Understanding Roof Vents
We all want our homes to be warm in the winter and cool in the summer. One way to help achieve that goal is to have a properly insulated and vented attic. The insulation will help keep the cold winter or hot summer air out of your home, while the vents allow air into your attic, where it circulates and carries hot or cold air out. While we all get lots of information on how important it is to properly insulate an attic, the equally important part about allowing for airflow isn't as frequently talked about. Here's an overview of how attic ventilation should work.
(For more on attic insulation, look here.)
Why We Need Ventilation
When you first think about it, the idea of allowing cold winter or (hot summer) air into your attic just seems wrong. However, your attic provides a middle ground or buffer between the extremes of the outside temperatures and the relatively constant temperature inside your home, without adding to the floor space that you need to heat and cool.
Without air movement, moist air from inside your home escaping into the attic (around light fixtures, fan ducts, or attic entrances) would be trapped there. Over time this trapped moisture would get into the insulation (reducing its effectiveness) as well as wooden joists and rafters. Mold and mildew would start to grow and the wood would rot. Air movement gets that moisture out of the attic.
In the summer, even though the attic is hot, it's still slightly cooler as a result of air movement. This movement allows the insulation (that actually holds heat) to cool quicker so it won't transmit heat into your home after the sun goes down. The cooler attic temperature is also good for your roof since it helps prolong the life of your shingles.
Getting Proper Ventilation
Proper attic ventilation requires a steady flow of outside air from the lowest part of the attic, along the roofline, and out the highest part of the attic. This air movement helps the insulation work best and cools the underside of the roof.
Soffit vents (the area beneath the eaves of your roof) allow the air to enter at the lowest part of your attic and ridge vents (a screened vent that replaces the top shingles at the peak of your roof), or roof vents allow it to escape out the top.
Older homes often don't have proper ventilation, since they were built with just a few gable vents near the top of the roof. Building codes across the country often require roofers to install new roof vents when reroofing homes that only have gable vents.
Insulating Roof Vents
While roof vents need to be installed using a waterproof membrane and overlapping shingles so moisture can't get in under them, they shouldn't be insulated. Soffit vents also need to be left uninsulated to allow airflow.
A properly insulated attic will have insulation across the floor/ceiling joists, but where the floor meets the roof joists (over top of the soffits) there will be no insulation.
Contractors often use attic rafter vent/baffles installed between the roof joists to keep insulation from migrating into the area over the soffits and blocking airflow. These vent baffles are available at home and hardware stores for the DoItYourselfer installing insulation in their attic. And that's all you need to know.