Attic Ventilation

Proper ventilation helps water vapor to exit the attic, helping to dry out existing moisture.

The attic plays a key role in the durability of a house and the safety of the people living there. If you don't pay close attention to the attic's venting, moisture can propagate mold growth and cause the roof's wood to rot.

What's the best way to ventilate an attic?

The basic concept is to get the right amount of air circulating throughout, while preventing moist air from coming in. Proper ventilation creates an air current for the water vapor to ride on and exit the attic, which also helps to dry out any existing moisture.

The best practice is to create a continuous ridge-and-soffit ventilation system, which creates a continuous flow of air upward from the eaves or soffits out through the ridge.

Before you start, check with the local code official for any special requirements pertaining to attic venting in your area.

Here's how to create a ventilating system:

1. First, determine how large the vents need to be by calculating the "net free ventilating area" according to the following equation:

  • Divide the total square feet of the attic footprint by 300 to get the required net free ventilating area in square feet. For example, an attic floor that's 900 square feet would need 3 square feet of net free ventilating area.
  • Multiply the answer by 144 to convert the square feet into square inches, since most ventilation products are rated in square inches. So, 3 square feet becomes 432 square inches of net free ventilating area.

2. Select the dimension and type of materials for the ridge and soffit vents according to your required net free ventilating area, as well as local codes.

3. Keep in mind that at least 50 to 80 percent of this ventilating area must come from the ridge vent. For the example above, the ridge venting area would be 216 to 346 square inches.

4. Framing a roof with this kind of ventilation is, for the most part, traditional with only a few differences.

5. At the ridge, hold back the roof sheathing 1 1/2 inches on both sides, leaving a 3-inch gap running the length to cover the required net free area.

6. To create the soffit vent, cut a notch into the roof rafters, called a "birds mouth," and allow them to over-hang the exterior wall far enough to create the space needed for the net free area.

7. Cover the roof as usual and install the ridge vent last, as a cap, following the manufacturer's specifications.

8. Finish off the soffit vent by attaching a J channel and sliding in a perforated vinyl or aluminum soffit material. This completes the construction.

9. Seal any penetrations in the attic floor such as ceiling fans, recessed lights, plumbing vents, or HVAC ducts. Use a caulk or expanding foam to seal the openings.

10. Insert baffles, which look like w-shaped polystyrene sheets, at the eaves by the soffit. These baffles keep insulation from blocking the venting area.

11. Finally, insulate between the ceiling joists of the attic floor using insulation with an R value of 25 to 38.

Many builders used to use gable vents or hood vents to create air flow, but these practices didn't circulate air around all surfaces of the roof assembly. Instead, they would localize air flow, leaving areas of the attic susceptible to mold growth.

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