The principal source of summertime attic heat is direct sunlight on the roof of the home. This is radiated heat, and even on a cloudy day, there is an appreciable amount transmitted to a roof.
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Solar heat on the roof is transmitted through the roof material and, in turn, is radiated to the attic floor—or to the top surface of the ceiling insulation material. This surface becomes heated, and the attic air in contact with the underside of the roof and the top of the insulating material also becomes heated. Convection allows some circulation of the air so that more and more of the attic air is heated.
Gradually, the temperature increases until the entire attic—roof, floor, insulation, and air—are extremely hot. In an unventilated attic, the roof sheathing may reach a temperature in excess of 160 degrees Fahrenheit (F), and the attic floor 150 degrees (F) or more when the outside temperature is in the 90s.
When the sun goes down, the source of heat, of course, is depleted. The roof begins to reradiate the heat from the attic to the outside air. Sometimes the heat absorbed by the structural materials, including the insulation, may not be entirely removed during the cooler night hours. The heat then builds up over a long period of hot weather. The heavier the structural material, the thicker the insulation and the amount of stored items present, the greater the amount of heat may be stored.
Intense attic heat is transmitted to the ceiling surface of the living space below. The ceiling acts as a “hot plate,” not only warming the air in the rooms but radiating some of the heat to the occupants as well. This, in turn, adds to the air conditioning requirement—both in the size of the unit needed and in operating costs.
The portion of the solar heat that reaches the living area through the attic is proportional to the difference between the attic floor and room ceiling temperatures. Adequate ventilation can substantially reduce this temperature difference. Ceiling (attic floor) insulation retards the rate at which the heat flows to the rooms below. A cooler attic floor reduces the quantity of heat, which the insulation must keep out. Ventilation simply makes the insulation more effective. Ventilation also reduces the quantity of heat, which is stored within the insulation and other structural materials during the day. This ensures a quicker and more complete cooling of the attic during the night. Seasonal build-up of heat is then minimized or eliminated.
There are two sets of circumstances favorable to winter condensation of moisture in an attic:
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In cold climates, a combination of high, inside relative humidity (60% or above) and low outside temperature (30 degrees F or below) may cause condensation on the underside of the roof sheathing. Condensation develops from a combination of high relative humidity and temperature differentials. Condensation cannot form when the relative humidity is low, or the temperature is high.
Even in moderate climates with high relative humidity, the day-night temperature cycle, combined with high humidity, may cause condensation on the underside of the roof sheathing.
Effective attic ventilation is often more critical in newer than in older homes. Incongruous as it may seem, advances in home construction have created conditions that increase the possibility of winter moisture condensation. Modern homes are better insulated, thus easier to heat and cool. They are “tighter,” thus cleaner and less drafty. They are better planned and more compact. They incorporate more labor-saving appliances. All of these factors mean more comfortable living, but they have combined to increase the quantities of water vapor within smaller spaces and have made it more difficult for the vapor to escape.
The result is a series of problems such as wet (and consequently less effective) insulation, wood decay, and peeling paint. These conditions may go unnoticed until considerable damage has been done.
If little or no insulation is present, there is little possibility that a ventilation problem will exist because without adequate insulation, the heat that is lost to the attic will allow the air to control the rising relative humidity. Homes with little or no insulation are likely to have 2 to 10 times more air-changes per hour than modern, relatively tight homes. Since homeowners have become aware of the importance of insulating and tightening up their homes to conserve energy, condensation and ventilation problems have become widespread. Saving energy is recommended, but it is important to understand what happens to the moisture in the air when the relative humidity goes up and down.
During the summer, a poorly ventilated attic can reach or exceed a temperature of 150 degrees F. Even with insulation covering the attic floor, the rooms below may have excessive heat gains and, therefore, be less comfortable and increase air conditioning costs. Such a situation could also shorten the life of the air conditioning system as well as some roofing materials. The air conditioning system may suffer significant inefficiencies due to the heat, especially if the ductwork is located in the attic. Cool air may also be lost through the ductwork and the unit may have to work longer.
High attic temperatures may cause deterioration of many fire-retardant plywood roof sheathings, joist and truss members to split and deform, and truss plates to deteriorate and loosen.
Humidity primarily comes from within the house (i.e. from tubs and showers, unvented clothes dryers, humidifiers, cooking, basement and crawl spaces, etc.). It also comes from less obvious sources, such as plants, standing water in a sink and even a large number of people who may stay in the house for a prolonged period of time. The very act of breathing by a family of four can expel approximately 1/2 pint of water per hour into the atmosphere of a home. Mopping a kitchen floor of about 150 square feet can release approximately 4 ½ pints of water; washing the dinner dishes can release about 1/2 pint. A wind-blown rain can cause water to enter and evaporate into the attic area through roof leaks or poorly designed or installed ventilators.
Condensation in an attic is due to saturated air. The first place that the air will usually saturate is on the north side, at the lowest area in the attic, just above the insulation. The reason for this is two-fold:
The north side will be colder than the south side (northern hemisphere).
