Attics that are vented improperly allow heat and moisture to remain trapped in the attic and create potential problems like wood rot, mildew and deterioration to the home’s structure. The impact of ventilation on the performance of a roof is so great that all major shingle manufacturers VOID their warranties if their shingles are installed over improperly ventilated attics.

To avoid potential damage to your home and finances, it’s important for Kansas City homeowners to know the basics of roofing ventilation, and choose a roofing installation expert like Bordner that knows how to properly ventilate your home’s attic or rafter system.

Please contact Bordner online for more information or a free consultation, or read the sections below to learn more about roofing ventilation.

How Does Condensation Occur?

Condensation – when water vapor cools and becomes a liquid – can occur inside homes and buildings, and often leads to damage and deterioration. Condensation can occur when there is high interior humidity and the temperature of interior building components is at or below the dew point temperature.

Condensation that forms inside home attics and buildings can be caused by the use of washing machines, dish washers, bathtubs, showers, tumble dryers, humidifiers, hot tubs, and swimming pools unless these items are properly ventilated. In many instances the condensation is bad enough to be mistaken for a roof leak.

Condensation can be eliminated by one, or a combination, of the following:

  • Reducing or eliminating the source of moisture.
  • Raise the temperature of interior surfaces above the dew point temperature (this temperature will vary, depending on the relative humidity).
  • Install proper ventilation.

What’s the Best Type of Ventilation?

Attic ventilation is an important part of roofing. Proper attic ventilation extends the life of a roof and reduces problems because it minimizes the temperature differential between the attic and the air outside. Proper ventilation will remove moisture and heat from the attic. Trapped heat and moisture 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° F (65° C).

The best type of roof ventilation for most homes occurs when cooler air can enter the attic through soffit or fascia intake vents. These vents can be seen from the ground and are located behind the gutters. As the cooler air warms and rises, the warm air is vented out through exhaust vents mounted on or near the top of the roof. Both intake ventilation and exhaust ventilation installed at an approximate one to one (1:1) ratio are required for maximum air circulation. One square foot of ventilation per 300 square feet of attic floor space is a general guideline and is a code requirement in some areas.

Without these vents, heat and moisture will build up in the attic area and combine. This combination causes the sheathing to rot, shingles to age prematurely, and the roof insulation to lose its effectiveness.

Avoid Common Ventilation Misunderstandings

There are several common misconceptions about attic ventilation. Many people believe if they have only power vents or turbine vents working near the ridgeline, their attic is properly ventilated. In order for exhaust vents to properly function, they have to have intake vents working with them. Air has to enter somewhere. If there are no intake vents air will enter through some of the other exhaust vents. The result is circulation of only the air immediately surrounding the vents or in between the vents.

Another common misconception is that “more is better.” Many people believe they can improve the ventilation of their attic by installing vents throughout the roof surface. This can cause a “Ventilation Short Circuit.” For instance, if a roof has a full soffit and ridge vent system, and additional vents are installed halfway up the slope, the ventilation is now hampered because air is now exiting out the vents in the middle of the roof before it reaches the ridge, leaving the attic partially un-vented. Depending on wind pressure, air can also be taken in at the intermediate vents reducing the intake at the eaves.

Proper Roof Ventilation Consists of both Intake and Exhaust

To properly ventilate an attic, two types of vents are needed. Intake vents, which are located at the down slope edge of the roof (a.k.a. eaves) and allow fresh cooler air into the attic; and exhaust vents, which are located near or on the ridgeline of the roof and allow warmer air to leave the attic. The use of an exhaust vent in conjunction with an intake vent uses the natural forces of wind pressure and thermal effect, collectively known as the Stack Effect, to ventilate the attic space.

HINT: Make sure your attic insulation doesn’t block the intake vents. If necessary, use baffles to keep the insulation away from the intake vents.

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 has to be replaced. If the proper intake ventilation isn’t used, air will be brought into the attic through the exhaust vents and will, at times, bring moisture with it.

Know Your “Net Free Vent Area”

When determining the ventilation needs of your home, a professional roofer will calculate the required “net free” ventilation space needed for your roof, and select ventilation equipment that will meet these needs. A vent’s effectiveness is measured by its Net Free Vent Area. The Net Free Vent Area is the portion of the opening in the vent that actually ventilates.

For example, a vent can have an opening that measures 12 inches by 12 inches. This would appear as if it would yield 144 square inches of ventilation area. Because of louvers, an insect screen, or some other type of blockage, the actual ventilation area could be as little as 40 percent of that, yielding about 58 square inches of ventilation area. This 58 square inches is what’s known as the Net Free Vent Area, and is the amount that should be used when calculating how much venting you need.

Rafter Ventilation for Existing Buildings

The purpose of rafter ventilation is to reduce or eliminate heat build-up, and reduce or eliminate water vapor that can condense in the ventilation spaces.

Achieving rafter ventilation can be difficult or impossible for existing buildings, and can involve significant costs. However, every attempt should be made to design and install an adequate ventilation system.

Free Roof Inspection

To learn more about the condition of your roofing and ventilation, contact Bordner online or at 816-358-2102 to schedule a free, no obligation inspection, consultation and estimate.