Ventilation of Steep-Slope Roof Systems and Transitions

By James R Kirby AIA 10-14-2020
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Ventilation for steep-slope roof assemblies is often misunderstood. One must not only understand the code requirements, but be able to translate them into real-world installations.



  • Building codes have requirements for ventilation of steep-slope attics and enclosed rafter spaces.

  • Balanced ventilation — nearly equal amounts of intake and exhaust — typcially provides efficient ventilation.

  • Transitions between low-slope and steep-slope roof areas require more distinct intake and exhaust details than traditional eaves/soffits and ridges.


This blog provides information relating to ventilation for educational purposes only. Designing ventilation to meet the specific needs of a given project remains the responsibility of the architect, specifier, design professional or roofing contractor. Damage due to inadequate ventilation is typically excluded from coverage under manufacturer warranties.


Introduction


Residential attic ventilation was a requirement in the very first edition of the Building Officials Conference of America's (BOCA's) model building code that was published in 1948! Even though this requirement has been around for decades, it is still often misunderstood. Perhaps it's the words used and perhaps it's because the code isn't quite specific enough.


When discussing residential construction, we often hear something like "We need to vent the roof," when we really mean that we need to vent the attic. We don't ventilate steep-slope roofs themselves; we ventilate the space beneath the roof. More specifically, ventilation is needed for the space under the roof system that is above the insulation in the attic floor. That's the space we know most commonly as an attic (when the insulation is located in/on the floor of the attic).


Benefits of attic ventilation


Ventilation of an attic space provides a couple of benefits: it lowers the attic temperature and also helps reduce excess moisture that can accumulate. These benefits occur when the air in an attic space is replaced by outside air that is a lower temperature and has less moisture in it (i.e., lower relative humidity). While this seems obvious for most parts of the US, even in warm, humid locations like Miami and Houston, the majority of the time the ambient air is cooler and contains less moisture than the air in an unconditioned attic.


Code requirements


The International Residential Code (IRC) applies to one- and two-family dwellings, and because of that, most in the roofing industry relate attic and rafter ventilation with residential steep-slope construction, which is a valid and correct presumption. However, the International Building Code (IBC), which covers all buildings other than one- and two-family dwellings (e.g., commercial, industrial, institutional, large residential), also includes information about attic and rafter ventilation because a large number of these types of buildings also include steep-slope roof systems.


To that end, both the IBC and the IRC have requirements that apply to the ventilation of attics and enclosed rafter spaces. These requirements are included in Chapter 8, Section R806, Ventilation in the 2018 IRC, and in Chapter 12, Section 1202, Ventilation in the 2018 IBC. (Free versions of the codes are found here.)


Both the IRC and IBC include nearly identical requirements, albeit the code sections are arranged slightly differently. The following summarizes the requirements:



  • The requirements for ventilation are specific to enclosed attics (insulation on the floor of the attic) and enclosed rafter spaces (where ceilings are applied directly to the underside of roof rafters/framing members and insulation is between rafters above the ceiling).

  • Vents should not allow the entry of rain and snow.

  • Vents are to be protected from the entry of small 'creatures' such as birds and rodents.

  • Corrosion-resistant materials are to be used, and minimum and maximum sizes of vent openings are provided.

  • The minimum net free vent area is 1/150 of the vented space.

  • The minimum net free vent area can be reduced to 1/300 when both of the following conditions are met:

    • In climate zones 6, 7, and 8, a Class I or II vapor retarder1 is installed on the warm-in-winter side of the ceiling (i.e., attic floor).

    • A "balanced ventilation"2 method is used.



1Vapor retarders — An example of a Class I vapor retarder is a polyethylene sheet, and an example of a Class II vapor retarder is kraft-faced fiberglass batt insulation. The polyethylene sheet or the kraft-paper side of the insulation should be installed immediately below the attic floor insulation layer in order to meet the requirements shown above, regardless if it's a traditional attic or an enclosed rafter space. Importantly, but not specifically required in the codes, these vapor retarders should be installed and detailed to also act as air barriers to prevent warm, moist air from the interior spaces from leaking up into the attic.


