(281) 341-5779

Attic ventilation is an important part of roofing. Proper attic ventilation extends the life of a roof and reduces problems because it minimizes the temperature differential between the attic and the air outside. Proper ventilation will remove moisture and heat from the attic.

Trapped heat and moisture can raise energy costs, cause ice dams, and damage roof system components as well as structural and personal items located inside the attic where temperatures can easily reach 150° F (65° C). Condensation that forms inside attics can be caused by the use of washing machines, dish washers, bath tubs, showers, and tumble driers unless these items are properly ventilated through the roof. In some cases the condensation can be bad enough to be mistaken for a roof leak.

  • Sumps between rafters (deck deflection) can happen because after awhile (sometimes several years, sometimes only a couple years), a plywood roof deck can warp or deteriorate and become spongy and dangerous to walk on. This occurs because one side of plywood decking needs be able to "breathe" by being exposed to circulating air.

    The adhesives used in the plywood can deteriorate or Dry Rot can occur because of condensation.

  • Water vapor will condense first on anything metal inside the attic; this will eventually cause the metal to rust. Heads can rust off nails, metal plumbing straps or straps holding HVAC ducting can rust in two causing the ducting to crash down on top of the ceiling joists or through a suspended ceiling. This problem is more common in humid climates.

  • In colder climates – generally where the average January temperature is 32° F (0° C) or colder – high inside humidity (40% or greater) combined with low outside temperatures can cause frost to form on the bottom of the roof deck. See Dry Rot in the glossary.

  • Insulation can trap moisture which will reduce the R-value of the insulation and create a nice environment for the propagation of certain molds, spores, and fungi which will also cause problems. See Dry Rot in the glossary.

  • There is also the problem of mildew which is both damaging and can cause health problems.

  • The roof system itself will deteriorate prematurely.

  • Cooling units will need to be serviced or replaced prematurely because of excessive use.

  • Ice Dams - ice dams are the result of melting snow continually refreezing at the roof perimeter and then backing up under the shingles and causing leaks. Proper ventilation used in conjunction with heavy insulation and an air barrier can create a Cold Roof Assembly which will help eliminate ice dams.

There are many types of attic vents available today. There are static vents, power vents, ridge vents, turbine vents, soffit / cornice vents, gable vents, starter vents, and cupola vents. These all come in a wide variety of sizes, styles and shapes. Some will ventilate better than others depending on the roof configuration, attic size, climate, etc.

To properly ventilate an attic, two types of vents are needed. Intake vents, which are located at the downslope edge of the roof (a.k.a. eaves) and allow fresh air into the attic; and exhaust vents, which are located near or on the ridge line of the roof and allow air to leave the attic. The use of an exhaust vent in conjunction with an intake vent uses the natural forces of wind pressure and thermal effect, collectively known as the Stack Effect, to ventilate the attic space. HINT: Make sure your attic insulation doesn't block the intake vents. If necessary, use baffles to keep the insulation back from it.

A vent’s effectiveness is measured by its Net Free Vent Area. The Net Free Vent Area is the portion of the opening in the vent that actually ventilates. For instance, a vent can have an opening that measures 12 inches by 12 inches; this would appear as if it would yield 144 square inches of ventilation area. Because of louvers, an insect screen, or some other type of blockage, the actual ventilation area could be as little as 40 percent of that, yielding about 58 square inches of ventilation area. This 58 square inches is what’s known as the Net Free Vent Area, and is the amount that should be used when calculating how much venting you need.

Calculating how much venting your attic needs is relatively simple. All you need to know is the area of the attic floor. Include the garage, if you have one, and the soffited overhang because heat gets trapped above them, too. A common rule of thumb is the 1/300 rule, which means 1 square foot of net free vent area per 300 square feet of attic floor space. Let’s look at an example. Say you have an 1,800 square foot home with a garage that measures 20 feet by 22 feet. This will yield a total area of 2,240 square feet. You then divide this number by 300.

2,240 ÷300 @ 7.5

This tells us that we need 7.5 square feet of ventilation for the attic. Most attic vents are measured by square inches so we need to convert the 7.5 square feet to square inches. This is done by some simple multiplication. 1 square foot is equal to 144 square inches, so we multiply 7.5 by 144.

7.5 x 144 = 1,080

So we need 1,080 square inches of Net Free Vent Area. Divide this by two and we see that we need 540 square inches of intake ventilation and 540 square inches of exhaust ventilation.

There is always a lot of concern for what the best type of ventilation is. You have already read that you need both intake ventilation and exhaust ventilation installed at an approximate one to one (1:1) ratio. Now remember that the idea behind this is for maximum air circulation. Installing more than 1 square foot of ventilation per 300 square feet of attic floor space will not hurt anything – it’s a general guideline and code requirement in some areas. Most roofing professionals will agree that the best type of ventilation is continuous soffit and ridge ventilation. If a continuous exhaust vent and an equal or slightly greater amount of intake vent is installed, then the attic will be ventilated for its entire length.
There are several common misconceptions about attic ventilation. One is that many people think that if they have only power vents or turbine vents working near the ridgeline, then their attic is properly ventilated. Remember that in order for an exhaust vent to properly function, it has to have intake vents working with it. If there are no intake vents, then air has to enter somewhere so it will enter through some exhaust vents and exit through others. The result is circulation of only the air immediately surrounding the vents or in between the vents.

This picture shows what happens if you have no intake vents along the eaves. Air will circulate only between the exhaust vents leaving the remainder of the attic space unventilated.

Another common misconception is "more is better". Many people think that they can improve ventilation of their attic by installing vents all throughout the roof surface. What they don’t know is this causes a Ventilation Short Circuit. For instance, let’s say that Fred has a full soffit and ridge vent system installed. But thinking that more is better, Fred decides to install a couple of vents about halfway up the slope. Instead of improving his ventilation, he has now hampered it because air is now exiting out the vents in the middle of the roof, before it reaches the ridge leaving the attic partially unvented. Depending on wind pressure, air will also be taken in at the intermediate vents reducing the intake at the eaves where it should be.

There is also the problem of weather infiltration. Wind blowing across a roof surface creates a negative air pressure. Nature will automatically try to compensate for it by moving air from a location of higher pressure, such as inside the attic. When the air is then removed from the attic in this manner, it has to be replaced. If the proper intake ventilation isn’t used, then air will be brought into the attic through the exhaust vents and will at times bring moisture with it.