The Importance of Attic Ventilation

An extremely important, but often misunderstood part of good home design,is attic ventilation. Proper attic ventilation can save homeowners energy, extend the life of their roofing materials and reduce moisture problems like mold, mildew and other types of water damage.

Since 1942, the Federal Housing Administration has had minimum attic ventilation requirements, and the building codes have included attic ventilation, for almost as long.

So why do we even need to worry about attic ventilation? Unfortunately, some homebuilders have promoted non-ventilated attics to save pennies, and Canada, with its cold weather, has downplayed attic ventilation, causing confusion to the homeowner. Here in Houston, attic ventilation is vitallyimportant.

Here's why; RSI Magazine’s 2005 State of the Industry Report shows that 90% of residential roofing contractors say that they check attic ventilation before re-roofing. And when they do, 66% say the existing attic ventilation is inadequate.

So, if you’re currently contemplating a new roof, there is a greater than 50% chance that your attic is poorly ventilated. This wastes money, energy, and can lead to premature roof failure, water damage and possibly, mold growth.

Why Ventilate?

By ventilating the attic we accomplish two things. One, we move hot air out of the attic during the summer and keep the attic temperature down, which will extend the life of the roof and keep the house cooler. Two, we move moisture that has come from inside the house out of the attic before it can condense, cause mold or other damage.

We can ventilate the attic using simple static vents, wind powered ventilators or motor driven ventilators. The simplest, most common and least expensive are the static vents. These consist of vents in the soffits and vents high on the roof, commonly at the top of the ridge, called ridge vents.

Air is moved by natural convectionup from the soffit vents to the ridge level vents. Additional ventilation takes place due to wind not only pressurizing the attic but also creating a vacuum on the opposite side of the roof from the wind direction. The best of ridge vents contain baffles on the exterior to keep the wind from just blowing through the vents and not providing the extra ventilation.

When powered ventilators, either wind or electricare used, we still need the soffit vents to let the air into the attic for the powered vents to exhaust. The design should allow for up to one air change per minute. More ventilation is generally better than less, and warmer climates, such as Houston, would benefit by the higher ventilation rate.

Most building codes require a minimum of one square foot of net open area for every 150 square feet of attic ‘floor.’ The venting area should be evenly split between the soffit and ridge area vents. Sometimes local building codes will allow half the vent area – when special conditions are met, however Magnum Roofing does not recommend this practice.

To discuss why ventilation works, we will look at what it does during the hot summer and cold winter months.

Summer Months

During the summer, we have high air temperatures and long hours of sunlight, which causes the roof surface to become hot. This heat is then transferred to the attic. If the attic is not ventilated the attic temperature can easily reach 185 degrees, while a properly vented attic will reach about 115 degrees.

Even with insulation, the heat in the attic will flow down into the house, because heat does not follow gravity but rather flows from hot locations to colder locations. The rate of heat flow is related to the temperature difference. Thus if the house temperature was 80 degrees, a vented attic would be 1/3 the amount of heat into the house as a non-vented attic. For example, a 2,000 square foot house in Houston, Texas would save about $225 per year in energy costs between a vented and non-vented attic.

Added to the increased energy cost is shortened life of the air conditioner unit due to additional operational hours.

The added heat in the non-vented attic will cause the roofing material to get hotter as well. Heat causes increased aging rates for organic materials like wood and asphalt, thus shorting their life. This problem is well recognized by the asphalt shingle industry, and many manufactures do not cover shingle damage due to non-vented attics in their warranties.

During the winter months, we have much lower air temperatures and fewer hours of sunlight. These changes in the weather change the role of attic ventilation.

The primary purpose of attic ventilation in winter is to remove excess moisture from the attic. If the moist air that gets into the attic is not removed quickly, water will condense onto the wood surfaces and in the insulation. Water on the wood surfaces will cause it to rot and mold. Water in the insulation will reduce its ability to act as a thermal barrier, increasing utility costs. In addition to damage in the attic from condensed water, the water can also flow into the house acting just like a leak, causing stains and water damage.

Fixing ventilation issues

If your house does not have any ventilation, it can be added. First, Magnum Roofing would need to install the appropriate amount of soffit venting for your house and make sure that the attic insulation does not block the airflow of the soffit vent. Since installing ridge vents or static vents require major roofing work, which should only be done by a roofingprofessional, it may be more economical to install a powered vent near the ridge, as typically only one hole would be needed.

If your experiencing some of the problems discussed above and you have attic vents, it is possible that the attic insulation was incorrectly installed so that it either covers the soffit vents or blocks the air flow from the soffit vent to the attic.

If this is the case, removing or pulling back the insulation from the eave and soffit areas will solve the problem. Either, the homeowner or Magnum Roofing can do this work. There are cardboard or foam attic rafter vents that fit between the rafters and prevent insulation from blocking the air flow from the soffit vents.

In summary, attic ventilation is important for a long lasting house in Houston, especially during the summer.