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 Minimizing Moisture Problems in Manufactured Homes Located in Hot, Humid Climates:
Response of Interior Air Pressures to Various Operating Conditions
You may download the report here (765 kb)

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:
  1. Duct leakage

  2. Shell leakage

  3. Interior airflow

  4. Exterior air barriers

  5. Air supply balance

  6. Return air handler grille opening size

  7. Exhaust fan operation

  8. 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.

 Moisture Guide
more information about this project

 Measured Permeance Values
more information about this project