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How To Properly Ventilate Your Attic

How to Properly Ventilate Your Attic

ice-damProper attic ventilation is an important part of roofing. Attic temperatures can easily reach 150° F (65° C) on summer days, causing both heat and moisture to become trapped. This will not only raise your energy costs and cause ice dams in the winter, but will ultimately damage your roof.  By minimizing the temperature differential between the attic and air outside, proper attic ventilation will remove moisture and heat from the attic, thereby reducing the chance for potentially irreversible damage and extending the life of your roof exponentially. In this post we’ll take a look at the all of the potential problems you’ll encounter and the steps you can take to properly ventilate your attic.

3 Problems Caused by Improper Ventilation

1). Condensation
Condensation is by far one of the leading causes of roof damage due to improper ventilation. Condensation that forms inside attics can be caused by washing machines, dish washers, bath tubs, showers, etc. 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.

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

2). Deck Deflection
After a number of years, a plywood roof deck can warp or deteriorate and become spongy and dangerous to walk on causing humps between rafters, otherwise known as deck deflection. This occurs because one side of plywood decking needs to be able to “breathe” by being exposed to circulating air.The adhesives used in the plywood can deteriorate or condensation can cause dry rot. This is a potentially hazardous problem for your roof.

3). 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.


How to Properly Ventilate Your Attic

To properly ventilate an attic, two types of vents are needed: 

1). Intake vents, which are located at the down slope edge of the roof (aka eaves) and allow fresh air into the attic.
2). 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.


How Much Venting Do I 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 square feet

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 square inches
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.


Misconceptions About Ventilation

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 ridge line, 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. 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.

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 (snow or rain) with it.

Which Type of Ventilation is the Best?

There is always a lot of concern for what the best type of ventilation is. When determining ventilation, you only need to remember the following: Intake and exhaust ventilation should be installed at an approximate one to one (1:1) ratio. More at the eaves is better if it can be attained. The 1/300 rule – 1 square foot of attic ventilation (Net Free Vent Area) per 300 square feet of attic floor space.


FHA Guidelines recommend the 1/300 rule. Some building codes require the 1/150 rule. When using the example above, adjust your numbers accordingly.

The Home Ventilating Institute (HVI) recommends a 60/40 ratio where 60% of the total ventilation is for intake and the remaining 40% is for exhaust. For requirements in your area, consult your local building code authority.

So is your attic properly ventilated? As we head into warmer, rainier months, it might be time to check your attic to ensure that it is ventilated as it should be.

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