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This is the most neglected aspect of roofing and most houses are grossly under vented. Inadequate ventilation will cause the life of your roof to be shortened dramatically and can lead to dry rot and material cupping or curling.
Simply put, the purpose of ventilation is to reduce summer heat and moisture from the attic and rafter spaces. The uniform building code requires, and most manufacturers recommend, a minimum of one square foot of venting for every one hundred fifty square feet of ceiling space. Fifty percent of that amount must be in the form of high vents, located near or at the ridge of the house. The balance can be in lower areas, such as the soffits. The vents should be equally distributed so as to provide for proper venting to all portions of the roof.
A proper roof venting system allows for a continuous air flow between the low (soffit) vents and the ridge vents. The air flow is “powered” by convection (i.e., the warmer air will rise and escape out of the high vents and ‘pull’ cooler and dryer air from the low vents). This works equally well in high summer temperatures and low winter temperatures.
Different types of vents available:
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, 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 roof and wall to permit free air movement.
If it’s in your budget, you can then place insulation batts between the roof rafters and the walls, but that will have considerably less impact on protecting the house than putting insulation between ceiling joists.