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What Is the Best Type of Attic Ventilation for My House?

Not only is ventilation recommended for your home, it is also required. However, having improperly installed ventilation can be just as bad as having no ventilation at all. If you are in the process of re-roofing your home, make sure to schedule a Ventilation Assessment to determine the correct ventilation type for your home. In this blog post, the expert roofing contractors at Bauer Roofing will walk you through:

  • Ventilation requirements
  • The difference between ventilation types
  • How to calculate how much ventilation you need in your home.

Ventilation Requirements

According to most building codes, you need one square foot of vent area for each 150 square feet of attic floor space. The minimum is one square foot for every 300 square feet of attic floor space if there is a vapor retarder or the space is balanced between the ridge and intake vents. A balanced ventilation system means about 50 percent of the required ventilating area should be provided by exhaust vents in the upper portion of your attic with the remaining 50 percent provided by intake vents. Ventilation requirements vary slightly from state to state so someone from South Carolina may have different needs than someone from California. The main thing is to make sure you have a place for air to escape your attic as well as a space for air to enter.

Ventilation Types - Exhaust vs. Intake

Just like a car engine has intake and exhaust, an attic has to have the same thing. If the air in your attic sits still it will condensate and cause serious damage to the shingles, sheathing, insulation and ceilings. Ventilation works best if there is at least as much room for air to enter the attic as there is for it to leave.

  • Exhaust - Air exhausts through power fans, dormers, turbines or ridge vent.
  • Intake - Air comes in through soffit vents in the overhang of the house.

Most homes that were built in the last 30 years will have eaves that either have soffit vents or room to add them. In older homes or bungalow style houses, you may have to put in a vented drip edge or plugs to be able to install a vent. If you aren’t sure what your house has, look out the window and look up. If you see vents then you should be fine; if not, then ask your roofing contractor to evaluate the situation.

Which Type of Attic Ventilation is Best?

The type of attic ventilation that is best for your home depends on what type of roof your home has.

  • Hip Roof - If you have a house with a "hip" roof (and I don't mean groovy), then the ridge of your house does not extend all the way to the edges. Instead, it is cut short and - depending on the pitch of the roof - it could be extremely short. On a house with this type of roof, the best ventilation option is a power fan. This fan can either be wired or solar-powered. The fans come in different strengths and material types. As for solar-powered vs. wired, that is for another day. Just understand that with a wired power fan, unless it is also regulated by humidity, won’t run in the winter when moisture can be a problem. For that reason, some prefer a solar-powered fan –plus it doesn’t take electricity to run it.
  • Gable Roof - If you have a house with a gable roof, the ridge vent is most likely your best option for ventilation because there appears to be enough ridge to meet the ventilation requirements from the roofing manufacturer. You won't know for sure until your attic is inspected by a professional who can determine that the house has the ability for a ridge vent to be installed. This is not something you will want to take a guess on.

    You could also put turbines on these types of roofs as well, but you would need about 8-10+ of them. Air Vent Inc says 42 linear feet of the ridge is equal to 5 turbines, so on a house that is 65 feet wide then you’d need about 8. (Remember that 15 roof louvers or 5 turbine vents would be needed to provide the same ventilation as 42 feet of ridge vent on the same house).That would make your house look like a mushroom field.

Roof Types

Mixing Different Types of Ventilation

Generally you should stick with the same type of ventilation for your home. If you have a roofer that is suggesting to mix different types of vents you should get a second opinion. Exhaust ventilation –ridge vents, turbines, and power fans– pull air from the nearest air source. If there is no other type of exhaust vents on the roof then the ridge vent will pull air in through the soffit vents in the eaves, up the wood and back out the ridge. But if you add a turbine or power fan, it becomes the nearest air source and the ridge vent will merely circulate air at the top; and the bottom 2/3 of the house will be stagnate. It will essentially short circuit the air flow and negate any advantage it could have provided.

Calculating the Amount of Ventilation You Need

Air Vent recommends 1 square foot of attic ventilation for every 150 square feet of attic floor space divided evenly between intake vents at the roof’s edge or in the overhang or undereave and exhaust vents at or near the peak of the roof. Here’s what the formula looks like for a 1200 square foot attic space: 1200 divided by 150 = 8 square feet of attic ventilation. Then divide that number by 2 to provide half the ventilation for intake and half for exhaust. Thus, 8 divided by 2 = 4 square feet of attic ventilation for intake and 4 square feet of attic ventilation for exhaust.

The final step is to figure out how many vents would be needed to provide 4 square feet of attic ventilation. To do this let’s start by converting the number to square inches by multiplying by 144. Thus, 4 x 144 = 576 square inches of attic ventilation for intake and 576 for exhaust. Air Vent ridge vents provide 18 square inches of Net Free Area per linear foot. To determine how many feet of ridge vent would be needed the formula looks like this: 576 divided by 18 = 32 feet of ridge vent. A typical 8″ x 16″ undereave vent provides 56 square inches of Net Free Area per vent. To determine how many undereave vents would be needed, the formula looks like this: 576 divided by 56 = 10.2 (which can be rounded up to 11).

As for power fans: unless you have a huge attic, one is enough. Power fans can vent attics upwards of 2100 square feet.

Call Bauer Roofing today at (803) 994-9498 or contact us online to schedule a ventilation assessment to your home in Columbia, SC!