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   Attic Ventilation Design Strategies for Manufactured Homes
You may download the report here


The intent of this project was to investigate attic ventilation strategies that may improve the performance of manufactured homes. Up-to-date, more complete information on the performance of various attic designs for manufactured homes will enable designers to make better informed decisions and provide a foundation upon which to base future changes to the HUD standards.

A committee was formed to guide this research project and to provide input from a variety of perspectives. Committee members were drawn from home manufacturers, ventilation equipment and roofing materials manufacturers, third party regulatory agencies (DAPIAs), academia, and the U.S. Department of Housing and Urban Development. These stakeholders discussed and debated the issues surrounding this research in a series of conference calls, individual communications, and responses to drafts of this document. The document that was created is largely based on the scientific literature reviewed during the course of the project, and represents a general consensus of the Committee. The end-product of this phase of the work is a defined research program assessing specific attic designs that may vary by climate.


Airflow through homes, and its impact on building performance, is one of the least well understood building science phenomenon. For a manufactured home, the air space can be broadly segregated into three regions: the living space, the attic cavity, and the crawlspace (or basement). This paper focuses on the attic airspace. Since the 1940s conventional wisdom has maintained that for proper moisture control in the attic cavity, attics should be ventilated continuously with outside air. In addition to moisture control, other arguments for ventilating attics were subsequently made, including energy conservation, mitigation of high roofing surface temperatures, and prevention of ice dams. While the initial research that established attic ventilation as a standard practice was conducted exclusively in cold and simulated-cold climates, the practice (and building codes to enforce the practice) spread throughout the U.S. into all climate regions. This paper makes the case that attic design in manufactured homes should not be a one-size-fits-all solution and must be examined individually for each climate type.

Attic ventilation has been effective in controlling moisture problems in predominately cold climates where the objective is to maintain cold attic temperatures in winter to avoid ice dams created by melting snow and to vent moisture that moves from the conditioned space into the attic. However, attic ventilation may cause moisture problems in other climates. For example, in hot, humid climates humid outdoor air that comes in contact with cold surfaces in the attic may condense - particularly if low interior temperatures are maintained during summer. Recent research suggests that in hot, humid areas, the best approach to avoiding moisture condensation in attics may be to keep the moisture out of the attic altogether by sealing the attic from the outdoors.

Furthermore, complex ceiling designs in new homes make air sealing between the attic cavity and living space more difficult; and resulting incomplete attic air barriers often allow humid air to contact cooled surfaces potentially causing moisture-related problems like mold and mildew. Ceilings in new homes are often a series of horizontal, vertical, and sloped planes, with mechanical chases, recessed lights, fireplace flues, and penetrations for plumbing, electrical, and space conditioning equipment. In reality, it is often impractical to try to maintain continuity of the air or vapor retarder at all of these locations. Air-tight recessed lights rated for insulation contact, foam sealing of penetrations, and full-depth blown insulation to cover the variations in ceiling plane can help to alleviate the problems, but at significant added cost. The most cost-effective location for moisture and infiltration control and insulation may in fact be at the roof plane rather than the interior ceiling plane, thus providing additional support for sealed, cathedralized attic strategies.

The Manufactured Housing Construction and Safety Standards (MHCSS) requires that attics in all double-section and shingled, single-section homes be ventilated. As noted above, this may not be the practice in many locations.

The published literature relating to attic ventilation suggests that unvented attics can work in all climates if designed and constructed properly. Unvented attics provide the most advantage over vented attics in humid climates. In fact, attic ventilation became a standard practice based solely on research conducted in the 1930s and 1940s in cold and simulated-cold climates. No scientific claims have ever been made for venting attics in hot/humid climates.

Most of the available published research on attic ventilation has focused on site-built homes or generic computer models, however a number of significant studies on HUD-code homes have also been conducted. Some of the most convincing research supporting unvented attics was conducted in part or exclusively on HUD-code homes. This research is summarized in an appendix to this report.

 Whole House Ventilation Strategies
more information about this project