Attic Ventilation in Accessory Structures

Construction Code Requirements for Proper Attic Ventilation Should Not Be Overlooked in Buildings That Don’t Contain Conditioned Space

The 2015 International Residential Code and International Building Code, published by the International Code Council, include requirements for attic ventilation to help manage temperature and moisture that could accumulate in attic spaces. Although the code requirements are understood to apply to habitable buildings, not everyone understands how the code addresses accessory structures, like workshops, storage buildings, detached garages and other buildings. What’s the answer? The code treats all attic spaces the same, whether the space below the attic is conditioned or not. (A conditioned space is a space that is heated and/or cooled.)

The 2015 International Residential Code and International Building Code include requirements for attic ventilation to help manage temperature and moisture that could accumulate in attic spaces. Although the code requirements are understood to apply to habitable buildings, not everyone understands the code also addresses accessory structures, like workshops, storage buildings, detached garages and other buildings.

The 2015 International Residential Code and International Building Code include requirements for attic ventilation to help manage temperature and moisture that could accumulate in attic spaces. Although the code requirements are understood to apply to habitable buildings, not everyone understands the code also addresses accessory structures, like workshops, storage buildings, detached garages and other buildings.


The administrative provisions of the IRC that set the scope for the code are found in Chapter 1. Section R101.2 and read:

    The provisions of the International Residential Code for One- and Two-family Dwellings shall apply to the construction, alteration, movement, enlargement, replacement, repair, equipment, use and occupancy, location, removal and demolition of detached one- and two-family dwellings and townhouses not more than three stories above grade plane in height with a separate means of egress and their accessory structures not more than three stories above grade plane in height.

Let’s clear up any confusion about the code. The ventilated attic requirements in the 2015 IRC include the following language in Section R806.1:

    Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow.

An accessory structure is actually defined in the IRC:

    ACCESSORY STRUCTURE. A structure that is accessory to and incidental to that of the dwelling(s) and that is located on the same lot.

The IBC also includes attic ventilation requirements that are essentially the same as the IRC. Section 101.2 of the 2015 IBC contains this text:

    The provisions of this code shall apply to the construction, alteration, relocation, enlargement, replacement, repair, equipment, use and occupancy, location, maintenance, removal and demolition of every building or structure or any appurtenances connected or attached to such buildings or structures.

This requirement for ventilated at-tics in accessory structures in the IBC and IRC is mandatory unless the attic is part of the conditioned space and is sealed within the building envelope. Unvented, or sealed, attics allow any ducts located in the attic to be inside the conditioned space, which can have beneficial effects on energy efficiency. For accessory structures, which are typically unheated, that provision does not apply.

It’s important to note the codes do contain detailed requirements for the design and construction of sealed at-tics to reduce the chance of moisture accumulation in the attic. These requirements have been in the codes for a relatively short time and remain the subject of continued debate at ICC as advocates of sealed attics work to improve the code language in response to concerns about performance issues from the field.

Traditional construction methods for wood-framed buildings include ventilated attics (with insulation at the ceiling level) as a means of isolating the roof assembly from the heated and cooled space inside the building. Attic ventilation makes sense for a variety of reasons. Allowing outside air into the attic helps equalize the temperature of the attic with outdoor space. This equalization has several benefits, including lower roof deck and roof covering temperatures, which can extend the life of the deck and roof covering. However, it is not just temperature that can be equalized by a properly ventilated attic. Relative humidity differences can also be addressed by vented attics. Moisture from activity in dwelling units including single-family residences and other commercial occupancies can lead to humidity entering the attic space by diffusion or airflow. It is important to ensure moisture is removed or it can remain in the attic and lead to premature deterioration and decay of the structure and corrosion of metal components, including fasteners and connectors.

In northern climate zones, a ventilated attic can isolate heat flow escaping from the conditioned space and reduce the chance of uneven snow melt, ice dams, and icicle formation on the roof and eaves. Ice damming can lead to all kinds of moisture problems for roof assemblies; it is bad enough that roof assemblies have to deal with moisture coming from inside the attic, but ice damming can allow water to find its way into roof covering assemblies by interrupting the normal water-shedding process. For buildings with conditioned space, the attic can isolate the roof assembly from the heat source but only if there is sufficient ceiling insulation, properly installed over the top of the wall assemblies to form a continuous envelope. Failure to ensure continuity in the thermal envelope is a recipe for disaster in parts of the country where snow can accumulate on the roof.

Accessory buildings, like workshops, that occasionally may be heated with space heaters or other sources are less likely to have insulation to block heat flow to the roof, which can result in ice damming. Ventilating the attic can prevent this phenomenon.

Accessory buildings, like workshops, that occasionally may be heated with space heaters or other sources are less likely to have insulation to block heat flow to the roof, which can result in ice damming. Ventilating the attic can prevent this phenomenon.


