Proper Roof Ventilation

ATTIC VENTILATION FOR HOMES

B. R. Stewart

Agricultural engineer-environmental control ~ Agricultural Extension Service ~ Texas A&M University

Ventilation of the home attic is important for two reasons. During the summer, excess heat that builds up in the attic during the day results in high energy costs for cooling. Also, moisture produced within the home may move into the attic if ceiling vapor barriers are not used. If this moisture is not exhausted from the attic it can condense and cause insulation and construction materials to deteriorate. Thus, temperature and moisture control are the major reasons for providing attic ventilation.

Ventilation Quantities

How much attic ventilation is required to provide proper temperature and moisture control? A number of studies sponsored by federal energy funds are under way to look at ventilation rates and methods. However, several studies previously made can help with this decision. The maximum ventilation rate is required to remove heat during the summer cooling months. Attics can reach temperatures of 150 to 160 degrees F during a summer day, although outside air temperatures are only 95 to 97 degrees F. The cooling load for a home air conditioner depends on the difference in temperature between the inside and outside air, and reduction of attic temperatures from 155 degrees to 105 degrees F will result in a significant reduction in cooling load. In a home with poor ceiling insulation, heat movement through ceilings may account for 30 percent or more of the total cooling cost. With a well-insulated ceiling, this source of heat may account for only 12 to 15 percent of the total cooling cost. Thus, high attic ventilation rates are most important for poorly insulated ceilings. A poorly insulated ceiling is one whose R rating is less than 14 or one with fewer than 4 inches of fiberglass, rockwool or cellulose insulation.
Attic temperature depends on the amount of solar radiation, construction details and the rate of ventilation. Calculations indicate that on a July day in Texas, a ventilation rate of one air change per minute for a typical attic using 95-degree F air will lower the peak attic temperature to about 101 degrees F. Providing half air change per minute will lower the temperature to about 106 degrees F. Thus, the first half change per minute is most effective and a doubling of this rate only achieves about 5 degrees F additional cooling. Studies indicate that further increases in ventilation are not effective in significantly reducing attic temperatures.
Winter attic ventilation must be sufficient to remove moisture vapor moving from the living space to the attic. In general, ventilation adequate for summer cooling is more than adequate for winter ventilation. Winter rates need not be more than about a tenth of the summer rate.
Calculate the required summer ventilation rate by determining the volume of attic space and dividing by 2. This will be the cfm (cubic feet per minute) of ventilation air needed. The volume is determined approximately for a rectangular house by multiplying the height from the ceiling to the peak/ridge (H) times the width of the house (W) times the length (L) and dividing by 2 -- ( H x W x L / 2 ). For a gable roof, this will be reasonably accurate. For a hip roof house, the volume will be overestimated but adequate.

Ventilation Methods

Attic ventilation can be accomplished by gravity ventilators, wind assisted ventilators or power ventilator.. Regardless of the method used, the purpose is to provide uniform ventilation of the attic for proper temperature and moisture control.
Natural ventilation is the most common and energy-efficient method of achieving attic temperature and moisture control. This method takes advantage of two principles. First, as air is heated it becomes less dense and rises. Second, wind movement over and around a home creates areas of high and low pressure. If a space has high air outlets in conjunction with low inlets, ventilation occurs as the air within the space is heated. The greater the vertical distance between the outlet and inlet, the greater the ventilation rate will be. Thus, roof outlets should be at or very near the ridge, and inlets should be under the roof overhang or near the ceiling line.
Ventilation caused by wind pressure differences requires less vent area to achieve the same ventilation rate as ventilation by gravity. One problem with wind ventilation is that the areas of high and low pressure change with wind direction, causing difficulty in locating inlets and outlets so that ventilation will take place regardless of wind direction. The best system is one in which the outlet is near the ridge and the inlets are in the soffit area. The hip roof is best suited to this system, since there is soffit area on all sides of the house. Outlet vents can be provided by roof louvers, gable end louvers, turbine ventilators or continuous ridge vents. These should be constructed so that rain and snow cannot enter. Water leakage is more likely with the turbine ventilator and gable end louver than with fixed roof or ridge ventilators.
Inlet vents may be of the manufactured type, either slotted or perforated, and installed as individual units or in a continuous strip. The slotted opening seems to resist clogging by fibers and dust better than the perforated opening. Screen wire is sometimes used for soffit vents, but again clogging is a problem. In some localities frequent cleaning may be necessary with either type.

