Roof Ventilation


Watch Video »  |  View a Case Study »

The importance of an attic ventilation system cannot be overemphasized. Ventilation is the most efficient way to remove heat and moisture from an attic space. Proper design and installation of roof vents can make your home more comfortable and prevent a number of roof problems. There are a wide variety of roof vents that are designed to work in conjunction with one another to achieve the required amount of ventilation for most houses.

Why do attics need to be ventilated?

 Unventilated Attic Ventilated Attic
 Summer Temperature 160 F 105 F
 Cooling Costs 15% Higher 15% Lower
 Thermal Expansion of Shingles Greater Reduced
 Moisture Retention in Winter Elevated Minimal
 Living Conditions Uncomfortable Comfortable

  1. Attics that are properly ventilated allow hot air to exhaust from the attic in the summer. The chart above illustrates the advantages of a well-ventilated attic system. Hot air can infiltrate the living space as the attic temperature increases. This makes living conditions uncomfortable and requires the cooling system to work harder. The increased attic temperature also causes greater thermal expansion of the shingles and roof deck. This is a problem that will be discussed in greater detail in regard to the shingle manufacturer’s warranty.
  2. Properly ventilated attics allow warm and humid air to exhaust in the winter. The warm air that is trapped in the attic in the winter months can cause several problems, including condensation and ice dams. Condensation occurs when the moist air from inside the house (often from bathroom vents) comes in contact with the cold attic air. Condensation can cause damage to the insulation, wood framing, wood roof deck, and drywall. Ice dams can be eliminated (in some cases) by improving attic ventilation. For details on the causes and effects of ice dams, click here.
  3. The life of the roof is extended if the attic is properly ventilated. Most shingle manufacturers require proper ventilation to validate the shingle warranty. It is not uncommon for an unventilated roof system to fail prematurely. Thermal expansion (and contraction) of the shingles and roof deck is the major cause. Higher attic temperatures increase the rate of thermal expansion for the shingles. The shingles that are exposed to higher rates of expansion tend to weaken around the fasteners and the adhesive strips. When the roof deck is exposed to higher temperatures, the resulting thermal expansion can cause the shingles to buckle.

Types of Roof Vents

Balanced attic ventilation will consist of two types of roof vent:
  • Air Intake or Soffit Vents (which unfortunately do not exist on some houses)
  • Air Exhaust Vents
The amount of air that flows through the vent is called the Net Free Vent Area or NFVA.

Continuous Soffit Vent

Vented Vinyl Soffit

Rectangular Soffit Vent

If Intake or soffit vents have been installed on a house, they can be found on the lower part of the roof system. The four main types of intake vents are continuous soffit vents, rectangular soffit vents, vented vinyl soffit, and specialty intake vents. Continuous soffit vents are the most desirable. A minimum of 50% of the NFVA should be installed as intake vents, if possible. Intake vents should be installed with air baffles which prevent attic insulation from blocking the airflow. It is important to make sure that soffit vents are unobstructed for them to function properly. To do this, look in the attic on a sunny day — if daylight is visible through the vents, they were properly installed.

Ridge Vent

Gable Vent

Roof Louvers or 750 Vent

Power Vent

Exhaust vents are always installed near the ridge or upper part of the roof system. There are many types of roof exhaust vents, including continuous ridge vents, louvers or 750 vents, gable vents, and power vents. The selection of the exhaust vent is a very important consideration that is usually determined by the architectural style of the house. For homes with good intake ventilation, a continuous ridge vent is usually the best choice. The Owens Corning VentSure continuous ridge vent system allows 20 inches of NFVA per foot, making it one of the most efficient ridge vents on the market today.