2Balanced ventilation — "Balanced ventilation" means 40% to 50% of the required ventilation area is located in the upper portion of the attic, and the remainder is used for intake at the eave or within the bottom 1/3 of the attic area. Commonly, exhaust vents consist of continuous ridge vents or static vents no more than 3 feet from the ridge (measured vertically). Intake vents within soffits or eaves are common, and in-plane intake vents (such as GAF Cobra IntakePro®) are used when eaves and soffits are not built to include intake vents.


Current construction methods commonly incorporate the balanced ventilation method for residential attic construction and, therefore, the 1/300 ratio is used to calculate ventilation amounts. The 1/300 ratio means 1 square foot of attic ventilation (evenly split between intake and exhaust) is needed for every 300 square feet of attic floor space.


The intent of the requirements for balanced ventilation is that there is more intake than exhaust. This is quite important! Having more intake than exhaust means there will be proper convective flow from eave to ridge. Because warm, moist air is more buoyant than dry air, the warm, moist air rises and is exhausted at the upper portion of the attic. When there is less intake than exhaust, the lack of intake can "choke" the system, reducing the overall effectiveness of the attic ventilation system.


Balanced ventilation and reroofing


Balanced ventilation is not only important for new construction, but it is an important objective for steep-slope reroofing projects, especially for residential construction. During reroofing, if the amount of exhaust is increased (e.g., by adding a ridge vent with more total exhaust capacity than the previous static exhaust vents), the amount of intake ventilation should be determined and increased as necessary to create a balanced system. If the amount of intake is too little, intake air will come from other sources! A lack of intake at the eave/soffit can lead to air being drawn into the attic from the interior of a residence through can-lights, ceiling vents, and attic-access locations. Believe it or not, air can be pulled from basements and crawl spaces through the cavities in interior walls up into the attic spaces. These "interior" sources of air can contain warm, moist air that can be detrimental to attics, causing condensation and other moisture problems that didn't previously exist. The interior air may not have been drawn into the attic if the system was previously balanced, even if undersized. So, be cautious when increasing the exhaust amounts on existing buildings without assessing the intake amounts. Addressing any 'intake' deficiencies during steep-slope reroofing projects can help ensure that ventilation is balanced and functioning as intended.




This post isn't going to dive into calculating the required amounts of ventilation. To better familiarize yourself with that calculation, use the GAF Attic Ventilation Calculator. The calculator determines the minimum amount of exhaust and intake, and the minimum lineal feet of specific GAF products, such as Cobra Rigid Vent 3 for warmer climates, Cobra SnowCountry for cold and snow climates, and Master Flow Undereave Vents, is provided to meet those calculated amounts per the 1/300 ratio.


Modern changes to construction: Cathedral ceilings


Historically, given that attic ventilation requirements go back decades, the code originally applied only to the traditional attic space under a steep-slope roof — that is, attics with insulation located at the floor of the attic/in the ceiling of the upper floor of a residence. Today, and in the recent past, the traditional attic space is often now a usable, conditioned space. That means the ceiling is attached to the underside of the sloped rafters creating a cathedral ceiling, or some form of that. The traditional attic is turned into occupied space, and the result is an enclosed rafter space. (Remember the code language from earlier that says "attics and enclosed rafter spaces"?)


Chapter 8, Section R806, Ventilation in the 2018 IRC, and Chapter 12, Section 1202, Ventilation in the 2018 IBC provide an option for ventilation when a cathedral ceiling is installed with insulation under the roof deck in the enclosed rafter space. The specific requirement for this type of construction states that there must be a minimum 1" vent space in each rafter space directly beneath the roof deck above the insulation. This can be somewhat difficult to construct and maintain continuous air flow. Also, once constructed, inspection and repair is difficult without removal of interior drywall and/or exterior soffits and eave components. The graphic, from the International Association of Certified Home Inspectors, is an example of ventilation of the construction method that incorporates enclosed rafter spaces.



The 1" minimum required air space (under the deck between the rafters) is considered to be the vented space, and that means the requirements for the protection of openings from snow, rain, and small creatures, as well as corrosion resistance and sizes of vent openings, are applicable.


