Just as a roof must keep water from entering a house, moisture buildup under the roof must be removed through proper ventilation. Improper ventilation results in warm, moist air generated in the house meeting with colder air in the attic, which causes condensation. Ventilation problems can be seen in a variety of ways, such as blistering and peeling paint on the gable end exteriors because of excessive heat and moisture. Buckling roof shingles are another sign of poor ventilation, as are ice dams in the Midwest and Northeast. Water stains on a ceiling may actually be a result of condensation moisture dripping from under the roof rather than a leak.
Ideally, air circulates under the roof by being drawn up through a continuous soffit vent, through the attic space, and then out the ridge through a continuous ridge vent. This is the smoothest and most efficient system. But few houses have continuous soffit and ridge vents. Most houses, however, do have some type of vent in the roof or an opening at gable ends. Houses with no rafter overhang, or a very short one, may not have soffits. Older houses may not have vents between the rafters leading into the attic space. Venting was not a major problem in older houses because they leaked air everywhere. But with the advent of better roofs and tighter house construction, moisture became a more significant issue.
Calculating the number and size of roof vents is not an exact science, but instead is influenced by the climate, the roof pitch, available locations for vents, and the house's orientation to prevailing winds. The Asphalt Roofing Manufacturers Association states:
"In most cases, a minimum free-flow ventilation area equal to 1 square foot per 150 square feet of attic floor area must be designed and properly installed to provide proper ventilation. Where a properly designed and installed eave and ridge ventilation system is employed, a free-flow ventilation area equal to at least 1 square foot per 300 square feet of attic floor area is often sufficient. Combination eave and ridge venting is generally recognized as a superior venting technique."
Vent screens should be cleaned regularly because accumulated dust and grime can significantly restrict air movement. Fans connected to thermostats can be placed at gable end openings to draw hot air out of the attic space when temperatures reach a preset level.
Types of Vents
SOFFIT VENTS: These vents are installed in the soffit (the enclosed portion under the roof overhang) and permit air to flow up under the roof and into the attic. They range in style from 6 inch round stainless steel vent covers that are placed in the soffit between each rafter to continuous vents that run the entire length of the soffit.
RIDGE VENTS: These vents run the length of the ridge and replace the ridge shingles or tiles. They are designed with interior baffles that permit air to flow out but prevent rain from blowing in.
TURBINE VENTS: Common on many roofs, the vent top spins on ball bearings. The slightest wind turns the vent, which in turn draws air from the attic.
EYEBROW VENTS: Also called turtle vents, they provide curved openings on roof slopes. They should be used in pairs with one on each side of the roof to facilitate air movement. Insulation is part of the whole roof system, and is designed to prevent both cold and heat from entering the house. Insulation is rated by R factors, which is a measurement of resistance to heat flow. Heat is energy and it always moves toward cooler areas—out of the house in winter, into the house in summer.
The temperature in a well-ventilated attic ideally should only be a few degrees different than the outside temperature. Therefore, to protect the house, the first place to install insulation in an attic is between ceiling joists. If using ‘batts’ (styrofoam inserts stapled to the ceiling, which allow air to flow between the insulation and your roof) place the paper or vapor retarder side, against the ceiling to resist the movement of water vapor. If using loose fill, a vapor retarder ideally should have been placed over the bottom of the ceiling joists before the drywall was installed.
Be sure that insulation between ceiling joists does not block any soffit vents. If your house has them, keep the insulation about a foot short of the connection between the roof and wall to permit free air movement.
If it’s in your budget, you can then place insulation ‘batts’ also between the roof rafters and the walls, but that will have considerably less impact on protecting the house than putting insulation between ceiling joists.
If you have a cathedral ceiling with little room for standard insulation without blocking air movement, then use ‘baffles’ to create a space between the roof deck and the insulation. If your cathedral ceiling is formed by tongue and groove lumber on top of rafter beams, you can also use rigid insulation on top of the roof deck, under the roof covering