How the Vents Work

Builders, Architects, and Roofing Contractors should always try to design a balanced system of intake and exhaust vents. Hot air accumulates in the attic when the sun radiates on the surface of the roof. The hot attic air rises and exhausts through the vents at the upper portion of the roof, creating a suction or negative pressure in the attic. The negative pressure at the exhaust vents causes the cooler outside air to be pulled in through the soffit vents. This process is called natural convection.
Soffit vents do not exist in many homes, even though they are the most important component of a roof ventilation system. The process of natural convection is not efficient if soffit vents are not installed or installed improperly. The following scenario is a demonstration of the convection process with and without soffit vents:
 Exhaust Vents InstalledIntake Vents InstalledAir is Replaced From:Energy Costs
YesYesOutside AirDecreased
YesNoLiving SpaceIncreased
The air intake will always occur at the path of least resistance. When soffit vents are not installed, the path of least resistance can come from interior spaces such as attic or wall penetrations. The most common areas that enable air to escape into the attic include light fixtures, electrical outlets, pipe penetrations, attic hatches, and heating and cooling ducts. When air is pulled out of the living space and into the attic, the heating and cooling system is required to work harder and energy efficiency is decreased.
The process of forced convection occurs when a power vent is installed in the attic. In Cincinnati, it is important to install a power vent with a humidistat and thermostat setting. A power vent such as the Lomanco Model 2000H has settings to automatically start the fan when the temperature is above 90 degrees F. or humidity is above 80%. The power vent is designed to depressurize the attic space by exhausting warm or moist air and replace it with cool, dry air (from the soffit vents). Power vents can also pull air out of the living space if soffit vents are not installed or installed improperly. Power vents are the best choice for some hip roofs since the ridge area is too small to achieve the required NFVA by using a ridge vent or louvers.

Common Problems with Roof Vents

Individuals with an understanding of roof ventilation systems can go to great lengths to design a balance of intake and exhaust and the correct combination of roof vents. Unfortunately, it is not uncommon to see roof vents improperly installed. Here are a few examples:
This photo shows the attic insulation blocking the soffit vents. Typically, baffles are installed to prevent vents from being covered by insulation.
In most cases, vented vinyl soffit is a bad idea. The tiny holes in the soffit panels do not provide enough NFVA. We have seen cases where the vented vinyl soffit is installed over solid wood, eliminating airflow altogether.
This house contains both ridge vents and 750 vents. Only one type of exhaust vent should be installed to ensure the air intake occurs at the lower (and cooler) roof area — the soffit vents. This is known as a short circuit.
This is a picture of a house with a well designed continuous soffit and ridge vent system. The felt paper was installed in a careless manner, covering the vent opening and preventing air flow. The ridge vent served as nothing more than a decoration on this roof.
Attic inspections often reveal that the holes in the roof vents were not cut to the proper size. The desired NFVA is compromised when the vent hole is too small. The photo on the right is a demonstration of the proper hole size for this roof louver.

Designing the Right System

Many older houses have unusual attic spaces that were designed before the movement towards greater energy efficiency had begun. A balance of intake and exhaust ventilation can be very challenging on these types of projects and requires an expert design. Homes that were constructed without intake vents are one of these unusual projects and will be discussed later.
The Asphalt Roofing Manufacturers Association or ARMA outlines three simple steps to properly ventilate an attic space.
  1. Calculate the square footage of the attic floor.
  2. Determine the placement of the intake and exhaust vents.
  3. Calculate the NFVA for each vent.
ARMA recommends an NFVA of 1 square foot for every 300 square feet of attic floor space if both intake and exhaust vents are installed. A typical home with an attic floor space of 2000 square feet will require the 480 square inches of intake vents and 480 square inches of exhaust vents. These calculations were determined by using the 1:300 ratio as follows:
  • 2000 Sq Ft of attic floor space = 288,000 square inches of attic floor space (2000 x 144)
  • 288,000 sq inches/ 300 sq inches = 960 square inches of NFVA required
  • Since it is our goal to achieve a balanced system, 960 / 2 = 480 square inches of both intake and exhaust vents should be installed.
The rectangular soffit vents and the continuous ridge vent are commonly combined to provide a balanced system of intake and exhaust. In the case of the 2000 sq ft attic floor space, a minimum of 18 rectangular soffit vents should be installed (with 28 square inches of NFVA each) and 24 Feet of ridge vent (with 20 inches of NFVA per foot).
The Chart below documents the number of roof vents required in order to achieve a balanced system.

Houses without Soffit Area

The architectural style of some houses makes it is very difficult to achieve a balanced ventilation system. Victorian and Cape Cod houses usually have insulation that is blocking the airflow from the soffit area to the exhaust vents. Removing or decreasing the thermal barrier (insulation) in order to achieve better ventilation is not recommended. For more information on how to ventilate these types of homes click here.

