• Building Science and Moisture Problems in Manufactured Housing

Figure 35 Palm Harbor HUD Code Manufactured Housing factory – production line.

Figure 36 Completed HUD Code Manufactured Home, Palm Harbor Homes

Manufactured homes have a permanent steel chassis attached below the floor and are constructed in a factory (Figure 35) to meet a national code maintained by the U.S. Department of Housing and Urban Development (HUD). After production, homes may travel a few hundred miles, hauled by truck, before final setup. The homes are setup by placing blocks under the steel I-beams and anchoring the beams firmly to the ground. A skirting covers the blocks and steel frame in a fully setup home (Figure 36).

Manufactured homes are typically heated or cooled by a system of ductwork, which delivers hot or cold air from the air handler unit (AHU). The ductwork can be in the attic or in the belly cavity of the home. The ducts are typically made of aluminum or fiberglass trunk lines which supply air to the floor registers through in-line boots or flex ducts. The boots or ducts terminate at perimeter registers on the floor. Supply duct leaks represent one of the biggest causes of moisture problems in manufactured homes. (Figures 37 and 38). Poor design and construction leave holes at the AHU connection to the main trunk, and where the boots connect to the trunk, supply registers, end caps, cross-over duct connections, and other connection points. When the AHU blows air, some air leaks into the belly and eventually to the outside through belly board tears. This loss of air creates a negative pressure inside the house and a positive pressure in the belly. The negative pressure pulls outside or attic air into the house through cracks and crevices which connect the inside of the house to the outside or to the attic. During northern winters, this outside air is cold and dry and its entry increases occupant discomfort and heating energy use.

Figure 37 Pressure field and unintentional air flow created by supply duct leaks.

During summer in the Southeastern US, the air is consistently at or above the dewpoint of 75. If a homeowner keeps their home thermostat set below this 75 F dewpoint, the moisture laden outside air condenses as it comes into contact with the cold inside surfaces. If it condenses behind an impermeable surface such as vinyl flooring or wallpaper, serious mold, mildew, and floor buckling problems can result.

Many manufactured and site-built homes have only a single return and, therefore, very little return air transfer from the bedrooms (basically via the undercut at the bottom of interior doors). When interior doors are closed, rooms off the main body (e.g., bedrooms) become pressurized and the main body of the house depressurizes. Even though negative pressures are usually only one to three pascals (Pa) - they can cause serious problems in a home.

Researchers use a calibrated fan called a ductblaster to measure duct leakage. The ductblaster is attached to the return grill or the crossover duct opening (Figure 39) and all supply registers are masked off and the fan is turned on. Once the house ductwork reaches –25 Pa, airflow through the fan is read (in CFM). The resultant measure is the total duct leakage. In good airtight ductwork, total duct leakage (CFM@25 Pa) should be less than 6% of the homes square footage.

Figure 38 Cross section showing foundation support, crossover duct, and one type of ventilation system in a manufactured home.

A second duct leakage test measures leakage to the outside. This leakage is calculated by depressurizing the entire house to –25 Pa with a blower door, then adjusting the ductblaster flow so there is no pressure difference between the house and the ducts. This measurement is a true indicator of duct air loss to the outside and is used in energy calculations for estimating the energy loss from leaky ducts. In good duct systems, duct leakage to the outside (in CFM) is less than 3% of the home’s square footage.

Figure 39 Floor and belly area with supply ducts. These ducts supply conditioned air to all rooms through floor vents, a common duct system layout in manufactured homes.

The battery of tests run in a problem house typically includes measuring the airtightness of the house with a blower door, depressurizing the house to –50 Pa. At that time, the house to belly and belly to crawlspace pressures also can be measured. Researchers also test pressure differentials caused by AHU operation and closed interior doors. An additional measurement of duct leakage, called pressure pan, is conducted on some houses to pinpoint specific registers which might have large leaks. In this measurement the house is first depressurized to –50 Pa and all the register vents are unmasked. Then the registers are covered one by one and the pressure difference between the covered register and the house is measured. A zero reading indicates no leakage at that register. Readings over one Pa indicate a sizeable leak that should be repaired.