Providing for Natural Ventilation

Natural ventilation caused by a chimney effect or by wind movement is the most economical ventilation method. The quantity of ventilation air depends on opening size, temperature rise and wind movement. Tests have shown that effective natural ventilation can be achieved by providing inlet and outlet open vent areas of approximately 1 square inch per square foot of attic area, when roof slopes of 3/12 to 5/12 are used. This vent area should be the net open area rather than gross vent size. Some prefabricated vent materials may have only 60 percent net open area. Thus; if a vent area of 10.4 square feet is required, the gross vent area would be 10.4 / 0.6 = 17.3 square feet. [National standards appear to require only 1/2 this amount of open vent area.  The open vent area calculated by these formulas is to be divided roughly equally between inlet and outlet.]
Suppose a 1,500-square-foot home requires the above soffit vent area. If the total soffit length of the home is 100 feet, the vent width for a continuous soffit vent would be (17.3 x 12) / 100 = 2.1 inches. Ventilators should be purchased on the basis of net ventilation opening.
Roof overhangs of 12 inches or more provide ample space for inlet (soffit) vents. Outlet vents should have the same net area as inlet vents. This is easily provided by continuous ridge vents or individual roof vents for gable-roofed homes. The roof ridge length may not be long enough on a hip roof to use a continuous ridge ventilator. In this case, several individual roof vents, well spaced near the ridge, should be used.

Power Ventilation

[For a safety and other reasons some authorities discourage the use of power ventilation. -- jim]

Power ventilation can be accomplished in two ways. In homes not mechanically cooled (air conditioned) the temperature can be controlled to some extent by the use of attic fans. These fans are usually ceiling mounted in a central hallway so that outside air is pulled through open windows and exhausted through the attic. Sufficient outlets must be installed in the attic to exhaust the air without creating high pressures against which the fan must operate. The net area of attic outlets should be 1/800th of the rated fan volume in cfm and 1/8 inch static pressure. The outlets should be distributed uniformly.
Air conditioned homes can use power attic ventilators by installing an exhaust fan through the roof or in the gable. Inlets for the ventilating air should be at the soffit, or the opposite gable, when no roof overhang exists.
Power ventilators have the advantage of providing good ventilation even when there is no wind. They also provide limited attic temperature control when installed in conjunction with a thermostat. For well-insulated ceilings (i.e. insulation levels of R-19 or above) it is doubtful that a power ventilation can be justified economically. 
However, there are some attics which cannot be ventilated by gravity or wind-assisted methods, and in these the power vent is necessary for moisture and temperature control.
For areas of Texas where blowing dust and sand are a problem, consideration should be given to the use of a power ventilator with automatic or motorized inlet louvers which close when the fan is not in operation. Ventilation can be shut off during a dust storm to prevent the buildup of dust and sand in the attic.
 






















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What Should I Expect After A Hail Storm?

What Should I Expect After A Hail  Storm?

Written by Administrator

Wednesday, 28 January 2009 23:20

Brought to you by:  //American Siding and Roofing, llc. Here is a list of everything else you should know, how to deal with things, and what to expect. The severity of these things will depend on the size of the storm and your location. Things you’ll see at home: Over the next several months you will most likely have dozens of door hangers, mailers, calls, and knocks at the door with people soliciting to repair your home. Some will be legitimate professional companies who are marketing the storm, while others and the majority will be broke storm chasers from other cities, janitors, bartenders, mechanics, painters, and people from all other professions who are getting into construction as a way to cash in on the storm. The best thing to do to avoid this is to put a USRCT sign in your front yard and put a do not silicate sign on your door. If they still knock, just politely tell them that you are being taking care of and shut the door. Do not let them talk. because they will not stop. Beware of gimmick: We will pay your deductable, free upgrades, $1,000 off. All of this is defrauding your insurance company and against the law. You don’t want to get mixed up in a situation like that. You will see billboards going up everywhere advertising storm damage, cars with ladders on top and want ads in every newspaper. Understand why people pitch locals: After about two months into the storm, everyone will start to pitch “use a local company” “I’m local” What happens is that the insurance company convince people of this, which in turn promotes the use of that type of marketing. Usually this happens later in the storm because storm chasers start to lease the names of local companies to use the name to compete against other storm chasers. By saying “were local” The truth is that the majority of local companies are either not local at start or new businesses popping up that are trying to benefit from the storm. After all how many local roofing, siding, or window companies did you have before the storm? Look for companies driving nicer vehicles: Usually you can tell the credit status of a company by what they drive, as companies who pay their bills usually drive nicer vehicles then companies that don’t. Look for vehicles that have their company name on them in permanent lettering: This assures you that they did not just run out and print a magnetic sign to instantly become a contractor. Do not try to profit from your insurance company: This is something that happens a lot during a Catastrophe storm. People think that they just won the lottery. You didn’t, you are being paid to repair damage on your property. If you use the money for something else or defraud your insurance company two things can happen. You can be guilty of a crime and and/or will not be covered in the event of a future storm. This is important because next time it could be a lot worse, and if your insurance company already paid for work to be done and it wasn’t, they won’t pay again. Beware of local companies who get more work than they can handle: During a storm most local companies who are used to producing 3-5 jobs a week start selling 50-100 per week. This in turn makes it so they do not have the staff in place to manage the jobs. They start hiring new inexperienced worker and quality starts to slip through the cracks. Workmanship gets very poor, and they have a hard time managing everything that is happening. This sometimes results in unpaid labor and/or materials and a possible lien against your home. No matter who you use, make sure they have experience managing the amount of jobs that come along with a weather catastrophe

How Hail is Formed?