The minimum net free vent area requirements may also apply when there is a vent cavity/air space under the deck and above the insulation between the rafters. In other words, the vent space size is calculated the same way as the traditional attic space. Specifically, the 1/150 ratio still applies, and in order to reduce the amount of ventilation to 1/300, the additional requirements for Class I and II vapor retarders in Climate Zones 6, 7, and 8, and balanced ventilation also apply. At no time can the vent space between the rafters above the insulation and below the roof deck have less net free vent area than is required for intake and exhaust vents. The depth of the air space may need to be greater than 1" deep to accommodate enough air flow to provide proper ventilation.


For example, if the 1/300 ratio determines that 10 square inches per lineal foot of net free vent area (NFVA) is required, a 1" deep air space is appropriate. However, if 20 square inches per lineal foot of NFVA is required, then a 2" deep air space is needed to provide appropriate air flow. Calculating the required depth of the air space to match the amount of NFVA for eave intake and ridge exhaust should take into account the ratio of rafter-to-open air space for continuous eave and ridge vents.


Tricky transitions


There are many options to vent eaves and ridges on traditional residential construction. However, where a steep-slope roof transitions to a low-slope roof (and vice-versa), the methods to provide intake and exhaust ventilation can be a bit trickier.


Where a low-slope roof abuts the low edge of a steep-slope roof, a good option for intake vents is to use a "deck-level" intake vent, such as GAF Cobra Intake Pro. This type of intake vent is intended for use where there are no eaves or soffits available to install traditional intake vents. Due to the potential for water to build-up at the transition from the low-slope roof to the steep-slope roof due to rain, sleet, or snow, or some combination thereof, it's logical to install a "deck-level" intake vent up-slope at least 2 courses. It is best to locate an intake vent far enough up-slope to help prevent snow from blocking the vents, as well.



The National Roofing Contractors Association (NRCA), in The NRCA Roofing Manual: Steep-slope Roof Systems—2017, provides the following detail for a "Steep- To Low-Slope Roof System Transition." A key element is that NRCA shows the bottom edge of the shingle roof is a minimum of 10" from the low slope transition point. This helps prevent water intrusion through the steep-slope roof. And if the "deck level" intake vent is up 2 courses, the intake is some 20" from the surface of the low-slope roof (albeit measured along the slope, not vertically).



Where a low-slope roof abuts the upper portion of a steep-slope roof, detailing and constructing the exhaust vent is needed in order to properly terminate the low-slope roof. The concept, in general, is to use one-half of a ridge vent, and that likely means this detail is built in place (it does not appear that there are pre-manufactured vent devices for this type of installation). A gap is needed at the top of the sloped deck to allow air to move from the attic or enclosed rafter space up and out the vent material. As shown in the detail below, wood blocking and vent materials are installed on top of and along the upper edge of the steep-slope roof covering. A nailable top layer (e.g., a 2x6) is installed to keep the vent material in place and to act as a nail base for the termination of the low-slope roof.



In addition to the ventilation details needed at these types of transitions, it's important to remember the transition details need to consider the continuation of the water, air, thermal, and vapor boundary conditions. You can refresh your knowledge with this GAF blog post.


What the codes mean but don't say


Simply put, ventilation of attics and enclosed rafter spaces occurs outside of the thermal layer. The code requirements have been developed and instituted based on this, but codes don't explicitly state it. That leads to confusion by some who ask if low-slope roofs need to include ventilation. Let's think about that. For membrane roofs with insulation above the deck (that is, compact roofs), where exactly would the ventilation space be located? Between the insulation and the membrane? That's not how low-slope roofs are constructed. The next possible location for a ventilation space would be under the deck, which means the ventilation is on the conditioned side of the thermal layer for a low-slope, compact roof system, and that is illogical. Expensive conditioned air would easily escape from the building, and unwanted exterior air would easily enter. That would be like leaving doors and windows wide open while air-conditioning or heating a space.


One very important point — even if there was a way to provide intake and exhaust vents as part of a low-slope roof system, a horizontal air space provides no path for warm moist air to rise to an exhaust vent. Another way to say it — natural convective flow does not really happen in a horizontal space.


In conclusion


We ventilate our attics and enclosed rafter spaces to remove unwanted heat and moisture. According to the GAF Pro Field Guide for Steep-slope Roofs, attics can reach up to 165° F, and for asphalt shingles, excessive heat can reduce shingle life. The Guide provides information why venting makes sense, and there are a couple other details available for review and use. Keep your ventilation balanced!

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