Presented by Roof Doctors
Roof ventilation has been around for a while now. With building codes updating and changing, it can be difficult to know if you should ventilate or not.
To better understand roof ventilation and whether you need it or not, it’s best to start with the basics.
Why do I need roof ventilation?
Roof/attic ventilation serves to lower temperatures and reduce mold and mildew buildup. By lowering the temperature in the attic, you are more likely to reduce air conditioning energy costs and roof deck temperature, optimize the life of a roof sheathing, and minimize ice damming. . It 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 caused by the tendency of warmer air to rise. In other words, the air circulates naturally in the attic, without the use of mechanical means.
Outside air enters the attic through soffit or eave vents, rises into the attic as it warms, and exits through vents at or near the top. For this method to be most effective, approximately equal amounts of venting should be placed at the soffits or eaves, and at or near the top of the attic.
Not sure you need it? Let’s look at the code
The amount of ventilation and opening size requirements can be found in Section 806, Roof Ventilation of Chapter 8, Roof-Ceiling Construction. Here are the requirements taken directly from the IRC:
R806.1 Ventilation required.
Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation to each space separated by ventilation openings protected from the entry of rain or snow. Ventilation openings must have a minimum dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings with a minimum dimension greater than 1/4 inch (6.4 mm) must be screened with corrosion-resistant metal cloth, hardware cloth, perforated vinyl, or similar material with openings having a minimum dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members must comply with the requirements of Section R802.7. Required ventilation openings should open directly to the outside air and should be shielded to prevent the entry of birds, rodents, snakes and other similar creatures.
R806.2 Minimum ventilation area.
The minimum net free vent area should be 1/150 of the area of the ventilated space.
Exception: The minimum net free ventilation area must be 1/300 of the ventilated space, provided the following two conditions are met:
- In climate zones 6, 7 and 8, a Class I or II vapor barrier is installed on the warm winter side of the ceiling.
- Not less than 40% and not more than 50% of the required ventilation area is provided by fans located in the upper part of the attic or rafter space. Top fans should be located no more than 3 feet (914 mm) below the ridge or the highest point of the space, measured vertically. The rest of the required ventilation should be located in the lower third of the attic space. Where the location of wall or roof framing members conflicts with the installation of overhead ventilators, installation more than 3 feet (914mm) below the ridge or highest point of the space must be allowed.
Although the primary code requirement is a 1:150 ratio, a 1:300 ratio is commonly used. To use 1:300, both conditions of the exception must be met. The first condition requires a Class I or II vapor barrier for buildings located in climate zones 6 through 8. An example of a Class I vapor barrier would be a sheet of polyethylene, and it should be installed on the warm side of attic insulation. A Class II vapor barrier could be kraft-faced fiberglass batt insulation installed on the attic floor with the kraft paper side down.
Does your roof need to be ventilated?
Attic ventilation is often considered a technical requirement for steep roofs, as well as a building code requirement. However, since the 2009 edition of the IRC, attics can be designed to be ventilated or unventilated. So the decision to ventilate an attic is no longer dictated by the building code, and ventilation is now more of a design choice.
Requirements for unventilated attics can be found in section R806.5 of IRC 2018. This section contains a long list of requirements and conditions that must be met in order to have an unventilated attic. This article will not discuss them, but readers are encouraged to familiarize themselves with these requirements.
To vent or not to vent?
That’s the question, and it’s not as complicated as it seems. If you experience harsh, cold winters, ventilation makes sense. It will minimize the formation of ice dams and also provide much needed ventilation during the summer months.
For more information and to speak to an expert, call Roof Doctors!
1214 S. Dickerson Road.
Goodlettsville, TN 37072
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