Older homes built in the 50's and 60's didn't need as much ventilation as today's homes because they weren't insulated. Air was free to move through the walls and up into the attic. Energy prices were much lower then. Now that energy prices are high we try to build much tighter homes to keep out the heat and cold. Insulation, weather stripping and house wraps are used to prevent outside air from entering your home. Attic ventilation is now very important. It is estimated that less than 10% of homes have the required amount of attic ventilation.
Without proper ventilation you can have moisture and rot problems as well as mold, peeling paint, excessive heat build up and shingle damage. The NRCA (National Roofing Contractors Association) recommends 1 square foot of free vent area for every 150 square foot of attic floor. If you have a 1500 sq ft house - divide 1500 by 150
= 10 sq feet of free vent area. About 60% should be intake such as soffit vents and 40% for exhaust such as ridge vents. If you have too much exhaust and no intake then expensive conditioned air will be sucked out of your house. Screens used in vents to keep out insects also cut down on the air flow. Manufacturers list the net free vent area on their products to account for the area taken up by screens.
Watch this video for more information. Though it is a product specific promotional video it contains some very good information.
The house pictured on the left is a very unique design but the roof line is one that is commonly seen in the Bear Creek area as well as other parts of Houston. You will notice there is no overhang, that means there are no soffit vents for air intake. Many of these homes originally had wood shingle roofs. Wood roofs do not have decking, they are built on open lattice to provide the ventilation that is needed for the underside of a wood shingle roof. When a new asphalt roof is installed, decking is put down and this attic can no longer breathe. Ventilation needs be added somewhere.
Often with this type of roof a power vent is put on the high side of the roof and a static vent installed on the low side for intake. Adding exhaust without proper air intake can result in the conditioned air being sucked out of your home and increase your cooling bills. There are other options you should consider if your home has no place to install soffit vents.
Radient barrier decking will help stop the heat build up in the first place. If you have high ceilings with little room for insulation you definitely need to check this out. LP Techshield can cut your electric bill by up to 20%. Since the radient barrier is on the decking, the labor costs for installation should be no more than regular roof decking. DuPont™ Tyvek® AtticWrap™ creates a channel that allows air to flow between the wrap and the roof decking. Looks like a great product but is much more labor intensive than a product like Techshield.
For intake ventilation on roofs with no overhangs take a look at Vented Drip Edge from Air Vent Inc. or Tamlyn, Vented Fascia board from Crane Products and CoraVent products. These unique products were designed especially for homes with no overhangs. The best time to install these products is when you have your roof replaced.
Icynene foam insulation is a spray foam insulation product that can be use in an unventilated attic. When used to seal the underside of your roof deck it is impossible for moisture to travel up and harm the roof deck. An attic sealed with Icynene doesn't need to be ventilated.
The main purpose of attic ventilation is to prevent moisture damage. A cooler attic and lower electric bills are just a much appreciated side effect. If you house previously had a wood shingle roof and now have asphalt, proper ventilation may not have been added.
