Avoiding
moisture-related problems - such as material degradation and staining - is a challenge
in all types of structures, particularly those located in the hot, humid Gulf
Coast region of the U.S. As part of a multiphase research program to better understand
the underlying causes of moisture-related problems in manufactured homes and to
develop effective abatement strategies, a series of tests were conducted on two
manufactured homes to profile airflows under varying conditions. Air movement
is a major mechanism for moisture transport. Therefore, understanding how a home's
design, construction and operation impact airflow is an important step in developing
moisture control
The goal of this phase of the research was to develop
sets of conditions that would result in a home having a neutral to slightly positive
pressure relative to the outside. Such conditions create a barrier to air, and
therefore moisture, movement from outside into the living spaces and the building
cavities.
Data was collected from a single and a double section home, of
typical design and configuration. Air pressures were logged continuously in various
spaces and cavities as a number of conditions in the homes were varied. Parametric
analysis was used to study the impact of eight factors on the homes' pressure
profiles. These eight factors, which together are largely responsible for airflow
within the home and between the home and the outside, are: - Duct leakage
- Shell leakage
- Interior airflow
- Exterior
air barriers
- Air supply balance
- Return air handler
grille opening size
- Exhaust fan operation
- Positive
ventilation
As a result of the testing, the following conclusions
and recommendations are made: - Duct leakage and shell leakage have
the most dramatic effects on a home's pressure profile.
- As duct
leakage increases, most pressures decrease and pressure imbalances between spaces
are exaggerated. Duct leakage should be limited to 5% to the outside to avoid
severe negative pressures within the home relative to the outside.
- Tight envelopes exacerbate negative and imbalanced pressures,
although it is possible to achieve near-neutral pressures with
a shell as tight as 7.2 ACH50 (estimated as 0.29 achnatural)if
ducts are tight and adequate return air pathways are provided.
Tight shells (a maximum of 8.8 ACH50, estimated as 0.35 achnatural)
should be maintained to control airflow through the envelope.
- Of the factors
that are controlled by the homeowner, those that have the biggest positive impact
on pressure balance within the home are maintaining adequate return air pathways
(such as keeping bedroom doors open if adequate return air grilles or jumper ducts
are not provided) and limiting the use of exhaust ventilation fans to only when
they are needed. In the event that adequate return air pathways are not provided,
closing of bedroom doors, in combination with a tight shell, is enough to create
significant negative pressures within the home. The size of return air pathways
from bedrooms should be maximized.
- Exhaust fan operation should
be limited to only when it is necessary to exhaust interior moisture.
- An
exterior wall sheathed with an effective air barrier can significantly affect
the pressure gradient across the wall, as compared to a wall cavity that easily
exchanges air with the exterior. Sheathing of exterior walls to create an air
barrier should be pursued to minimize intrusion of humid outside air.
- While
air distribution systems in manufactured homes are in some cases unbalanced, it
does not appear that balancing them will appreciably reduce negative or unbalanced
pressures within the home as a whole.
- Positive ventilation can
play a major role in creating a balanced and positive pressure profile in the
home. Future research is needed to identify the best strategies for accomplishing
this goal.
The two homes tested, both of which are from a single manufacturer,
are representative of homes produced in this industry, and thus the recommended
operating parameters are expected to achieve the desired pressure profile for
most HUD-Code homes.
Future testing will validate these recommendations,
measure actual moisture levels in homes built to effectively manage air pressures
and also provide a set of suggested practices for manufacturers, installers and
homeowners.
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