Attic Ventilation
by Nick Gromicko and Rob London
Perhaps the most misunderstood element of home maintenance is attic ventilation. Most people feel gable vents are adequate. They also believe it’s a good idea to close them off during the winter to save heat. And many home owners believe the soffits should be covered with insulation. They feel that seeing light in the attic is a bad idea. “It will let in insects,” is something I hear often. All of these beliefs are wrong dew point temperature and condensation occurs. The dew point is always a temperature. It is the point at which water vapor changes state and becomes a liquid and it is always contingent upon two variables—temperature and humidity.
You now have a wet attic. There are some obvious telltale signs that the home has a moisture problem. Rust forms on nails at the underside of the roof deck. There will also be stains on the floor of the attic from the nails dripping. When I was in the roofing business, home owners would swear this was evidence that “every nail in the attic was leaking.” The stains are the result of the moisture dripping off the nails. You may actually see small icicles hanging off the nails in winter. These stains are the result of excess moisture in the attic. A ridge vent will surely help, but if you have these stains, you have to take a look at moisture sources in the house.
Venting is not enough. Dealing with the excess moisture is often recommended. In reality, most homes are about 70 degrees in the winter, and attics are much colder than 47 degrees. If the air is cold and moving, the water vapor continues to move with the cold air and vents out through the ridge vent. Now if you close off the vents in the attic to save the heat that is now worthless, you trap the moisture in the attic and rot out your attic. You want nice even-balanced, continuously-working ventilation to carry out the moisture. It should be working round the clock and not running off electricity. Continuous soffit and ridge venting will give you the best defense against moisture damage in your attic. Keep in mind, there are other variables with attic moisture. If the home has more moisture than the system can vent out, you may still have moisture accumulate in the attic. A humidifier or a wet basement or crawl space, are a few examples of conditions that can put too much moisture in the home and create problems. Next, let’s take a look at ventilation in the summer.
We ventilate in the summer to let out heat, AND reduce the surface temperature of the roof. The most efficient form of attic ventilation in my opinion is continuous ridge venting, and continuous soffit venting as stated above. This will allow the most air flow over the underside of the roof. A well ventilated attic can help to keep your roof cooler. Heat is the enemy of your roof. I have recorded temperatures in excess of 160 degrees on the surface of the roof. A well-ventilated attic will help reduce the temperature of the shingles. Studies have shown you can reduce the surface temperature of the roof by 20 degrees by having proper attic ventilation. Another thing that happens with a well ventilated attic is it cools down better at night. The attic builds up heat during the day. If the attic is not properly ventilated the heat just sits there at night. The next morning comes and the attic is already hot. It has a running start on building up heat the next day. If it’s properly ventilated, it will cool at night. This cooling will reduce the heat on the 2nd floor and in the attic.
A well-ventilated attic should be close to the outside air temperature just before dawn in the summer. A poorly ventilated attic stays hot right through the night.
We all are familiar with the concept of heat rising. Warm air is thinner than cold air and therefore lighter. The heated air rises and takes the heat with it. This is a process known as convection. Another form of heat transfer that occurs in the attic is thermal radiation, which results from the sun’s rays bearing down on roof shingles. A lot of this heat raises the temperature of the underside of the roof and the shingles and some of it reflects down into the living area. The radiant heat from the attic raises the temperature of the 2nd floor generally about 5 degrees warmer than the first floor. If the roof is poorly ventilated, the 2nd floor may be 12 to 15 degrees hotter. Studies have shown the attic temperature to be as much as 25 degrees hotter in a poorly ventilated attic compared to a well-ventilated one.
If you add a ridge vent to a home that did not previously have one, it is recommended that all other vents, except for those at the soffits, be closed off. Openings such as those at gable vents and roof fans will allow air to enter the attic in a counter-productive manner and short-change the soffit venting. Rather than the air entering at the eaves, it will enter through the other openings and the attic won’t get the full benefit of the balanced system.
So what is proper attic ventilation? Building codes and shingle manufacturers have accepted a formula of 1/150th of the flat attic space to be an appropriate degree of ventilation. The air flow should be balanced equally between high and low areas, or eave
and ridge as discussed. This translates to about one square inch of ventilation for one square foot of attic space. This formula was adopted during World War II, although not based on scientific study, and remains the present-day standard of the construction industry. But it is not always easily attained. There are hip roofs, roofs with dormers, and a variety of other conditions that can make this formula difficult to achieve. A variety of products are on the market that can address unusual conditions. If you have a home where you don’t have an open and easily ventilated attic, you can do your own research. Air Vent, Inc. makes several products to address specific conditions. Their website is www.airvent.com. Another company that has specific materials for such applications is DCI, Inc. Their phone number is 1-800-622-4455.