How is Hail Formed?

http://www.ucar.edu/communications/factsheets/Hail.html

Brought to you by:///American Siding and Roofing

Nearly everyone welcomes the warm, sunny days of summer. But with summer come thunderstorms, bringing tornadoes, flash floods, and hail. Although tornadoes and flash floods are dramatic by-products of thunderstorms, hail can be far more devastating to property and crops.

Hail is formed in huge cumulonimbus clouds, commonly known as thunderheads. When the ground is heated during the day by the sun, the air close to the ground is heated as well. Hot air, being less dense and therefore lighter than cold air, rises and cools. As it cools, its capacity for holding moisture decreases. When the rising, warm air has cooled so much that it cannot retain all of its moisture, water vapor condenses, forming puffy-looking clouds. The condensing moisture releases heat of its own into the surrounding air, causing the air to rise faster and give up even more moisture.

NCAR scientist Nancy Knight holds a hailstone that fell in Coffeyville, Kansas, in 1970. The largest hailstone ever documented, it weighs 0.75 kilograms (1.67 pounds), and spans 14.4 centimeters (5.67 inches).

Cumulonimbus clouds contain vast amounts of energy in the form of updrafts and downdrafts. These vertical winds can reach speeds over 176 kilometers (110 miles) per hour. Hail grows in the storm cloud's main updraft, where most of the cloud is in the form of "supercooled" water. This is water that remains liquid although its temperature is at or below 0 degrees Celsius (32 degrees Fahrenheit). At temperatures higher than -40 degrees C (-40 degrees F), a supercooled water drop needs something on which to freeze, or it remains liquid. Ice crystals, frozen raindrops, dust, and salt from the ocean are also present in the cloud. On collision, supercooled water will freeze onto any of these hosts, creating new hailstones or enlarging those that already exist.

Cross sections of hailstones often reveal layers, much like those of an onion. These layers are caused by the different rates of accumulation and freezing of supercooled water, as the hailstone forms. When there is a great deal of supercooled liquid in the air through which the hailstone falls, water accumulates faster than it can freeze, so a coat of liquid forms. This becomes a layer of clear ice when it does freeze. When a hailstone falls through air with a smaller amount of liquid, the liquid freezes on contact with the hailstone, forming small air bubbles in the opaque layers. The more supercooled water a hailstone makes contact with, the larger and heavier the stone is likely to become. When the hailstone becomes so heavy that the updraft can no longer support it, it falls from the sky.

Hail falls along paths scientists call hail swaths. These vary from a few square acres to large belts 16 kilometers (10 miles) wide and 160 kilometers (100 miles) long. Swaths can leave hail piled so deep it has to be removed with a snow plow. In Orient, Iowa, in August 1980, hail drifts were reported to be 2 meters (6 feet) deep. On 11 July 1990, softball-sized hail in Denver, Colorado, caused $625 million in property damage, mostly to automobiles and roofs. Forty-seven people at an amusement park were seriously injured when a power failure trapped them on a Ferris wheel and they were battered by softball-sized hail.

The emergence of the Insurance Recovery Specialist has given rise to an entirely new arena- Storm chasers.  If you suspect that you have a Hail Damage Roof contact a BBB reputable roofing company ASAP and have them work to have your insurance claim roof settled timely and in an professional manner.

Hail also does a great deal of damage to crops. U.S. costs run into hundreds of millions of dollars annually. While hailstones have been found weighing as much as 0.75 kilograms (1.67 pounds), even much smaller hail can destroy crops, slicing corn and other plants to ribbons in a matter of minutes. Farmers cope with the hail hazard by purchasing insurance. Illinois farmers lead the United States in crop-hail insurance, spending more than $600 million annually. However, U.S. hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley." Parts of this region average between seven and nine hail days a year.

Corn crop damage caused by hail.

Today, farmers seek monetary compensation for hail damage, but in the past, farmers had no recourse when their crops were destroyed. They were left to their own ingenuity to try to suppress hail. In the 14th century, people in Europe attempted to ward off hail by ringing church bells and firing cannons. Hail cannons were especially famous in the wine-producing regions of Europe during the 19th century, and modern versions of them are still used in parts of Italy.

After World War II, scientists across the world experimented with cloud "seeding" as a means of reducing hail size. In Soviet Georgia, scientists fired silver iodide into thunderclouds from the ground. Such methods supposedly stimulated the formation of large numbers of small hailstones, which would melt before they reached the ground, but comparable experiments performed in Switzerland and the United States did not confirm Soviet theory.

While hail suppression continues to elude scientists, sophisticated radar has been developed that can detect the presence of hail before it falls to the ground. Eventually, warnings may be issued as much as 15 minutes before hail strikes, allowing pilots to avoid threatening air space, people to seek shelter, and property to be protected.

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