The biggest temperature change takes place just above the insulation. There is also a smaller volume of air at this point than there is closer to the center of the attic or roof system. Mold will form at this north side (lowest area first); it progresses up the north side, and when it gets up about halfway, it starts at the lowest area of the south side. If the conditions are serious enough, the mold will continue to rise on both sides until all of the sheathing is black with mold.
- Deteriorated, sagging sheathing
- Rust on nail heads at sheathing
- Ventilation fan inoperative or doesn't come on when manually operated
- FRT plywood (some)
- No provision for ventilation (usually older homes)
- Soffit vents blocked or undersized
- Gable vents blocked or undersized
- Ridge vents blocked or not fully cut through the sheathing
- Inadequate or insufficient ventilation configuration
- Vent fans (bath, dryer, kitchen) do not discharge to exterior
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How can you tell if an attic has condensation problems, and to an exacting degree?
Assume that it is the middle of the summer, knowing that there can be no condensation in an attic. A simple knowledge of physics is needed to easily understand this issue.
The warmer the air, the greater ability it has to retain moisture. The colder the air, the less ability it has to retain moisture.
During the summer months, an attic can get very warm. The warm air will eliminate the formation of condensation. During the winter months, attics can get very cold, especially in colder climates. Note: Homes in warmer southern states will usually not experience winter condensation.
Clue # 1: Since roofing nails are usually galvanized steel and are in direct contact with the exterior, and because their density will communicate the cold, the nails are the first place where moisture vapors turn to liquid. Rust forming on the nails is your first indication. This is very minor, and action is not necessary.
Clue # 2: Since the nails are the first place where moisture forms, the wood sheathing adjacent to the nails will absorb the water and dark stains will appear. This is still minor, however, a recommendation to improve the ventilation is appropriate.
Clue # 3: Depending on the amount of water that forms on the nails, stains may be apparent on the flooring (or holes/erosions in the insulation if there is no flooring). This would receive the same recommendation as clue #2.
Clue # 4: This is the first clue that will require action to reduce or eliminate condensation. The roof sheathing closest to the eaves on the northerly side of the house will start to form a light gray fungus/mold.
Clue # 5: The fungus will get darker and form at points higher on the northerly side. This condition should be addressed as soon as possible.
Clue # 6: The fungus starts to form on the southerly side of the roof sheathing and becomes even darker on the northerly side. This is a serious issue and the sheathing, especially on the north side, will soon begin to delaminate.
Clue # 7: The sheathing becomes worse and worse until most or all of the sheathing is black, wet and delaminated. When conditions are this extreme, there is a good possibility that the sheathing will have to be replaced. This will require the roofing to be replaced as well.
Poorly ventilated attics contribute to ice damming after snowfalls. The (relatively) warm air in the attic causes accelerated melting of the snow on the roof. The melted snow flows down the roof until it reaches the eaves or soffit area. This area is beyond the living space and not heated, which will allow freezing. The build-up of ice over this unheated area creates a dam. The water from melted snow pools behind the ice dam and is forced up, under the shingles to the sheathing above the heated space and drips into the attic. It wets the sheathing, insulation, interior wall and ceiling building materials and finished components. When insulation gets wet, its effectiveness is compromised.
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A properly vented attic keeps the roof closer to the outside temperature in the winter, slowing the melting of snow and greatly reducing the chance of an ice dam forming. Ice damming mostly occurs on low-sloped roofing and on the north side. The south side gets the sun, which usually keeps it warm enough to eliminate ice-damming problems.
One method to determine if existing ventilation is adequate is by placing a thermometer in the attic on a warm, windless day to determine if the temperature is more than 10 to 15 degrees F above the outside temperature. If it is, more ventilation is recommended.
The minimum ventilation requirement is a total net ventilating area not less than one square foot for each 150 square feet of attic floor area. This ratio can be reduced to one square foot for each 300 square feet of attic floor space, provided at least 50%, and not more than 80% of the ventilation is high, and the balance of the required ventilation provided is low.
