Some common ventilation myths that misinformed contractors are guilty of spreading are:
-You can add ridge vent to the existing ventilation system. No.
Mixing different types of vents can cause indefinable draft patterns that interrupt the free flow of air. The different vent types can actually cancel each other out. The existing turbines or low profile vent holes should be covered if ridge vent is going to be installed.
-Installing low profile vents on both sides of a roof peak doubles the exhaust. No.
If you place the vents across the ridge from one another, one side will act as an intake for the other and actually push the warm air beneath them down against the attic floor (living space ceiling).
– Power vents are the best thing out there. No.
Power vents are heat activated with a thermostat and generally do not work to move air in the coldest months of winter, allowing moisture to accumulate. The thermostats in the power vents are very temperamental, and usually break within the first two years. They require an electrician to install and they run off of your electricity, which makes them an ongoing cost. On the other hand, solar powered vents are superior products by far, and they posses none of the negative traits of the traditional power vents when installed correctly.
– More ventilation is better. Not necessarily.
Too much attic ventilation can suck the conditioned air from the living area and become counter productive. The ratio accepted by the UBC, FHA, and SBCCI (major building code agencies) is 1/300, however 1/150 is recommended for warmer year round climates. That is, one square foot of ventilation for every 150 square feet of attic floor space. This number is then divided evenly between the intake and exhaust. By this formula, a 2000 square foot attic needs thirteen square feet of ventilation (2000/150). This number needs to be divided by two, 6.5 square feet for intake and 6.5 for exhaust. For reference, one wind turbine or four feet of ridge vent is worth one square foot of exhaust. Two soffit vents are worth one square foot of intake. For a 2000 square foot attic you would need thirteen soffit vents and 26 feet of ridge vent, or 7 turbines.
The most important thing to remember is that without air intake, the roof vents are worthless. If you do not have any soffit ventilation, call a trusted local contractor for an estimate. If you do have soffit vents, make sure they are clear of dirt and insulation. If you have special circumstances such as cathedral ceilings or open cornices you should ask a qualified roofing contractor for a more creative solution. You can also have more specific ventilation questions answered in the homeowner’s forum at http://www.hailandwind.com/