In typical residential construction, it has been common for decades to provide thermal protection at the top of the structure by insulating the top-floor ceiling. To prevent a build-up of moisture, the unconditioned attic space is vented to the exterior. In the steep-sloped (greater than 3:12 slope) roof systems commonly used on single-family houses, convection and wind-driven ventilation of the attic can make this an effective approach. However, when a similar design is adopted for structures with low-sloped roofs (such as 0.25:12 slope), convection and ventilation are far less effective, and condensation problems can occur.
Condensation occurs when moist air comes into contact with a surface colder than the air’s dew point temperature. Diagnosing condensation involves determining how moisture is entering the air within a space (particularly if it is added at a greater rate than it is removed) and what causes the surfaces to become cold and prone to condensation. The primary cause of condensation is airflow from warm areas to cold areas, rather than the diffusion of water vapor through permeable materials.
In recent decades, wood framing has been commonly used in construction of low-rise multi-family residential buildings. In many of these buildings, roof structures consist of engineered wood trusses. Attached to the top chord of the trusses is oriented strand board (OSB) sheathing and membrane roofing, while gypsum board is attached to the bottom chord of the trusses and serves as the top-floor ceiling. To meet energy code requirements, batt insulation is placed atop the gypsum board ceiling.
Prior to the adoption of the 2015 International Building Code (IBC), vents were required in the roof assembly to allow the “attic” space—between the underside of the roof deck and the top of the ceiling below—to vent to the exterior. However, the volume of air movement provided by these vents is rarely sufficient to mitigate moisture problems. Further, the air exhausted from the attic is usually replaced by air drawn upward from the interior, which generally has an elevated moisture content.
In typical construction, there are many paths of airflow from the conditioned top-floor spaces and the attic space. If provided at all, the vapor barrier beneath the trusses is rarely airtight. Recessed light fixtures and exhaust ducts penetrate the ceiling and, in some cases, ductwork is positioned above the ceiling.
Depending upon the detailing, partition walls may also create gaps in the ceiling construction. All these components are rarely fully airtight, allowing warm moist interior air to reach the unconditioned attic space, resulting in potential condensation, mold growth, and wood decay on the cold surfaces above the insulation (Figure 1).
In addition to the economics and familiarity of wood framing, code requirements also lead designers to select the assembly described above. Per NFPA 13, Standard for the Installation of Sprinkler Systems, concealed spaces filled with noncombustible insulation are not required to have sprinkler protection, and a 51-mm (2-in.) air gap at the top of the space is permitted. To control condensation, alternative approaches can be considered.
One option involves omitting batt insulation and installing a vapor retarder, rigid insulation, and membrane roof, all above the deck. Another option is to install closed-cell spray foam insulation at the underside of the roof deck. Both approaches increase the surface temperatures within the attic space, but also trigger the need to include sprinkler systems in the attic space.
To avoid the need for sprinklers, a hybrid system can be considered in which batt insulation is used in the attic space along with rigid insulation above the deck or closed-cell spray foam below the deck. In a hybrid system, careful study of predicted surface temperatures and insulation thicknesses in different weather conditions is critical; therefore, it is important for owners to be aware that there may remain some risk of condensation during periods of extremely cold weather.
Kenneth Itle, AIA, is an architect and associate principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation. He can be reached at email@example.com.
Elizabeth Pugh, PE, is an engineer and senior associate with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in building enclosure issues. She can be reached at firstname.lastname@example.org.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier or CSI.