• Ventilation of Attics for Homeowners

    Posted Feb 17th, 2011 By in Articles With | No Comments

    In the summer attics can get really hot. Even with proper insulation this high attic temperature will beat down into your home, increasing the interior temperature by as much as 30 degrees. This is uncomfortable, causing you to turn up the air conditioner. But the heat also does other, less obvious things. It dries up the shingles and it cooks the wood in your attic. If you were to look in the attic you would probably see the sap seeping out of the rafters (it resembles little amber bubbles).

    At night temperatures drop. The cool of the evening brings condensation. Water vapor is present in all air, whether it is cold or hot. But when the temperature falls and objects cool, the moisture in the air will condense onto these cooler surfaces (think of the dew on the grass in summer or the frost on your roof in winter).

    Moisture is a year round problem, but it is of special concern in the colder months. In the winter, furnace-warmed air circulates through the house. As it moves it collects humidity from the house created by bathing, cooking, laundry, etc. If you have a humidifier on your furnace there is even more water vapor held in the air. This heated, moisture-rich air moves up into the attic. The water condenses on the rafters and roof deck. The wood absorbs much of the moisture, and then mold and mildew develop. The amount of mold growing in today’s well-sealed homes is alarming. Once mold spores attach, they multiply rapidly.

    According to the Environmental Protection Agency, quoted from their publication titled Mold Remediation in Schools & Commercial Buildings:
    All molds have the potential to cause health effects. Molds can produce allergens that can trigger allergic reactions or even asthma attacks in people allergic to mold. Others are known to produce potent toxins and/or irritants. Potential health concerns are an important reason to prevent mold growth and to remediate/clean up any existing indoor mold growth.
    Since mold requires water to grow, it is important to prevent moisture problems in buildings. Moisture problems can have many causes, including uncontrolled humidity. Some moisture problems in buildings have been linked to changes in building construction practices during the 1970s, 80s, and 90s. Some of these changes have resulted in buildings that are tightly sealed, but may lack adequate ventilation, potentially leading to moisture buildup.”

    Proper attic ventilation will remove excess heat and moisture.

    What is attic ventilation? It is simply changing the air in the attic. A typical response to excessive attic heat is to throw in a few more roof vents so the hot air can escape, but this is not enough. Without intake ventilation, the attic will retain its heat. When you breathe you inhale and exhale; you can’t do one without the other. Your attic needs to inhale and exhale, too. Attics breathe in through soffit vents and breathe out through exhaust vents. The hot, moist air in your attic rises, and as it exits via the roof vents, it is replaced by new, cooler air flowing in through the soffit vents. But it can’t breath out without breathing in.

    Which type of ventilation is best for your home?

    Exhaust Vents
    Gable End Vents
    This system actually acts as both intake and exhaust. These need to be large vents, placed in both gable ends of the roof, and need to face into the prevailing wind. The wind blows through, ventilating the attic space. The distance between the vents needs to be relatively short, making the gable end vent ineffective for large homes, or even sprawling ranches. Gable end vents worked well in the simpler homes of yesteryear, but today’s densely built subdivisions and more complicated architecture severely limit their use.

    Can Vents
    The options in exhaust ventilation include can vents, ridge vents and power fans. The most commonly used is the can vent. These are installed about every four feet on the back of the house. They ventilate the attic poorly, allowing only pockets of hot air to escape, and sometimes not working at all because of air pressure and wind. Can vents are often clogged by wasp nests, which further proves that they don’t work as wasps build their nests in protected places. Beware that can vents cannot be used as intake vents. Because of the velocity of the water running off the roof, the vents can leak into the house. Even the manufactures warn against this practice.

    Ridge Vents
    Ridge vents are exactly what their name implies. They are vents that run along the ridge (top) of your roof. The ridge vent has come a long way since its inception. Originally ridge vents were just corrugated plastic strips about an inch thick that were used to keep the cap shingles away from the hole made in the top of the ridge. These older models were ineffective; the wind blew right through them, preventing hot air from escaping. Scientists discovered that if external baffles (like those plastic shields on car/truck hoods that divert the wind) were placed on the ridge vent, the wind could be used to create a negative pressure system in the attic. This enables the wind to affect the air in your attic as it affects the air outside. Now your attic temperature doesn’t have to build to uncomfortable levels before air is exchanged. The different pressure sucks the air out of your attic. Ridge vents have many benefits: 24 hour a day, silent operation, no moving parts to break and best of all, once installed, it doesn’t cost anything to operate.

