanes that catch wind and spin the turbine. As the vanes spin, they create an area of low pressure which pulls air from the attic.

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While this type of attic ventilation system not as effective as ridge vents, they do provide a low-cost alternative in areas with consistent wind speeds of at least 5 mph.

That’s the power cord for the evaporative cooler you see improperly routed through the turbine vent into the attic.

Attic Ventilation

delicious Passive Attic Ventilation Systems

Energy Efficiency With Aerogel Works to Control Heat Transfer

Energy Efficiency as an Insulation Material

Aerogel and silica aerogel are used as an insulation material which is good for reducing heat transfer and thermal conductivity. Controlling heat transfer and heat loss is becoming more and more of value in energy efficiency.

aerogeltorch Energy Efficiency with Aerogel Works to Control Heat Transfer

Aerogel is a class of porous, solid materials that exhibits an impressive array of extreme properties. Invented in 1931 and used for decades in scientific applications, aerogel is becoming increasingly feasible as a building insulation, largely due to a decrease in the price of the material.

Aerogel is still prohibitively costly for most homeowners, and the few who can afford it probably don’t know what it is. At expensive properties with environmentally friendly features, however, inspectors should be prepared to encounter the material. Also, the prevalence of aerogel is likely to increase in the coming years as it becomes more affordable and widely known.

Physical Properties and Identification of Aerogel

Aerogel holds 15 world records for material properties, a few of which are listed below. Aerogel is:

lightweight. It is, in fact, the lowest-density solid on the planet. Some types are composed of more than 99% air, yet they still function as solids;
extremely high in surface area. It can have a surface area up to 3,000 square meters per gram, meaning that a cubic inch of aerogel, if flattened out, could cover an entire football field; and
strong. It can support up to 4,000 times its own weight. In the picture at right, a 2-gram piece of the material is supporting a 5-pound brick.

The following qualities will also assist with identification. Aerogel:

appears blue due to Rayleigh scattering, the same phenomenon that colors the sky;
feels like Styrofoam® to the touch. Although a slight touch will not leave a mark, pressing more firmly will leave a lasting depression or even produce a catastrophic breakdown in the structure, causing it to shatter like glass; and
is rigid. Despite its name, it is hard and dry, little resembling the gel from which it was derived.

Energy Efficiency of Silica Aerogel Performance as an Insulator

Composed almost entirely of gas, which is a poor heat conductor, aerogel can almost nullify the three methods of heat transfer (conduction, convection and radiation). Boasting an R-value of 10 to 30, NASA has used the material to protect astronauts and equipment, such as the Mars Rover, from the extreme cold of space. As compared to conventional insulation material, the R-values of vermiculite, rockwool, fiberglass and cellulose are approximately 2.13, 3.1, 3 and 3.1, respectively. Silica aerogel is especially valuable insulation material because silica is also a poor conductor of heat and creates less heat transfer. A metallic aerogel, on the other hand, would be less useful as an insulator allowing greater heat transfer.

Production of Aerogel and Silica Aerogel

Aerogel is derived from gels, which are substances in which solid particles span a liquid medium. The first aerogel was produced from silica gels, although later work involved alumina, chromia, carbon and tin oxide. Through a process called super-critical drying, the liquid cabotranules Energy Efficiency with Aerogel Works to Control Heat Transfer component of the gel is removed, leaving behind the hollow, solid framework. The resulting aerogel is a porous, ultra-lightweight lattice composed of more than 90% air. Ordinarily, drying of a gel results in its shrinkage and collapse (think of Jell-O left out for a few days), but super-critical drying is performed under intense heat and pressure that preserve the structure of the gel.

Energy Efficiency

Manufacturers offer the material in a variety of forms, such as the granules pictured at right, made by Cabot, which are sometimes used as insulation in skylights. Aspen Aerogel® offers 57-inch wide rolls of the material in 0.2- and 0.4-inch thicknesses, while Thermoblok® comes in 1.5-inch wide strips that are used to cover framing studs and help prevent thermal bridging at a cost of about $1.99 per foot. Silica aerogel is also becoming more widely used.

Safety as an Insulation Material

Aerogel safety is dependent on the safety of the gel from which it was made; it will be carcinogenic, for instance, if the gel from which it was derived had this quality. Fortunately, silica-based aerogel is not known to be dangerous, although it may irritate skin, mucous membranes, eyes, the respiratory tract, and the digestive system. Aerogel is hydroscopic and extremely dry to the touch, which can, in turn, cause it to dry out unprotected skin. Gloves and goggles are recommended for inspectors and contractors who must handle the material.

Aerogel does not seem to be an environmental threat. Aspen Aerogel’s® website states: “Aerogel blankets do not meet any of the characteristics of a U.S. EPA hazardous waste,” and further notes that scrap aerogel may be disposed of in landfills that are approved to accept industrial waste.

Controlling thermal conductivity, supercritical drying and energy efficiency are all benefits of Aerogel.

In summary, aerogel is a safe, remarkably effective thermal insulator whose use should become more widespread as it becomes more affordable with greater energy efficiency.

Thanks To Nick Gromicko and Rob London

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Attic Ventilation Systems