Roof Ventilation: The Essential Explanation

Attic ventilation is an integral part of roofing. Proper attic ventilation extends the roof’s life and reduces problems because it minimizes the temperature differential between the attic and the air outside.

A proper roof ventilation system will remove moist air and heat from the attic. Trapped moisture and heat can raise energy costs, cause ice dams, and damage roof system components as well as structural and personal items located inside the attic where temperatures can easily reach 150 degrees Fahrenheit (65 degrees Celsius). Condensation that forms inside attics can be caused using washing machines, dishwashers, bathtubs, showers, and tumble dryers unless these items are properly ventilated through the roof. In some cases, the condensation can be bad enough to be mistaken for a roof leak.

Here are 8 problems associated with an improperly ventilated attic space:

1. Sumps between rafters can happen after several years, or sometimes only a few years. A plywood roof deck (or roof sheathing) can warp or deteriorate and become spongy and dangerous to walk on. This occurs because one side of plywood decking needs to “breathe” with proper airflow by being exposed to circulating air. The adhesives used in the plywood can deteriorate, or dry rot can occur because of condensation.
2. Water vapor will condense first on anything metal inside the attic, eventually causing the metal to rust. Heads can rust off nails. Metal plumbing straps or straps holding HVAC ducting can rust and break into two, causing the ducting to crash down on top of the ceiling joists or through a suspended ceiling. This problem is more common in humid climates than in Colorado.
3. In colder climates – generally where the average January temperature is 32 degrees Fahrenheit (0 degrees Celsius) or colder – high inside humidity (40% or greater) combined with low outside temperatures can cause frost to form on the bottom of the roof deck.
4. Insulation can trap moisture, which will reduce the insulation’s R-value and create a welcoming environment for propagating certain molds, spores, and fungi, which can also cause problems.
5. Mildew can do damage and cause health problems inside your living space.
6. The roofing system, such as an asphalt shingle roof, will deteriorate prematurely, impacting its lifespan.
7. Air conditioning cooling units will need to be serviced or replaced prematurely because of excessive use.
8. Ice dams result from melting snow continually refreezing at the roof perimeter and then backing up under the roof shingles and causing leaks. Proper ventilation used in conjunction with heavy insulation and an air barrier can create a Cold Roof Assembly, eliminating ice building or ice damming.

Explaining Ventilation

Everything You Need to Know About Roof Vents and How They Work

There are many types of attic vents available, including static vents, power vents, ridge vents, turbine vents, soffit vents, cornice vents, gable vents, starter vents, and cupola vents. These all come in a wide variety of sizes, styles, and shapes. Some will ventilate better than others, depending on the roof configuration, attic size, climate, etc.

To properly ventilate an attic, two types of vents are needed. Intake vents, located at the roof’s down-slope edge (also known as eaves), allow fresh, cool air into the attic. Exhaust vents, which are located near or on the roof’s ridgeline, let hot air to leave the vented attic.

In conjunction with an intake vent, an exhaust vent uses the natural forces of wind pressure and thermal effect, collectively known as the stack effect, venting the attic space.

It is essential to make sure your attic insulation does not block the intake vents.

A vent’s effectiveness is measured by its net free vent area, which is the portion of the vent’s opening that ventilates. A common rule of thumb is the 1/300 rule, which means 1 square foot of net free vent area per 300 square feet. There is often a concern for the best type of ventilation – and it’s important to understand that a home needs both intake ventilation and exhaust ventilation installed at an approximate one-to-one (1:1) ratio. The idea behind this is for maximum air circulation. Installing more than 1 square foot of ventilation per 300 square feet of attic space won’t hurt; it’s a general guideline and code requirements in some areas. Many roofing professionals will agree that the best type of ventilation is continuous soffit and ridge ventilation. If a continuous exhaust vent and an equal or slightly greater amount of intake vent are installed, the attic will be ventilated for its entire length.

There are several common misconceptions about attic ventilation. Many people think if they have only power vents or turbine vents working near the ridgeline, their attic is adequately ventilated. For an exhaust vent to function correctly, however, it must have intake vents working with it. If there are no intake vents, the outside air must enter somewhere, so it will enter through some exhaust vents, and exit through others. The result is the circulation of only the air immediately surrounding the vents or in between the vents.

Another common misconception is “more is better.” Many people think they can improve the ventilation of their attic by installing vents throughout the roof surface. Unfortunately, professional roofing contractors know that installing vents halfway up the roof’s slope can hamper ventilation because warm air is now exiting the vents in the middle of the roof before reaching the ridge, leaving the hot attic partially unvented. Depending on wind pressure, air will also be taken in at the intermediate vents, reducing the eaves’ intake, where it should be.

There is also the problem of weather infiltration. Wind blowing across a roof surface creates negative air pressure. Nature will automatically try to compensate for it by moving air from a location of higher pressure, such as inside the attic. When the air is removed from the attic in this manner, it must be replaced. Without the right amount of intake ventilation, air will be brought into the attic through the exhaust vents and will, at times, bring moisture with it.

When determining ventilation, there are a few critical summary points:

• Intake and exhaust ventilation should be installed at an approximate one-to-one (1:1) ratio. More at the eaves is better if it can be attained.
• The 1/300 rule – 1 square foot of attic ventilation (net free vent area) per 300 square feet of attic floor space.
• No attic vents should be installed between the intake and exhaust vents.
• There should be at least three feet of vertical distance between the intake vents and exhaust vents.

Attic Insulation is Integral to Your Entire Roofing System

Energy used in homes and buildings is the number-one source of greenhouse gas emissions – and in an under-insulated home, most of a home’s heat escapes through the attic and crawl space. Adding attic insulation and crawl space insulation is one of the easiest and most cost-effective ways to make your home more energy-efficient and improve your utility bills’ energy savings. Plus, it helps to reduce greenhouse gas emissions.