Cross ventilation is not as effective as high low ventilation, because there is no ongoing motivation to ventilate. Due to natural thermal convection and the fact that the temperature in the attic is almost always higher than the outside temperature, ridge and soffit venting is the most effective type of ventilation. To better explain how this works, consider the following:
The outside temperature is typically lower than the temperature in the attic every day of the year, especially in the summer months. The lower the outside temperature, the larger the temperature differential in the attic. If the attic is not ventilated properly, the attic temperature may be 20 or 30 degrees F warmer than the outside temperature. This may not be obvious to the casual observer because the attic will always feel cold in the winter when you access it from the living space. The temperature difference between the attic air and the outside air is impacted by the amount of insulation, the type and amount of ventilation, wind, holes, chases or openings from the living space and ductwork or other components that could contribute heat to the attic space. An alternative way to decrease the necessary ventilation would be as follows: The net, free cross ventilation area may be reduced to one square foot for each 300 square feet of attic space when a continuous vapor barrier is installed on the warm side of the ceiling. This is difficult in a house that already has insulation installed, because you would have to remove the insulation to install a continuous barrier below the insulation or at the warm side of the insulation. If the vapor barrier were installed on the cold side of the insulation, moisture vapors would become trapped on the underside of the plastic vapor barrier and in the insulation. If these vapors changed to liquid, there would likely be damage to the drywall or plaster. If you find a house with the vapor barrier installed on the wrong side, it should be cut or sliced with a knife every 12 to 24 inches or removed. In most cases, this is not a problem because the barrier is only on the insulation and not continuous. To be continuous, it would have to lap all of the joists or be a continuous sheet. Insulation should not block the free flow of air at the soffits. A minimum of a one-inch air space should exist between the insulation and the vents or roof sheathing at all locations. This is typically accomplished with baffles that are installed where the insulation contacts the roof sheathing. A vent’s effective area (net free ventilating area) is less than the actual size of the vent. Screens and louvers can reduce airflow through a vent by as much as 75%. Therefore, depending upon the type and construction of the louvers and screens, the overall size of the vents should be increased. Most vents provide 50% to 65% free air.
Soffit and Ridge Configuration
The best attic ventilation system is a soffit and ridge configuration. Up to half of the required clear air should be located under the eaves or lowest area of the roof, and the balance of vent area located at the roof ridge or the top of the roof. Since warm air rises, this type of system takes advantage of thermal convection or a natural chimney effect, and air movement will be created through the attic, even when there is no wind. The soffit and ridge configuration is compromised by additional vents or openings because the additional openings interrupt the natural convection (i.e. gable vents or roof vents, in addition to the soffit and ridge vents).
It is important that air flows freely over the underside of the roof sheathing. This is especially critical with cathedral ceilings. Insulation must not be allowed to block this flow. The amount of insulation in a cathedral ceiling should be approximately 1-2 inches less than the depth of the rafters. The insulation, rafters and sheathing, etc. may not be visible in a cathedral design. If ventilation were present at the top and bottom, it would be fair to assume that the cathedral ceiling system is acceptable.
If condensation is developing in a cathedral ceiling system and the building is 5 years old or older, there is probably evidence on the drywall. Assuming the ceiling has not been painted recently, look for faint stains that appear to be shadows along the rafters, mostly the lower area of the ceiling and on the north side. They will not look like water stains. Shine a flashlight on the ceiling to make sure they are not shadows. If they are stains from moisture at the joists, it is most likely condensation. These will not be visible in new construction or freshly painted ceilings, and may not be visible in situations with a minimal or modest amount of condensation.
Regardless of the roof geometry, there is usually a small amount of built-in ventilation where the roof and wall structures meet. This slight space allows for light to shine through and some amount of air circulation through the attic, however, it is difficult to calculate or depend on this area for ventilation. Check the insulation, regardless of type, on the attic floor for a minimum of a 1-2 inch space between the insulation and the roof sheathing. Insulation baffles should be installed between the sheathing and the insulation to open or ensure that this path is open.
Ventilating the attic area or cavity below flat or low-sloped roofs can be extremely difficult. If there are overhangs, continuous soffit venting can be employed. In some cases, louvers placed in the fascia board may be effective. Since there is usually very little space between the ceiling and the underside of the roof structure, insulation should be at least 1 ½" thinner than the roof cavity to prevent condensation from developing and being trapped in the insulation. This condition may compromise the insulation and establish conditions that may allow mildew and deterioration to develop. Flat roofs may require ventilation through the roof surface, perpendicular to the joists to vent each bay, however, it is rare when there is full, thick insulation in a flat roof and there is not some heat loss to this area.
In addition to the standard ventilators (e.g. gable, louvers, soffit vents, and ridge vents) for attic areas, there are wind turbine exhaust vents and motorized attic vent fans (not to be confused with a whole house fan). On a hot, still day, the heat rising up in the attic will start the turbine spinning. The more heat going out, the faster it will spin. Add a little wind, and something similar to a no-cost vacuum cleaner is drawing hot air out of the attic. Motorized attic vent fans are usually activated by a thermostat, which should be set at about 105 to110 degrees F. When the temperature reaches the setting on the thermostat, the fan automatically activates and continues running until the temperature drops below the setting. This type of fan can also be activated by a humidistat. Ideally, attic fans would be controlled by a thermostat at the highest area of the roof and a humidistat at the lowest area of the roof, on the north side.
Moisture from Exhaust Fans
Moisture from exhaust vents originating in the house (e.g. kitchen, bath, and laundry) should terminate to the exterior of the house. They should never terminate in the attic area due to potential for elevating the relative humidity, creating mold, compromising the insulation and, in extreme situations, causing moisture damage to the roof system and interior ceilings.
Moisture generated in a home will only cause condensation during the winter months. This condition can be aggravated if a homeowner seals the attic vents during the winter. Vents in an attic should never be closed during cold weather. With proper insulation at the attic floor/ living space ceiling, the ventilation will have little, if any, effect on heat loss.