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.

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About James R. Kirby, AIA

James R. Kirby, AIA, is director of Codes and Regulatory Affairs for the Kellen Co. He is a licensed architect in Illinois and has a Master of Architecture—Structures Option and a Bachelor of Science in Architectural Studies from the University of Illinois. Kirby also has a Graduate Certification in Sustainable Building Design and Construction from Boston Architectural College. Kirby has expertise in roof system design and construction, weatherproofing and energy-efficiency of the building envelope, as well as rooftop PV systems. He also is an accredited Green Roofing Professional. Kirby has a strong combination of association experience, as well as roofing expertise, which help serve the Asphalt Roofing Manufacturers Association (ARMA) and the Roof Coating Manufacturers Association (RCMA), among others.

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