For unheated buildings in the north, ice damming is less likely to occur, unless the structure is occasionally heated. Accessory buildings, like workshops, that might be heated from time to time with space heaters or other sources are less likely to have insulation to block heat flow to the roof. In these situations, a little heat can go a long way toward melting snow on the roof.

While the ice damming and related performance problems are a real concern even for accessory structures, it is the removal of humidity via convective airflow in the attic space that is the benefit of ventilated attics in accessory structures. We know that moisture will find its way into buildings. Providing a way for it to escape is a necessity, especially for enclosed areas like attics.

There are many types of accessory structures, and some will include conditioned space. Depending on the use of the structure, moisture accumulation within the building will vary. For residential dwelling units, building scientists understand the normal moisture drive arising from occupancy. Cooking, laundering and showering all contribute moisture to the interior environment.

The IRC and IBC include requirements for the net-free vent area of intake (lower) and exhaust (upper) vents and also require the vents be installed in accordance with the vent manufacturer’s installation instructions. The amount of required vent area is reduced when a balanced system is installed; most ventilation product manufacturers recommend a balance between intake and exhaust. The IRC recommends that balanced systems include intake vents with between 50 to 60 percent of the total vent area to reduce the chance of negative pressure in the attic system, which can draw conditioned air and moisture from conditioned space within the building. This is less of an issue for non-habitable spaces from an energy-efficiency perspective, but moisture accumulation is a concern in all structures.

PHOTOS: Lomanco Vents

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Improve Attic Ventilation Airflow

GAF has made available its Cobra IntakePro Rooftop Intake Vent, which promotes energy efficiency and helps guard against roof system rot, ice damming and mildew growth.

GAF has made available its Cobra IntakePro Rooftop Intake Vent, which promotes energy efficiency and helps guard against roof system rot, ice damming and mildew growth.

GAF has made available its Cobra IntakePro Rooftop Intake Vent, which promotes energy efficiency and helps guard against roof system rot, ice damming and mildew growth. It can also improve attic ventilation airflow by up to 20 percent when installed with existing soffit/undereave vents in a properly balanced ventilation system (based on GAF ASHRAE airflow testing at 5-mph wind speed). The vent, which rolls out and fastens with included 1 3/4-inch coil nails, is fully tested to withstand wind-driven rain, snow and ice-dam infiltration (under controlled GAF laboratory testing).

Asphalt Roofing Provides Comprehensive Weather Protection for a Luxury Condominium

Working with the unique variables of a region and its climate poses a significant challenge to roofing contractors. Installing a roof system that looks beautiful and can stand up to ice, snow and freezing temperatures takes an expertise that only comes with experience. This is the case in the snowy and picturesque mountains of Park City, Utah. The city is a winter haven for skiers who vacation there, but the extended snow and cold season can deliver a beating to a roof.

The concrete tile roof and poorly ventilated deck were causing major problems for the building owner, not to mention the residents who live and rent there. Heat was escaping through the roof, causing the snow to melt and refreeze at the eaves.

The concrete tile roof and poorly ventilated deck were causing major problems for the building owner, not to mention the residents who live and rent there. Heat was escaping through the roof, causing the snow to melt and refreeze at the eaves.


The Grand Lodge at Deer Valley Resort, a luxury condominium development at one of North America’s top-ranked ski resorts, sits at an elevation of 9,000 feet in the mountainous area. Along with the breathtaking views comes an average annual snowfall of 350 inches. As a result, the 40,000-square-foot concrete tile roof of the lodge had begun to fail after only six years of intense weather and needed to be replaced immediately.

The concrete tile roof and poorly ventilated deck were causing major problems for the building owner, not to mention the residents who live and rent there. Heat was escaping through the roof, causing the snow to melt and refreeze at the eaves. Dangerous icicles would form, and noisy chainsaws were frequently needed to cut through the ice on the 5-story building. In 2013, the owner of the condominium decided to completely redesign the roofing system. IronClad Exteriors Inc., a Sandy, Utah-based roofing company had successfully installed roofs for Deer Valley in the past and was contacted by Deer Valley Resort Management to consult on the new design.

“Due to insufficient insulation and poor ventilation on the existing roof, ice dams were forming, tiles were cracking and the roof was falling apart,” says Eric Kircher, owner of IronClad Exteriors. “There was an architect involved in redesigning the roof … . I was asked to take a look at the design, and I recommended an asphalt shingle roof instead.

Kircher advised that a full asphalt roofing system with proper insulation and moisture protection would be able to withstand the harsh weather of the ski resort.

“Asphalt was the ideal material for the reroof for reasons that involve aesthetics, safety, and the long-term health and viability of the roof,” he notes. “I recommended a shake style because it really fit the architecture and look of the roof while being able to protect the building and residents from the weather conditions.”

