Why You Should Not Mix Roof Exhaust Vents

Step by Step

1. 1Step 1: Count the exhaust vents on your roof

   From the ground, look for roof vents at the ridge and on the roof face, and gable vents at the ends. In the attic, look for fans or thermostats from power vents. Write down each type and how many there are.

   If there is more than one type of exhaust, the roof is mixed. Also check bath fans, see how to vent a bathroom fan through the roof, because a fan venting into the attic is a moisture source, and it is not an exhaust vent in this sense.

2. 2Step 2: Understand how the short circuit happens

   A ridge vent and a lower exhaust vent both work as outlets. The pressure difference between them is small but direct. Air is pulled in at the lower vent and leaves at the ridge, and the intake air from the soffit has little pull on it. The eaves stay dead.

   Power vents make it worse. A fan in the roof pulls hard and can draw air through the ridge vent and gable vents, or from the soffits or from the house through the ceiling, rather than where you want it.

   Tip: Rain and snow can also be pulled in through the lower vent when the ridge vent is pulling, since the lower vent is acting as an intake.

3. 3Step 3: Choose one exhaust type

   A continuous ridge vent gives the most even exhaust and is the usual first choice when the ridge is long enough. On a short or cut-up ridge, a row of box vents may be the better single choice, but use one or the other.

   Gable vents are a separate question. On roofs with gable vents and a ridge vent, the gable vents can short circuit the ridge, and they depend on wind direction. They are best removed or sealed when a ridge vent is installed, unless the design specifically accounts for them.

4. 4Step 4: Size the single exhaust by net free area

   Use the calculation in how to calculate attic ventilation. The BC Building Code asks for vent area not less than 1/300 of the insulated ceiling area, or 1/150 for low slope or joist roofs, split about half intake and half exhaust.

   If the ridge alone cannot give enough exhaust, choose a ridge vent with a higher rating per foot, or use a single type of box vents instead. Do not add a second type to make up the difference.

5. 5Step 5: Make sure intake is at least equal to exhaust

   If you remove exhaust vents, you may find there was too much exhaust already. Intake should be at least as large as exhaust. See how to add soffit intake vents and how to install insulation baffles.

   A good ridge vent with open soffit intake does the job of several mixed vents.

6. 6Step 6: Remove or seal the extra exhaust vents

   For box vents and turbines, remove the vent, patch the deck with plywood and shingle over the patch, lapping the new shingles correctly. For a power vent, disconnect the wiring safely, remove the unit and close the hole the same way.

   For gable vents, block them from inside with rigid foam or plywood, or cover them outside. Seal any vents that remain so they do not act as a second exhaust or intake.

7. 7Step 7: Check the result

   With one exhaust and a clear intake, you should feel air moving at the soffit, see daylight at the vents and find dry sheathing. Mold or damp that was present before may take a dry spell to go away. See how to fix attic mold caused by poor ventilation for treatment.

Combinations That Cause Trouble

Power Vents and Moisture

Common Mistakes

• Adding a ridge vent and keeping the old box vents. The two short circuit.
• Adding a power vent to a roof with other vents. It pulls air from the wrong places.
• Leaving gable vents open with a ridge vent. Wind passes straight through the attic.
• Closing old vent holes with only sealant. The openings need a proper patch and shingling.
• Adding more exhaust to fix a damp attic. The problem is usually intake or moisture sources.
• Ignoring intake. A single exhaust still needs soffit vents.