How to Improve Attic Ventilation

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.

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.

• 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 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.