Did you know that, just like you, an asphalt shingle roof needs to breathe? Cool, dry air must be drawn in along the eaves (usually through the soffit) and warm moist air must be exhausted as close to the ridgeline as possible, creating a flow that should completely turn over the air in the attic space. Having adequate balance of intake and exhaust venting is the most important thing a property manager or owner can do to get the optimum life from asphalt shingles. An abundance of either vent (intake or exhaust) does not make up for a lack of the other.
In addition to prolonging the useful life of shingles, a properly vented attic space provides more benefits. It prevents mold and deterioration on the underside of the roof deck and on the rafters or trusses. It safeguards items stored in the attic against mildew. Proper ventilation reduces attic condensation which can cause interior damage, cause rust on anything metal in the attic and ruin insulation. It can help minimize peeling and extend the life of interior paint. It can reduce heating and cooling costs and, my favorite, it will hinder the formation of ice dams in the winter. If a roof is not properly ventilated it may void your roofing material manufacturers warranty.
So, does your property have enough ventilation? How do you tell? I can give you the basic "rule of thumb,” but because almost every roof has its own unique characteristics, it may be necessary to have your properties inspected by a qualified roofing and/or insulation contractor. The Federal Housing Administration recommends a minimum of one square foot of ventilation for every 300 square feet of attic space. According to FHA, that one square foot should be half intake and half exhaust ventilation. For example, if an attic space is 1,200 square feet, it should have at least four square feet of ventilation, two square feet of intake and two square feet of exhaust. Notes: The Home Ventilating Institute actually recommends a 60/40 ratio where 60 percent of total ventilation is intake and 40 percent is exhaust. Also, some building codes and architectural specifications will require even more ventilation: one square foot for every 150 square feet of attic space.
It is often said that nine out of ten houses in America don’t have adequate attic ventilation. Where most of these homes are falling down is in the intake ventilation category. It’s easy to cut holes in a roof and stick some more exhaust vents on it. For proper intake ventilation, a few things need to happen. First, a vented soffit panel must be installed that allows enough air to be drawn. Most professionals agree that continuous soffit vent is preferred. Next, sheeting on the bottom of the soffit enclosure needs to be cut out to allow the vented soffit panels to function. Note: if your building doesn’t have adequate soffit for ventilation there are drip edge and fascia products that can be used instead. Inside the attic, chutes or baffles must be installed to ensure that air can flow from the soffit vents into the attic space without being blocked by insulation.
When it comes to exhaust vents, all of the varieties available to us will work. There are static vents like slant louvers (also called box vents, low profile vents, 750 vents and turtle vents), gable vents and ridge vents (still available in the classic metal style and the newer, better plastic "shingle over” style). Then there are vents which move to exhaust air, like turbine vents (whirlybirds) and power vents (which must be solar powered or hardwired to operate). Each type of exhaust vent has its own benefits and challenges. They each have their own abilities with regard to the square footage of attic space they are able to handle, but if enough are installed, any is capable of doing the job. Product literature from the manufacturers will show how many vents of a given type would be required for your roof.
One situation that must be avoided is to have a mix of these exhaust vent types in any one attic space. Mixing exhaust vent types can result in a situation where air is actually drawn in a vent that was designed to expel it. If that happens snow and rain can get sucked into the vent (and thus, into the attic space) along with the air.
Just because things look good on the outside does not mean your roof is properly ventilated. If your attic space is divided (for example: by firewalls), each separate space must have its own intake and exhaust ventilation. Two other things that can complicate proper ventilation are vaulted ceilings and knee walls. In these situations it will be necessary to work with your contractor to come up with the best possible solution for your specific needs.
If one of your properties is experiencing any of the problems mentioned above, or if you are in the market for a new roof, make sure you are talking to your contractor about the roof ventilation system.