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Ventilation Basics

Proper attic ventilation is an important part of a healthy home–both for the structure and its occupants. This document explains how attic ventilation protects a home from moisture and how to install vents that will keep your home in good condition.

There are a wide variety of sources of moisture in a home, from the building materials themselves to normal everyday activities. Cooking, bathing and washing clothes all release gallons of water vapor into the air, for example.

That vapor isn't a problem inside the average home because the temperature inside the home is warmer than outside for much of the year. Warm air holds more moisture, in the form of water vapor, than cool air.

The problem is that vapor gradually works its way out of the living area and into the structure. As warm, moist air cools, the vapor begins to condense into water droplets. If that happens inside an unfinished attic, for example, it can get insulation and framing materials wet. That not only reduces the value of your insulation but can cause mold, mildew and rot.

During the summer, when the outside temperature is typically much higher than the inside temperature, attic ventilation serves a different purpose. An unfinished attic builds up a tremendous amount of heat, and if that heated air has no place to escape, it can make the inside of the house much warmer or cause an air conditioning system to work much harder to cool the house.

Building codes specify the minimum amount of attic ventilation needed in a new home to prevent winter moisture buildup, but your summer needs are much greater. Also, older homes were often built with inadequate attic ventilation–at least by today's standards–and may need to be retrofitted with proper attic ventilation.

A good attic ventilation system is designed for summer needs. It includes two types of vents: intake vents are placed along the soffits to allow fresh air into the attic, and exhaust vents are installed in the upper third of the roof to allow attic air to escape. The object is to create a continuous "wash" of air along the underside of the roof sheathing. The rule of thumb in the summer is that you should provide enough ventilation to completely change the air in your attic every six minutes.

Types of Ventilation

  • Gable vents are triangular vents installed in the gable wall just below the peak of the roof. As a rule, gable vents are the least effective type of vent, because air circulates only near the gables and does not wash the entire roof.
  • Static vents, also known as roof line or eyebrow vents, consist of a sheet metal cylinder with a flashing collar and a metal hood to keep rain out. They are installed in rows along the face of the roof by cutting holes in the roof, nailing the flashing collars to the roof sheathing and shingling around the vents. Their effectiveness depends on how many are installed; probably their greatest disadvantage is that like any roof penetration, they may leak.
  • Soffit vents are made usually with a screen to keep insects out and of an aluminum panel with louvers punched into the face to allow air flow. They may be 4" or 8" wide and 14" or 22" long, so they will fit between 16" and 24" on center rafters. They are installed simply by cutting rectangular holes in the soffits and screwing the vent over the hole.
  • Continuous soffit vent is of similar construction, 4" wide, and 96" long. Cutting a long slot in the soffits and screwing the vent over the hole install it.
  • Circular vents range from 1" to 8" in diameter. Drilling holes in the soffits and pressing the vent into the hole install them.
  • Exhaust vents fall into two basic categories: Static vents simply allow air to escape while Power ventilators actively suck air out of the attic.
  • Ridge vents are installed along the peak of the roof and replace the ridge singles.
  • Power ventilators are turbine vents that consist of a turbine mounted on a sheet metal cylinder. They are installed like roof line vents along the face of the roof. When the wind blows, it spins the turbine, which in turn draws air up out of the attic. Their effectiveness, naturally, depends on whether or not the wind is blowing.

Chimney & Vent Flashing

Because closed valleys aren't visible from the roof, the only sign of damage is usually a leak directly under the valley. A professional roofer should repair this kind of valley.

Vent Flashing from Rosselli Roofing & Siding
Vent Flashing from Rosselli Roofing & Siding
Vent Flashing
Vent Flashing
Custom Copper Chimney Flashing from Rosselli Roofing & Siding
Rubberized Chimney Flashing from Rosselli Roofing & Siding
Custom Copper Chimney Flashing
Rubberized Chimney Flashing

Metal flashing is used to seal out water around the chimney, at vent pipes, along the valleys where two roof pitches meet, and sometimes over exposed windows. To prevent leaks at the flashing, inspect it every spring. If you see thin spots or gaps along a flashing joint, spread roof cement over the entire joint, applying it generously with a trowel. The flashing edge should be covered completely.

Power Vents

 GAF Master Flow Roof Mount Power VentGAF Master Flow Roof Mount Power Vent

  • Made of Aluminum
  • 1/5-hp Motor
  • Reduce Heat Build-up in Your Attic
  • Save Energy Costs and Prolong the Life of Your Roof
  • Weathered Wood Finish
  • Heavy-duty Screen to Keep Animals, Birds and Insects Away

 GAF Master Flow Solar Powered Roof Mount Power VentGAF Master Flow Solar Powered Roof Mount Power Vent

  • No Wiring
  • Brushless Motor
  • No Electricity
  • Extra Wide Flashing
  • Prolongs Roof Life
  • Environmentally-friendly Operation
  • Adjustable Solar Panel for Optimum Exposure
  • Removes Dangerous Heat and Moisture from Attic
  • One Unit Handles Approximately 800 Square Feet of Attic
  • Solar Panel is Hail and Foreign Object Impact resistant

Ridge Vents

A ridge vent runs the entire length of the roof peak, blending into the roof line for a more attractive home. Years of research prove that a ridge vent with external baffles and an internal weather filter, combined with under-eave venting, is the most efficient system you can install.

Ridge Vent on Shingles
Ridge Vent on Cedar Shakes
Ridge Vent on Metal Roof
Ridge Vent on Shingles
Ridge Vent on Cedar Shakes
Ridge Vent on Metal Roof

Ridge Vent Benefits

  • Works Year-round
  • Provides Evenly Distributed Ventilation Along the Entire Underside of the Roof
  • 18 Square Inches of Net-free Area per Linear Foot (depending on type and model)
  • Slim Design, Visual Appealing
  • Provides a Higher Volume of Airflow per Square Foot of Attic Area than any Other Fixed-vent System
  • Design Maximizes Airflow Across the Entire Underside of Roof Sheathing
  • Changes in Wind Direction have No Significant Effect on Vent Performance

Design Considerations

  • Ridge vents must have an external baffle to deflect weather away from the attic and to create low pressure over the vent openings to help pull air out of the attic
  • May not provide enough ventilation area on steep hip roofs
  • With all roofs, install ridge vents along the entire length of ridge for best performance and appearance.
  • With vaulted or cathedral ceilings Each attic [joist] cavity must be ventilated, leaving 1-1/2" between sheathing and insulation
  • Intake vents must be installed to provide proper airflow.

When you have a leak, you often think the roof itself is the culprit. But proper drainage via roof accessories -- gutters and leaders -- is paramount to preventing water from backing up and into your home.

Rosselli Roofing & Siding specialize in difficult assignments, frequently using extraordinary materials and demanding techniques. So, if you're looking for something unique or just a project done correctly, you've come to the right professionals to do the jobWe've Got You Covered!

Call or contact us today for a free estimate or more information.