Attic Ventilation Basics
The concept of attic ventilation is pretty simple, if it's explained correctly. My goal here is to explain how attic ventilation works. And if I do this right, you'll also learn how the ventilation of your attic affects a roof structure, roof covering, and the comfort of those living in the house. A few simple rules of thumb can prolong the life of your shingles, roof sheathing, and prevent disastrous moisture and mold problems in the attic. Misconceptions about the air flow in your attic can lead to thousands of dollars in damage and repair costs.
A Different Animal
First, think of your attic and roof as a separate entity from your home. Your home is a heated (conditioned) area that is designed to keep air IN. We insulate, moisture-proof and throw up every barrier to heat and cold that we can muster. The exchange of air is discouraged in everything we do in the construction process...taping of fiberglass insulation seams, installation of Tyvek wrap, plastic moisture barriers on the floor of crawl spaces, etc.
Your attic area is just the opposite, in many ways. Your living space effectively ENDS at the top of your upstairs ceiling. Beyond that, the structure is not living space, but strictly utilitarian. There is a "buffer" area that we call the attic between the living space and the roof structure. If your home doesn't have an open area that can be called an attic, then it likely has a cathedral ceiling or a finished attic that is used for living space. Above the attic, we have a structure that is designed to keep moisture out: the roof. If you think of your attic and roof in these terms, you're miles ahead of most people's thinking.
Many of people make the mistake of TREATING their attic as if it were living space. By that I mean, they think it should be as air-tight as their living room. Not so. Attics should be designed to breathe, and breathe deeply.
Air Flow is Key
As a former Home Inspector, I've seen the bewildered look in my clients' eyes as I explain that their attic is smothering. No air movement, no fresh air, and nothing to cool the roof surface to outdoor temperature. I can look at the roof surface of a home and tell you whether there is mold in the attic, without setting foot up there. Clues like cracked or curled shingles, absence of ventilated soffit, gable vents or ridge vents adds up to no air flow, and big problems.
In a perfect world, the insulation above your living space (ceiling / floor of attic) keeps heat in. Of course, in warmer weather, you would hope that it would keep the COOL air in. Either way, beyond that point, you should let the outdoor temperature take over. Your attic should be at or below the outdoor temperature.
To accomplish this, your attic needs a lot of air exchange. Perforated soffit is designed to allow air to flow in, and upward. In case you're not familiar with the term, soffit is the material on the underside of the eaves (overhang) of your roof. After entering the soffit, air needs to flow upward and exit through gable vents or ridge vents, cooling the roof structure and roof covering to outdoor temperature. Again, terminology....gable vents are vents that are installed on the "ends" of the roof rise, and ridge vents are installed at the peak of the roof.
What happens WITHOUT enough air flow?
Well, a lot of bad things. First, the roof surface will heat up in warm weather. This can cause asphalt and fiberglass shingles to crack and curl. Shingles can deteriorate in as little as 2 years without proper attic ventilation.
If air isn't allowed to escape the attic, the moisture that it contains will eventually condense onto attic surfaces as the air temperature drops in the evening. Moisture and warmth are the perfect recipe for mold, and mold is the most opportunistic organism there is. Mold spores can lie dormant for many years, waiting for moisture. All it takes is one instance of poor ventilation, and you'll be growing a Chia Pet in your attic.
Ice Dams
Another result of poor attic ventilation is ice dams. Of course, this is a problem normally seen in northern climates. Attics with poor ventilation allow the buildup of heat in the attic, usually from heat escaping the living space. The heated attic air warms the roof surface, melting snow on the roof. As the snowmelt runs down the roof, the water is cooled at the eaves where it freezes once again. After a few days of this, the ice at the eaves forms a barrier, and any further snowmelt collects behind the barrier. Eventually, this allows water to back up under the shingles, damaging the roof sheathing, attic insulation, and possibly causing a drip into the living space. Starting to see how important attic ventilation can be?
Prevention
Preventing a financial attic or roof catastrophe is pretty easy if you follow a few simple rules:
1: Make sure there is air flow from the eaves. This is normally the point where air ENTERS your attic. If the soffit of your home isn't perforated to allow air movement, either replace it with perforated soffit, or install round air vents under the eaves to allow air flow.
2: Make sure the air can exit. Either gable vents or ridge vents should be present, or ideally...both. If you have gable vents, be sure that they're unobstructed. If your home lacks a perforated ridge vent along the peak, install ridge ventilation so that warmed air can escape at the highest point of the attic. This can be done without replacing much of the roof covering in most cases.
3: Check your attic insulation. In frigid areas, experts recommend 12' of fiberglass insulation between the living space and attic, giving you an R-value of 38. Sounds excessive, but it isn't. The more heat you can prevent from escaping into your attic, the healthier your attic and roof will be.
Be sure that the FLOOR of your attic has uniform coverage: it doesn't help you a bit to have a foot of insulation if an entire batt of insulation has been rolled back, allowing warm air to enter the attic.
Insulation should never be compressed into a space, but the edges should fit snugly between the ceiling joists. If your home has a cathedral ceiling or a finished attic space, the same rules apply. Air should be "tunneled" from the eaves, behind the insulation that is installed between the roof rafters, and allowed to exit at the peak or at gable vents. Plastic, cardboard, and styrofoam baffles are available at home stores, which can be installed between the roof sheathing and insulation to allow unobstructed air flow from the eaves to the unheated space.
Hopefully this article has helped you gain a better understanding of how your attic area is designed to work. Feel free to email me if you're experiencing an attic issue. Knowledge is contagious...let's spread it.
Attic Ventilation