Over the span of six months, IronClad Exteriors tore off the tile roof and installed a system they had used many times to help homeowners in the area protect their homes from ice and snow.

Over the span of six months, IronClad Exteriors tore off the tile roof and installed a system they had used many times to help homeowners in the area protect their homes from ice and snow.

Installation

Over the span of six months, IronClad Exteriors tore off the tile roof and installed a system they had used many times to help homeowners in the area protect their homes from ice and snow. FlintBoard ISO NB (Nail Base) Composite Polyisocyanurate/OSB Roof insulation was installed over the plywood deck, followed by a 3- by 10-inch fascia board. WinterGuard HT advanced waterproofing underlayment and DiamondDeck High Performance Synthetic Underlayment were then added to provide important moisture resistance. Finally, the Presidential Shake TL asphalt shingles provided a beautiful look that matched the lodge’s breathtaking surroundings. The project was completed in November 2014.

The Grand Lodge’s new asphalt roof also contains a unique feature that sets it apart in form and function. IronClad installed 11,000 copper snow guards that offer another layer of weather protection. Snow freezes around the copper pieces and keeps it from sliding down the roof to form dangerous ice dams at the eaves. Lodge residents no longer have to walk underneath potentially hazardous icicles or listen to the sounds of manlifts and chainsaws that are used to remove them.

“The roofing system we designed had the unique ability to withstand that type of cold environment,” Kircher notes. “There will be no heat loss contributing to ice and snow on the eaves, and the insulation protects the interior of the lodge. These are high-end condominiums with finished ceilings and no attic space at the top where you can put more insulation, so the insulation had to be installed on the existing roof deck to prevent ice dams.”

The installation process went smoothly despite the challenges brought on by Park City’s weather. Snow can begin to fall as early as September and lasts through the spring, providing little time for construction projects to take place. Fortunately, IronClad had extensive experience with the roofing systems needed in Park City.

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Snow-retention System Designed Specifically for New Elementary School

Alton Hall Elementary School, Galloway, Ohio, recognized the need for snow retention and specified the Sno Barricade from Sno Gem Inc. to be attached to the standing-seam roof.

Alton Hall Elementary School, Galloway, Ohio, recognized the need for snow retention and specified the Sno Barricade from Sno Gem Inc. to be attached to the standing-seam roof.

Providing a safe and healthy environment for students is clearly a high-ranking consideration in the construction of an elementary school. Architects for the Alton Hall Elementary School in Galloway, Ohio, recognized the need for snow retention and specified the Sno Barricade from Sno Gem Inc. to be attached to the standing-seam roof.

“We specified the Sno Barricade because of its proven durability and performance,” says Mike Parkinson, associate project manager at SHP Leading Design of Cincinnati. “We’ve used the Sno Barricade on dozens of projects. I can’t remember the last time it wasn’t on one of our projects. The system is designed specifically for each project by Sno Gem. With the design criteria, they run calculations for the project and prescribed a two-rail system around the entire roof to protect the occupants from potential sliding snow and ice.”

With a layout of the standing-seam metal roof, considering slope, length of run, panel width, annual snowfall and other factors, Sno Gem calculates the best snow-retention solution. “Every metal roofing layout is different and each one requires its own calculations,” notes Jim Carpenter, vice president of Operations at Sno Gem. “Our calculations are based on results obtained from extensive testing of our clamps.”

After receiving the design criteria, Sno Gem ran calculations for the project and prescribed a two-rail system around the entire roof.

After receiving the design criteria, Sno Gem ran calculations for the project and prescribed a two-rail system around the entire roof.

For the Alton Hall Elementary School, the Sno Barricade was prescribed by Sno Gem. Rush Architectural Metal Erectors Inc. of Washington, Pa., installed 1,850 linear feet of the Sno Barricade around 100 percent of the perimeter of the building. R.A.M.E. also installed the Barricade Plate on Alton Hall. The Barricade Plate is an accessory designed to hold back thinner amounts of ice and snow that could pass beneath the bar. The Barricade Plate is installed on the upslope side of the bar in the middle of the panel. It’s held in place by a tek screw, not visible from the ground. Like the Sno Barricade, the Barricade Plate is available in a color to match any roofing panels.

The Barricade Plate is an accessory designed to hold back thinner amounts of ice and snow that could pass beneath the bar.

The Barricade Plate is an accessory designed to hold back thinner amounts of ice and snow that could pass beneath the bar.

“Sliding snow and ice is a dangerous problem building owners don’t have to deal with any more because of engineered snow-retention systems,” adds Albert Rush, owner of R.A.M.E. “The Sno Barricade attaches easily and securely without penetrating the panel, so it doesn’t compromise any roofing warranties. The addition of the Barricade Plate provides peace of mind for the occupants, as well as the school district.”