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Introduction
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.
Background
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.
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