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Roof ventilation has been around for a while now. With building codes updating and changing it can be tough to know whether you need to ventilate or not.
To better understand roof ventilation and whether you need it or not, it’s best to start with the basics.
Roof/attic ventilation serves the purpose of lowering temperatures and reducing the buildup of mildew and mold. By lowering the temperature in the attic you’re more likely to reduce air-conditioning energy costs and roof deck temperature, optimize the service life of a roof covering, and minimize ice damming. This benefits your home in every season of the year.
Some benefits of removing excess moisture include reducing the possibility of mold and mildew growth and minimizing the potential for wood rot.
The two methods used to ventilate roofs are static and mechanical.
The more common of the two is the static method. This method relies on convection, which is a mode of heat transfer that is caused by the tendency of warmer air to rise. In other words, air flows through the attic space naturally, without the use of mechanical means.
Outside air enters the attic space through soffit or eave vents, rises through the attic space as it warms, and exits through vents that are positioned at or near the top. For this method to be most effective, approximately equal amounts of ventilation should be placed at the soffits or eave level, and at or near the top of the attic space.
The ventilation amount and opening size requirements can be found in Section 806, Roof Ventilation of Chapter 8, Roof-Ceiling Construction. The following are the requirements taken directly from the IRC:
Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl, or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air and shall be protected to prevent the entry of birds, rodents, snakes, and other similar creatures.
The minimum net free ventilation area shall be 1/150 of the area of the vented space.
Exception: The minimum net free ventilation area shall be 1/300 of the vented space, provided both of the following conditions are met:
Even though the primary code requirement is the 1:150 ratio, a 1:300 ratio is commonly used. In order to use 1:300, one must meet the two conditions in the exception. The first condition requires a Class I or II vapor retarder for buildings located in Climate Zones 6 through 8. An example of a Class I vapor retarder would be a polyethylene sheet, and it should be installed on the warm side of the attic insulation. A Class II vapor retarder could be kraft-faced fiberglass batt insulation installed on the attic floor with the kraft paper side facing down.
Ventilating attic spaces is often viewed as a technical requirement for steep-slope roof assemblies, as well as a building code requirement. However, since the 2009 edition of the IRC, attics can be designed to be either vented or unvented. So, the decision to vent an attic space is no longer dictated by building code, and ventilation is more of a design choice now.
Requirements for unvented attics can be found in Section R806.5 of the 2018 IRC. This section contains an extensive list of requirements and conditions that have to be met in order to have an unvented attic. This article will not discuss them, but readers are encouraged to familiarize themselves with those requirements.
This is the question, and it’s not as complicated as it seems. If you experience harsh, cold winters, ventilation makes sense. It’ll keep ice damming to a minimum and provide much needed ventilation during the summer months too.
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