    Power Fans
    These come in 2 or 3 different sizes and are very effective at moving large amounts of air. The use of power fans must be given careful consideration. Power fans draw air from every available source. This includes every hole between your attic and the interior of your house. A powerful attic fan can be a concern if you have an improperly installed hot water heater or furnace. If your power vent is too powerful it can cause a back draft of either of these. Fire and carbon monoxide are the scariest consequences, but the more likely effect is conditioned air from your home being drawn up into your attic. When that happens the air in your house needs to be replaced with air from outside. Then your air conditioner or furnace must cool or heat the new air, using more energy. Another drawback is that they are programmed to run in conjunction with thermostat and humidistat measurements. They react to high temperature or high humidity, rather than preventing a rise in heat or moisture. Despite their drawbacks, power fans are sometimes the best option, but there must be a lot of intake ventilation. Recently we have tried power fans that run on solar energy. These turn on when the sun comes up and turn off when the sun goes down. This eliminates the reactive issue with power vents, but in Michigan there is not enough sunshine to run them effectively. They are also very expensive and their warranties last only for a year or two.

    Intake Vents
    Intake ventilation consists of soffit vents in combination with attic baffles. The exact type of soffit vent you need depends on attic volume, the architecture of your home and the type of materials you have chosen to use. Intake vents are where most people make mistakes. They can be a real challenge. Intake vents need to be watertight. They need to be located down low in the attic (preferably in the soffit). And they need to provide at least as much air flow as the exhaust vents. An attic baffle is a Styrofoam or plastic channel that slips between your attic insulation and the wood of the roof deck that the insulation is smashed up against. It allows the air to flow into the attic from the soffit. It is important that these are placed in every rafter space. Without them the air will not flow from the soffit vents into your attic

    Aluminum louvers
    Typically 8”x16”, 4”x16”, or a continuous strip about 2” wide, properly installed aluminum louvers are great for houses with wood soffit.

    Aluminum or Vinyl Soffit
    This is aluminum or vinyl siding made for the soffit area. Because of the restrictions on airflow from the nature of this material, these need to be continuous vents, not spaced every few panels. There is perforated, slotted, and hidden vent soffit. This is a great product for maintenance free exteriors.

    Intake Vents for Houses with no overhangs
    There is basically 3 ways to vent a house with no overhangs. The easiest way for the roofer is to cut a slot in the roof deck close to the eave and cover it with something like a corrugated piece of plastic. The ease of installation is nice, and the cost is not bad at all, but there is a risk of a leak. In Michigan we have snow. The snow piles up on the eaves and ice dams might force water into these vents. The manufacturers have done extensive field testing and have assured us that they will not leak. In various ventilation seminars they have repeatedly told us that they will back up their products and pay for all costs associated with such leaks, including interior damage. This is not in writing, as a matter of fact their written warranties only cover manufacturing defects in the product. However, these products have been in use in Michigan and other similar climates since 2008. When this vent is installed, you can see a slight bump on the bottom 16” of the roof. It is about 3/4 of an inch high. It is not blatantly obvious, but it is there.

    A second option is vented drip edge. Drip edge is a piece of aluminum that is installed on the edges of the roof deck to prevent water from dripping around and under the shingles. On the eave edge you can install drip edge that has a slotted face that allows air to flow into the attic. With vented drip edge you cannot have gutters. The water and ice in the gutter will back up into the attic. Vented drip edge also tends to be a little delicate and gets bent up over time.

    A third (and expensive) option is to remove the gutters, cut a slot in the fascia board and the edge of the roof deck, install a thick, plastic filter and a second fascia board, then cover the area with a strip of aluminum before you roof it. You can then re-install the gutters. Fortunately there is a product that combines the above process with a plastic composite material that is relatively inexpensive and relatively easy to install. It does however look unsightly if you do not install gutters on top of it, and it’s net free airflow is limited.

    A cooler attic means a cooler, more comfortable house and lower utility bills.

    All shingle manufacturers require attic ventilation to meet or exceed the U.S. Dept. of Housing and Urban Development’s (HUD) Minimum Property Standards for Housing. The (simplified) minimum standards for attic ventilation are as follows: a. Cross ventilation shall be provided for each separate space. b. The ratio of the total net free ventilation area to the area of ceiling shall be not less than 1/150, except that the ratio may be 1/300 if: (1) A vapor retarder is installed on the warm side of the ceiling; and (2) Between 25 and 50 percent of the required ventilating area is provided by vents located in the eaves. It is our experience that 1/300 is very rarely enough.

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