Keep Heat Under Control With Proper Attic Ventilation

Fine, Mr. Attic.  Breathe in deeply... breathe out.  Now, cough...

Since the energy crisis of the '70's, the climate of our attics has taken on more importance as we have insulated, weather-stripped and sealed our homes into virtual mausoleums.  The idea of actually allowing a part of our home to be cold... and to do it intentionally... can give one the chills!  It seems a contradiction to all the brainwashing we have received regarding the absolute need for an energy efficient home.   But your attic is unlike other areas of your home, performs different functions and, to do its job, must be allowed to... breathe!!

Your attic is your friend...  understand it, respect it, appreciate it and care for it!

Think of your attic as a buffer zone.  Outside in the "real world" meet Mother Nature in all her glory, with temperature swings and moody weather.  Inside your home, you have relatively constant temperature and humidity hopefully matching the health needs of the folks inside.  The attic allows for a transition between the wild and the wonderful through the cooperation of your roof, insulation, vapor barriers and ventilation.

  • The attic roof, of course, keeps rain and melting snow from ruining your Persian carpets.
  • Insulation in the attic slows down the movement of heat up from your living space, trying its darndest to keep the inside of your home warm or cool regardless of the attic temperature. 

  • Vapor barriers in the attic keep moisture rising from your living space away from the insulation.  They are needed because damp insulation loses much of its value. Vapor barriers can take the form of plastic sheets installed on the attic floor (under the insulation), a built-in paper or aluminum foil facing on the insulation itself (always installed towards the living space), or in the form of special vapor barrier paints (See article on insulation for more information). 

  • Sufficient ventilation helps to keep attic temperature and moisture at their optimum levels for the season.  In winter, the perfect attic temperature is the outside temperature... cold and dry.  In summer, the ideal is to have lots of air movement so the attic is as cool as possible... without adding refrigeration!

Proper attic ventilation has year-round benefits...

The obvious benefit of increased ventilation in the warmer months is a lower attic temperature, which can decrease cooling costs in the home.  This is true regardless of how much insulation there is in the attic.  In fact, attic insulation can actually cause increases in temperature in the home as the insulation holds the heat long after the sun is gone, continuing to transfer it slowly through the ceilings into your living space.  Though we think of attic insulation as a barrier to the movement of heat, the oppressive heat of an underventilated attic can make your insulation a "fair weather" friend.

In the warmer months, a not-so-obvious effect of lower attic temperatures is increased roof shingle life.   Though asphalt shingles are designed to take the abuse of the sun for many years, they are subjected to greater temperatures when the roof decking is allowed to overheat... which is just what happens in an underventilated attic.

In the cooler months, moisture is the most serious concern.  Even with the use of vapor barriers to keep moisture from entering the attic, there are air leaks around ceiling light fixtures, bathroom exhaust fans,  access panels and fold-down attic stairways that allow unwanted moisture into the attic.  Just going into the attic to bring down that old Monopoly game can increase the moisture level in the attic a hundred fold.

Moisture condensing on the framing members and the inside of the roof deck can lead to the growth of mold, mildew and rot in the roof deck and framing.  And for those of you who use the attic area for storage, watch those roofing nails and any metal reinforcements!  Have you ever noticed that items in the attic seem to have evidence of slight water drips, but there doesn't seem to be a roof leak?  Glance at the business end of the roof nails sticking through the roof deck.  Are they rusty... or is the wood around them stained?  If so, you almost definitely have an attic moisture problem.  Moisture from the living area is entering the attic and freezing on the nails.  When the attic warms slightly, the ice melts and drips onto your stuff.  Mystery solved!

As if that wasn't enough, relative warm and cold spots on the inside of the roof deck can lead to the nightmare of ice dams and their associated roof leaks when the roof is snow-covered (see article on ice dams for more information).

Understanding non-mechanical attic venting...

One common type of vent, especially in homes built prior to 1980, is the gable vent, which is a louvered and screened vent located at or near the peak in the sidewall of the attic.   Gable vents come in different sizes, materials, and shapes... triangular, rectangular, wood, plastic or metal.  They offer a reasonable amount of ventilation in the upper-most areas of the attic but they do not produce uniform temperatures throughout the attic, leading to the formation of "hot spots" of overheated motionless air.

By far the best non-mechanical venting system is the combination of soffit vents and ridge vents.  This type of ventilation requires a specific roof design with an overhanging area at the lower edge of the roof called a soffit.  Vents are installed into the underside of the soffit overhang to allow air to move into the attic.  The graphic shows a continuous soffit vent which runs the entire length of the soffit.

To complement the soffit vents, another vent is installed at the peak of the roof, called a ridge vent.  This is a screened replacement for the uppermost shingles bridging the peak of the roof, allowing air to flow in or out of the attic along the entire peak.  The system is devilishly simple in function yet effective... warm air in the attic rises and exits through the ridge vents to be replaced by cooler air entering the soffit vents.  Because the path of the cool air is along the underside of the roof deck, it provides uniform air movement and the best possible ventilating action.

Living in an imperfectly vented  world...

Oh if only the world was perfect... but it's not.  And if only all the builders took attic ventilation seriously years ago.  Dream on!  They didn't.  As usual, we all have to make compromises and "make do" with what we have.  Here are a few suggestions to help you with some of these imperfect situations...

  • If your home has gable vents but unvented soffits, the installation of soffit venting will be a great improvement over gable vents alone.  Though you might be able to install continuous soffit vents, it is much easier for the homeowner to install circular aluminum vents.  Just bore the right size hole and press them into place!  They are available in sizes from 2" up to over 16".  Just make sure there is no insulation blocking the inside opening to the soffit, or the vents will be useless.  By the way, the addition of a ridge vent would make this retrofitting perfect... see below. (Vent louver pictured left manufactured by Seiho International.)
  • Installation of a vent or vents in the lower sidewall of the attic can also increase the air flow to a gable or ridge vent.  Don't use a gable-type vent in this low location... it's not too aesthetically pleasing.  A few circular aluminum vents painted to match the house are much less obvious.

  • Installation of a ridge vent in a home with gable vents will dramatically increase the ventilation of the attic, though not as much as if soffit vents were installed. However, if your moisture problem is not extreme, it may give you just enough ventilation to avoid more drastic measures, such as a powered ventilator, discussed in the next section.

  • Unfortunately, installation of soffits onto an existing house is not a small task.  If you just simply must have them, I suggest pricing out a new house first!

  • Do not allow your attic insulation to come in contact with any part of the roof, or to cover your soffits.

  • If you use your attic for storage, be sure to leave space around your vents for air movement.

Continue to Part 2 - Mechanical attic ventilation...

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