Attic ventilation is an important and critical component of a building system. Because of the tightening of the home with insulation, double pane, weather-stripped windows and doors, and the many improvements buildings have undergone, adequate ventilation is an important improvement.
Older buildings with leaky windows and doors, and little to no insulation, have many air exchanges with the exterior, especially in the winter. The cold air in the winter holds much less humidity than warmer, summer air. The large number of exchanges of air with the exterior brings in dry, cool air. This air has to be re-heated and in the process becomes even drier. Older structures are often filled with plaster and large wood beams, brick and structure, that also can absorb higher levels of moisture.
Thermo pane (double glazed) windows, insulated doors, caulk and building insulation has become typical not only in new buildings but in older retrofitted or renovated buildings. Moisture entering from below grade areas such as the basement or crawlspaces, cooking, laundering, showering, even breathing, all contributes to elevated moisture within a building especially when a home is tightly insulated. As the moisture begins to accumulate in the lower building spaces, that moisture source combines with other sources and increases as it rises through the structure into the attic. This is called the stack effect. Ventilation of the attic space becomes crucial as if this moisture then becomes trapped in the attic, and if not well ventilated, a number of problems and adverse conditions will occur.
In older buildings, often the windows in the attic are used as the only source of ventilation however many people close the windows in the winter believing the air flow to be detrimental to the heating of the house. Proper insulation in the floor of an attic space keeps the heat within the conditioned space, the rooms below. Keeping the windows open will allow air flow to travel through the spaces. With windows only, however, the ventilation may be uneven and often the prevailing winds influence the attic, and if the house is positioned incorrectly, the air flow may be negligible. Other buildings and trees may also block or divert air flow. Windows also allowed wind driven rain into the attic, something that can create significant damage.
Gable vents, slotted, inverted V or U shaped shutter like wooden or metal devices can be installed at the peak in the siding at the gable ends. The gable vents function similar to windows with the advantage of the sloped wood slots diverting wind driven rain but has the same air flow restrictions as windows. Ladders, scaffolding and tools are needed to cut siding and install the gable vents. If you don’t like working in high places, your girlfriend should do it.
For sloped roofing, the better methods of ventilation is the installation of roof vents or ridge vents as an upper air source and soffit vents as a lower air source. Both the upper sources are needed along with the lower sources, for if either is lacking, no air will flow. Roof vents, generally square black or silver vented boxes are placed directly over cut openings into the upper areas, generally on the rear of the roof. Ridge vents are placed over a cut opening extending about 1 ½ inches along either side of the peak of the roof. No better excuse can be found to buy a cordless circular or reciprocating saw, or both. As working on the roof may be a hair raising experience, the soffit vent installation may also be, especially if the building is two or more stories in height. Ladders or scaffolding is needed as the soffit areas are the areas at the roof overhang, at the exterior, where the roof structure extends out beyond the siding. Safety is critical and ropes and harnesses are often used. The horizontal under side of the roof extension is the soffit. These areas, more often than not, are filled with solid wood, metal or vinyl and the goal is to remove the solid material and replace with a continuous, metal, or vinyl perforated soffit venting. Hidden soffit can be purchased for higher end jobs but most houses use the typical type found at most building suppliers. If soffit vent is already present, check to make sure the perforated material is not installed over solid material, a condition that is found all too often.
With the upper and lower ventilation sources installed, the attic spaces should be checked to insure the insulation is not blocking the soffit venting. Depending on the attic design, a baffling material may be purchased and installed between the insulation and the roof sheathing to insure the air flows without blockage or restriction. The baffles are styrofoam or cardboard, elongated U shaped sections that handle easily. The sections may be slid into the cavities between the rafters or trusses and stapled as needed. Different sized baffles are available as the rafters are usually 16 inches apart and trusses usually 24 inches apart. Crawling down the lower perimeters can is another interesting challenge so being careful not to place any weight between horizontal roof/ceiling members on the ceiling below. With the lower and upper ventilation sources in place, the baffles installed, the air will flow between the two sources. The air is drawn in at the bottom, rises to the top and exits the house, along with excessive moisture and heat.
Fans can be installed to force air out of the attic space, at either a gable end or the roof itself, but are often connected to a thermostat only. That is fine for lowering hot summer temperatures but does nothing for moisture concerns that develop in the winter. Most fans are now connected to both a thermostat and a humidistat for control of both conditions, a much better choice. A disadvantage to thermostatically controlled fans is the reaction to high temperatures, and this may help a fire that is developing spread much faster.
Flat roofs are the more difficult to ventilate properly. The installation of numerous gable type vents, installed in the side and end walls, above the insulation level and below the roof surface, are needed. Sometimes a fancy older facade can be worked with screening behind the facade materials to act as a gable vent. A series of turbine type vents are installed at the flat roof areas. Turbines are similar to roof vents but these devices have a longer shaft, and have a series of circular blades that draw air out of the attic with any wind action, forcing the blades to move and create suction.
Poorly ventilated attic will exhibit moisture staining, frost buildup in the winter, and often mold growth. Even new construction, depending on the design, may create concerns and problems, as the influence of design on the ventilation sources creates situations that are often not fully understood by the designers. In the quest for each building to exhibit a unique or sometimes elaborate design, this had added a layer of complexity to the other systems within a home, such as the ventilation. New homes, exposed to the elements during construction, will let off tens to possibly hundreds of gallons of water, especially in the first year of occupancy. Some homes have become so tightly insulated that simple kitchen fan operation may cause the heating system or fireplace to back draft, and literally pull the exhaust back down the chimney and disperse it throughout a building.
Remember the stack effect and the movement of moisture within a structure. Keeping a basement dry with a dehumidifier is important. All crawlspaces should be properly insulated and ventilated. If a humidifier is used at the furnace, the setting should be kept as low as possible. And also, make sure the attic is well insulated and ventilated.