Designing the Right System

The architectural style of some houses makes it is very difficult to achieve a balanced ventilation system. Victorian and Cape Cod houses usually have insulation that is blocking the airflow from the soffit area to the exhaust vents. Removing or decreasing the thermal barrier (insulation) in order to achieve better ventilation is not recommended. For more information on how to ventilate these types of homes click here.
Competent design and installation is the key to maximizing the performance of your attic ventilation.
A Deer Park Roofing Staff member can inspect your house to determine any necessary improvements. You can request an estimate online or email us for more information.

News & Resources

Installed Counter Flashing before solder

Roof Flashing: different types, how to install them, and when you should replace them

If we look at a pie chart of our monthly service requests in Louisville Ky, leaks at roof flashings are the biggest piece of the pie.  A poorly installed flashing repair compromises the integrity of the roof.  At Deer Park…

Seamless gutters

10 Components of a Successful Gutter System

Gutters and Downspouts are designed to drain rainwater away from your home or building without causing damage to your foundation, basement, landscaping, or other building materials.  An effective gutter and downspout system includes a number of different accessories, including splash…

metal thermal movement

Understanding Thermal Movement

Understanding thermal movement in residential metal roof systems is key to long-term performance Roofing contractors often are amazed at the time-tested craftsmanship of sheet-metal systems that are removed from historical structures. It’s not uncommon for some of these systems to…

ice dam gutters

Ice Dams

Several factors contributed to problem roofing area in the photo below. Even though the shingles were in good condition and installed properly, leaks were occurring in the winter months due to ice dams. To remedy the situation, the homeowner installed…

Hail Damage to Roof Systems

Hail Damage to Roof Systems Hail storms can cause significant damage to all types of roofing systems, including asphalt shingles. Hail damage to roofs can be identified by discovering distress marks on the surface of the roof. In the case…

Chimney Flashing Soldering

Solder Versus Caulk

Solder A metal alloy that is used for permanently bonding metal flashings together. The word solder is derived from the Latin term solidare which means “to make solid”. Most solder, especially used in a roofing application, is made up of…

Residential Roof Repair or Replacement

Roof Leaks Roof failures occur for a number of different reasons. The main reasons are poor design, installation errors, age of roof, and flashing defects. The most common types of failures are noticed in roof leaks. These are normally discovered…

Louisville Drip edge

5 Types of Drip Edges on Metal Flashings

5 Types of Drip Edges on Metal Flashings Drip edge can be found on many metal flashings and components that are installed on your roof system, including: Gutter apron Rake edge flashing Gravel stop Chimney caps Coping The outward projecting lower…


Dissimilar Metals in Roofing Applications

Be Cautious When Using Dissimilar Metals in the Same Roof System Many different types of metals are used in various roofing materials, flashings, and fasteners. Problems can occur when two different types of metals (also known as dissimilar metals) are…

Algae on roofs

Algae Roof Stains

Roof discoloration from algae or fungus growth made its way to the Cincinnati area about 10 years ago. It originated in the coastal areas of the southeastern United States and traveled north. The name of the algae is Gloeocapsa Magma.…

Soffit and Eave Intake Vents

Traditional homes, like the one pictured above, contain an overhang near the eave area where intake vents can be installed. The intake vents are the most important component to any roof ventilation system. As warm air in the attic rises,…

Insulating and Ventilating a Cape Cod House

Cape Cod Stye homes are very common in the Louisville KY area. These are typically smaller 1 story homes. Insulating and ventilating cape cod style houses can be very challenging. Anyone who has lived in one of these types of…

Metal Patio Roof – Cincinnati and Northern Kentucky

Deer Park Roofing replaced the asphalt shingles on this patio roof with a beautiful new standing seam metal roof. This customer wanted to remove the asphalt shingles from their patio roof and install a standing seam metal roof. Since the…

Deck Waterproofing Project – Cincinnati and Northern Kentucky

Deer Park Roofing designed and installed this beautiful new deck to create a covered, waterproof patio underneath. This is a photo of the deck that was removed. A new deck is being installed with the goal of achieving a waterproof…

Mold in Attic

A close examination of the photo above reveals large amounts of mold on the underside of the roof sheathing. What caused the mold and why did it thrive in this attic space? 1. The insulation was improperly installed over the…

Shingle Installation Methods: Hand Nailing vs. Nail Gun

Which Shingle Installation Method is Better – Hand Nailing or Nail Gun? The question is often asked as to which nailing method will yield the best results when installing a new roof. Is it better to fasten the shingles by…