If you’re like most homeowners, you know the importance of insulating and tightening up your house to conserve energy. But what you may not know is that certain areas of  a house need to  breathe.

One area in particular is the attic, which requires proper ventilation not only to maintain comfort below, but also to keep the very roof over your head solid and secure!

There are two particular villains that proper ventilation will fend off. These are heat and humidity.

attic ventialtion


Heat comes from the sun, and in summer a poorly ventilated attic can reach temperatures as high as 150 F.—which means that even with insulation in the attic floor, the rooms below will be hotter than is necessary, therefore less comfortable, and more expensive to air condition. Excess heat can also shorten the life of some roofing materials.


Humidity comes primarily from within the house, drifting upward from showers, unvented clothes dryers, humidifiers and kitchen ranges. It also comes from other, not so obvious sources. The very act of breathing expels water into the atmosphere-at the rate of 1/2 pint per hour for the average family of four! Mopping the kitchen floor (about 150 square feet) releases 4 1/ pints of water, and washing the dinner dishes-1/2 pint. A 2 windblown rain can also cause water to enter and evaporate into the attic area through roof leaks. During cold weather, water vapour may condense in various areas of an insufficiently ventilated attic, seeping into wooden rafters or roof sheathing and rotting them. Moisture in the attic area can cause roof shingles to buckle and insulation to lose its effectiveness. It also creates an environment that is conducive to mildew.

In short, you need proper attic ventilation to help:

  • Prevent structural damage caused by moisture
  • Increase the life of the roofing material
  • Reduce energy consumption
  • Enhance the comfort level of the rooms below the attic

Now that you know why it is crucial to maintain adequate ventilation in your attic, how do you do it? There are a variety of ways, and the right one will depend on the style and structure of your own roof.

Check  Your Attic

First, it is wise to determine whether or not the existing ventilation is adequate. You can do this by placing a thermometer in the attic on a warm, windless day to see if the temperature that is being maintained is more than 10o to 15o F warmer than the outside temperature. If it is, then more ventilation is needed.

In addition, if periodic inspections during the winter reveal any signs of condensation—such as moisture, rot or mildew—then improved ventilation would be helpful.

Regardless of roof geometry, there is usually a small amount of built-in ventilation where the roof and wall structures meet. That slight space allows for some amount of air circulation through the attic. But it is not enough.

In addition to the free flow of air, insulation plays a key role in proper attic ventilation. In fact, the ideal attic has:

  • a gap-free layer of insulation on the floor to protect the house below from heat gain or loss
  • a vapour barrier under the insulation next to the warm ceiling below to stop moisture from rising into the attic
  • enough open, vented spaces, properly located, to allow air to pass in and out freely
  • a minimum of 1½” between the insulation and the roof sheathing.

Rule of Thumb:

The requirements for proper attic ventilation may vary greatly, depending upon the part of the country in which the home is located as well as the conditions at the home site, such as exposure to the sun, shade and atmospheric humidity. Nevertheless, the general formula is based on the length and width of the attic itself.

No vapour barrier under insulation:

One square foot of free vent area for each 150 sq. ft. of attic floor.

With vapour barrier:

1/2 square foot free vent area for each 150 sq. ft. of attic floor. Eg. 1200 sq. ft. attic floor = 8 sq. ft. free vent area or 4 sq. ft. if there is a vapour barrier.

You don’t need to crawl around the attic to determine the square footage of the floor. Simply measure the length and width of the house itself and multiply them to get the necessary square footage figure.

The next question is: What determines free vent area? It’s not as simple as the size of the opening the vent sits in. Louvres and screening—which are necessary to keep out rain, insects and so on—decrease the amount of air that can pass through, and that must be taken into account in calculating adequate ventilation. Therefore, if louvres and screen are present, multiply the needed vent area by three.

Helpful Hint: If you’re buying new vents, most now on the market are labelled with the free vent area they provide.

Out With The Old Air, In With The  New

Once you’ve determined your ideal total free vent area, then you need to divide it roughly in half for:

  • Inlet vents, which should be located under the eaves (called the “soffit” area) or low on the roof
  • Outlet vents, which should be located at the roof ridge, in gables or cupolas, or otherwise near the top of the

Since hot air rises, this type of system takes advantage of a natural “Chimney effect” and air movement will be created through the attic, even when there is no wind. (Wind will cause an even greater movement of air.)

The “ridge and soffit” vent combination can be applied to the majority of roofs in this country, which are gable style, or pitched. In most cases, houses of this type feature louvred openings in the end walls of the roofs; but unless these vents are perpendicular to the predominant breezes, their effectiveness is limited.

Roofs with Shallow Pitches

Flat roofs and roofs with shallow pitches are another story. It can be difficult to ventilate properly the cavity beneath flat roofs. If there are overhangs, continuous soffit venting can be employed. In some cases, louvres placed in the fascia board will do the job.

Another important point about flat or slightly pitched roofs—since there is very little air space between the lower ceiling and the underside of the roof structure, your insulation should be at least 1½” thinner than the roof cavity. Otherwise, water condensed from moist house air can be trapped in the insulation, making it useless, and allowing rot and mildew to get a foothold.

Some Final Points Regarding Attic Ventilation

To maintain the most efficient attic ventilation, make sure that vents from your bath, kitchen and laundry are not routed to the attic, but instead go directly to the  outside.

Never block off your attic ventilation in winter, since moisture generated inside the house that rises to the attic can cause more problems in winter than in summer. With proper insulation between the attic floor and ceiling below, the ventilation will not lower the temperature in the house.