A good system must have a way for the air to get in (usually the soffit vent), and get out (usually the ridge vent). A chimney effect helps the air circulate through the attic, cool air in through the soffits and hot air out the top ridge. Wind blowing across the roof peak outside creates negative pressure, helping the chimney effect and air circulation.
So what does attic and roof ventilation do?
- Helps prevent ice dams in the winter. An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas. The heat inside the attic causes the top part of the roof to be warmer than the lower edge - snow melts at the peak, drips down the roof and ices up at the bottom edge. Ventilation in the attic keeps the temperature at the top and bottom levels more consistent and reduces the formation of ice (insulation helps also).
- Reduces condensation and moisture in the attic and the home. Prevents wood rot, peeling paint, mold and mildew.
- Prevents premature aging and failure of roof shingles and decking. High temps bake the roofing materials from the attic side, ventilation helps the attic stay cooler. Most roofing manufacturer's warranties require proper roof ventilation to ensure that the materials perform as specified. If you want that new 30 year roof to actually last 30 years the attic (including the roof decking and the underside of the shingles) needs ventilation.
- Helps keep the entire home cooler in warm weather.
Some guidelines regarding attic and roof ventilation:
- There should be vents at the bottom (soffits) and the top (ridge) of the roof for proper airflow, but anything is better than nothing.
- As a rule you want the air temp of the attic to be about the same as the outdoors. If there is a wide difference you need to increase ventilation.
- Don't block vents. Covering your attic gable vents in winter doesn't keep your house warmer (increase the insulation in the floor of the attic to achieve this). Don't let the attic floor insulation block the soffit vents at the lower roof edge.
- Soffit and ridge vents are considered a balanced system. Ask a ventilation contractor for advice if your attic also has gable vents - the gable vents may negate the airflow from the soffits and ridge and may need correction.
- Consider adding an attic fan. Many cost below $100, are easy to install, and run on a thermostat that kicks in when attic temperatures get too high.
- Make the ventilation uniform throughout the attic, with no dead spaces. The ridge and soffit vents should be continuous if possible. If using gable vents bigger is better.
