Building Code
The standard building code requires 1 square foot of net free ventilation per 300 square feet of attic. The building code was updated in 2000 to recommend 1 square foot of net free ventilation per 150 square feet of attic.
Historically, the benefits of proper attic ventilation have not been understood by most builders or building inspectors. Many older homes (30+years) have no attic ventilation at all. Homes built in the past 30 years, have better ventilation. Newer homes (last couple years) should meet the minimum spec of 1 sq. ft. of net free ventilation per 300 sq. ft. of attic, but we still see a variety of problems even in relatively new homes.
Net free ventilation is not just the size of the opening of the vent; you must deduct the area of the vent covered by screening or louvers. Most static vents have a “net-free” rating. Static vents include soffit, eave, ridge and roof-mount roof caps.
Benefit of Proper Attic Ventilation
Most of us consider attic ventilation necessary just in the summer months to reduce heat in the attic. However, proper ventilation is important year round to remove unwanted heat and moisture.
In the summer, the temperature at the underneath side of the sheathing (roof deck) can reach 170 degrees, and the temperature at the attic floor level can reach 140 degrees. Adequate ceiling insulation helps reduce heat build-up to the ceiling below the attic.
By reducing the temperature in your attic, you reduce the temperature of your ceiling and reduce the cost to cool your dwelling’s interior. Reducing the temperature in your attic reduces the temperature of the sheathing and prolongs the life of the shingles on the roof. Shingles that overheat become brittle, crack easier and also warp.
Proper ventilation is equally important to dispel unwanted moisture, especially in the winter and in humid climates. Moisture created in the home from showers, dishwashing and laundry stays trapped in the home sealed tight during the winter months. This moisture permeates the ceiling and ends up in the attic. Moisture in the attic condenses on cold roof rafters and ceiling joists and can rot the structure – Mold and mildew forms easily in a dark and poorly ventilated attic. The droplets of moisture falling from the roof compress the insulation, over time, reducing the effectiveness of your ceiling insulation.
Static Ventilation
A static ventilation system takes advantage of thermal dynamics as hot air rises. As the temperature in the attic rises, the air in the attic begins to rise and escapes through exhaust vents located in the ridge or near ridge of the roof. Intake vents equally placed around the base of the attic space are required to elimate the vacuum and allow new air into the attic to replace the exhausted air. The net free ventilation required for the attic space should be divided equally between intake and exhaust vents, otherwise the system performs to the lesser of the two.
The effectiveness of the static system can be reduced by other influences such as the wind. A slight breeze can greatly reduce the air flow through the exhaust vent. In addition to the improper ratio on intake to exhaust ventilation, the common problems found with static systems include:
- The eave decking behind the intake vents is not cut-out to the full size of the screened or louvered area reducing the air flow.
- The intake vents are covered by blown in insulation.
- Vinyl siding contractors cover eave vents with perforated vinyl and do not remove the original vent. This results in a tremendous reduction of net free ventilation.
- Ridge vents are improperly installed. The sheathing must be cut back 1.5” on each side of the ridge pole. Many times this is not done at all or less than 3” total is provided. Equally important is the roof felt must also be cut back before installation of the ridge vent. When the roof is being installed, it is standard practice to cover the ridge fully with felt to dry-in the building. Unfortunately, we frequently find ridge vents installed over the ridge openings still covered with felt paper, completely eliminating the effectiveness of the vent.
- Ridge vents on hip roofs. In most cases, a hip roof does not provide enough lineal footage of ridge to properly exhaust the attic space with ridge vents.
Electric Powered Ventilators
Powered ventilators come in many different sizes and are rated by CFM (cubic feet per minute). To determine the proper CFM, multiply your attic square footage by .7. For example, a 1500 sq.ft., one story home requires a 1050 CFM fan.
Powered ventilation has become popular since it is more reliable in breezy conditions than ridge vents.
They also require no breeze like a turbine vent and require less roof penetrations than static exhaust vents. They are also very popular in homes with hip roofs because there is not enough lineal footage of ridge to properly exhaust hipped roofs.
The draw back with electric-powered ventilators is that they are expensive to install (due to electric hookup) They cost just as much or more money to operate than is saved in cooling efficiency and generally require replacement in 3-5 years. Having an electric powered ventilator professionally installed cost between $500.00 and $700.00. Your operating costs will be between $10 and $20 per month. If it only runs in the summer for heat, your annual operation cost will be about $100.00. So the cost of electric-powered ventilator over 5 years is $1,000-$1,200. Your cost will be higher if your fan provided a hemostat to run during periods of high relative humidity.
Solar Star Solar-Powered Attic Ventilator
Using solar-power to drive an attic ventilation fan has many advantages. It is not dependent on or impaired by wind. It runs daily to remove heat in the summer and moisture in the winter and there is no cost of operation. The solar-panel DC current motor runs much quieter than an AC motor. The solar panel used is of the highest quality and is expected to last 15 years (Solatube International warrants all housing components for 10 years and all electric components for 5 years). The Solar Stars’ DC motor can be renewed with replaceable brushes (5 year service interval recommended).
The Solar Star starts working as soon as the sun comes up instead of waiting until the temperature of the attic space reaches a preset temperature, as does a regular powered ventilator. This keeps attic temperature more moderate for more of the day. The performance of the Solar Star Fan is dependent on the amount of sunshine, corresponding nicely to the ventilation needed. The fan runs slower on cloudy days, faster on sunny days.
Solar Star is rated at 850 CFM and therefore sufficient for up to a 1200 square foot attic. Two or more Solar Stars may be required for larger homes. Each Solar Star requires a 383 sq. inches of net free intake ventilation.
Call today to schedule the installation/delivery of your energy-efficient, Solar Star Attic Fan.
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