Attic Ventilation and Water Damage


The following are some techniques used to reduce the likelihood of ice dams and attic condensation:
  • Prevent warm, moist air in living spaces from infiltrating into the attic with a good air barrier and appropriate water vapor control at the base of the attic. Properly taped drywall or plaster on ceilings make good air barriers and are required at all ceiling and wall surfaces adjacent to attic spaces.
  • Seal all openings rising into the attic, such as open wall cavities. Open wall cavities can allow warm air laden with moisture to rise up from the living spaces below.
  • Avoid designing ceiling mounted fixtures below the attic that create the need for holes in the drywall or plaster ceiling. If this cannot be done, seal around all penetrations to make them airtight. Ceiling mounted light fixtures and ceiling fans have electrical junction boxes mounted flush in the ceiling. These have a number of holes in them that need to be sealed.

    (Caution -- Fire Hazard! All light bulbs give off heat when turned on. Some recessed light fixtures are designed to dissipate this heat through holes and openings usually on or near the top of the fixture. If the holes are sealed or if insulation is placed over the light fixture, the light fixture can overheat and cause a fire. Before sealing around and placing insulation on top of recessed light fixtures in the ceiling, an electrician should determine if the existing light fixture is designed for that or if it has to be replaced.)

  • Avoid attic access from conditioned spaces. Access panels or doors can be located in unconditioned spaces, such as the garage, reducing the chance of warm, humid air rising up into the attic if there is not an airtight seal. No matter where the attic access panels or doors are located, make sure they are insulated and as air tight as possible.
  • Avoid penetrating the ceilings below attics with ductwork or piping.
  • Never exhaust air into the attic space. Exhaust ducts from kitchen and bathroom exhaust fans, as well as exhaust ducts from clothes dryers, should be terminated outside the home.
  • Limit indoor relative humidity in the living space to no more than 35 percent during the winter.
  • Depending on the climate, provide a vapor retarder or vapor barrier at all ceilings and walls that about attic spaces.
Vapor Retarder -- This is a material that allows a limited amount of water vapor to pass through. An example is the brown kraft paper on the back of rolls of fiberglass insulation.

Vapor Barrier -- This is a material that prevents water vapor from passing through. An example is a polyethylene plastic sheeting. In mixed climates with cold winters and hot summers, use a vapor retarder. This allows the system to work in both summer and winter conditions. In extremely cold climates without long, hot summers (regions with heating degree days greater than 8,000 - see the map below), it is possible to use a vapor barrier or vapor retarder.

Provide good attic ventilation to replace warm air in the attic with cold outside air.
  • All attic spaces must be ventilated.
  • The best ventilation solution is through natural convection by installing an equal amount of vents in the soffits (low point of the attic and at or near the ridge (highest point in the attic). It is generally accepted that using a combination of soffit vents and roof ridge vents is the best way to achieve ventilation by natural air movement. In cases where the home design does not have soffits, it may be possible to replace the drip edge at the bottom edge of the roof with a vented drip edge. Gable end vents (i.e. vents in the triangular section of attic wall bounded by the pitched roof) may be an option for some homes. Consult a building professional for the best option for your home.
  • Open attics are usually easier to vent than cathedral ceilings. If cathedral ceilings are used, maintain an air space of at least 1 1/2 inches to 2 inches (for most situations) between the top of the insulation and the bottom of the roof deck. The amount of air space needed varies depending on the length of the ceiling between the soffit and ridge vents, the slope of the roof, the venting capabilities of the soffit and ridge vents, and the thermal insulating value of the insulation below the airspace. Sometimes up to a 4 inch air space is needed if the run of the ceiling is long, there is a limited amount of venting, and there is a limited amount of insulation.1
  • Use rigid plastic air chutes to keep attic insulation from blocking airflow from soffit vents to roof ridge vents. Do not use cardboard or thin polystyrene air chutes which can easily lose their shape or be flattened, blocking airflow.
  • Avoid using mechanically-assisted attic ventilation unless specifically recommended by a knowledgeable building professional. Incorrectly designed fans can negatively pressurize the attic and pull warm, moist air into the attic from the living spaces.
1 Ventilating Cathedral Ceilings to Prevent Problematic Icing at Their Eaves; Wayne Tobiasson, Thomas Tantillo, James Buska; Cold Regions Research and Engineering Laboratory (CRREL); Hanover, NH; Proceedings of the North American Conference on Roofing Technologies; September 16-17, 1999

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