• BAIHP Field Visits to Moisture Problem Homes

A significant number of new manufactured houses built to HUD code and located in the hot, humid Southeast have exhibited moisture problems. Soft wallboards, buckled floors, damaged wood molding, and extensive mold growth are the most common symptoms. These problems do not respond to the standard service and repair strategies for water intrusion. (Please see Appendix B for sample problem home inspection trip reports.)

Summary of 1 st-4 th Budget Period Field Visits to Moisture Problem Homes

At the request of six manufacturers, 69 such moisture damaged homes were investigated from 1999 to the end of reporting year four (through March 31, 2003) to determine likely causes. In Year 4 alone, 18 homes were investigated by FSEC. One-time blower door, duct tightness, and pressure differential measurements were performed on all homes. Field data on ambient, crawlspace, belly and house temperatures, plus relative humidity levels were collected on a few of the homes. Recommendations and reports were prepared for the manufacturers’ service, production, and design staff. Field repairs were performed in most of these homes. A general theme was found in the houses investigated.

• Air conditioner thermostat settings (typically 68 to73 F) set below the ambient dew point.
• Negative pressures across the envelope from high supply duct leakage (CFM @25Pa >10 per 100 square feet of conditioned floor area), inadequate return air paths, interior door closures, exhaust fans, or a combination thereof.
• Inadequate moisture removal from disconnected return ducts, continuous fan operation (air handler or ventilation), inadequate condensate drainage, oversized air conditioners, or a combination thereof.
• Moisture diffusion from the ground into the house because of poor site drainage, inadequate crawl space ventilation, tears in the belly board, or a combination thereof.
• Vapor-retardant in the wrong location (i.e., vinyl or other impermeable wall or floor coverings located on the colder surfaces).

Recommended solutions provided to the manufacturers to eliminate moisture problems included:

• Maintain air conditioning thermostat settings above the ambient dew point (at least 75 F).
• Eliminate long-term negative pressures created by air handler fans or ventilation equipment.
• Tightly seal all ductwork and provide adequate return air pathways.
• Enhance moisture removal from the conditioned space by correct equipment sizing and maintenance.
• Eliminate ground source water and provide an adequate moisture barrier for the floor assembly.
• If possible, remove vapor barriers located on the wrong surfaces.

Research continues to determine if these steps will be sufficient to prevent problems even when vapor barriers are incorrectly located in homes in the hot, humid climate. Preliminary results are encouraging. One manufacturer has not reported a single new moisture problem in any of the homes produced since 2000 in a factory that previously had a significant number of problem homes. Steps taken by the factory were inclusion of airtight duct systems (a zero net-cost increase), right-sized cooling systems (a negative cost), return air ducts from all bedrooms (a cost of about $15), installation of a ground vapor barrier (no change from previous practice).

Figure 40 Flow lines under house, indicating running water under the house. Also note the “tide line” on the support column.

Summary of 5 th Budget Period Field Visits to Moisture Problem Homes

BAIHP researchers at FSEC received fewer requests in the 5 th budget period for assistance with moisture damaged homes (Table 22), reflecting improvement of duct construction and sealing, addition of return air pathways from bedrooms, and reduction of vapor impermeable interior surfaces. Additionally, service personnel who have attended BAIHP training and participated in
field work with BAIHP are more prepared to resolve problems without assistance. Service personnel report installing passive return air vents in bedrooms, providing appropriate moisture barriers, and sealing duct leaks to resolve humidity, comfort, and moisture damage call backs.

When service personnel have been unable to resolve a problem, they request assistance from BAIHP researchers who attend a service call and conduct various diagnostic tests to identify factors contributing to the moisture, comfort, or high energy bill problem. (MHRA has been providing similar services on a fee basis to the industry also.) After BAIHP researchers complete a field visit, a trip report is issued detailing the findings and recommendations, include basic building science background material.

It has been BAIHP’s experience that corrective measures from repeated moisture problem Diagnostics have been incorporated into the production process, resulting in thousands of improved manufactured homes. These are noted in Category D of Table 2.

A common problem that remains unresolved involves the combination of abundant crawl space moisture (Figure 40 and 41) and poorly vented skirting (Figure 42). In the hot-humid coastal regions, this combination raises vapor pressure across the belly to critical levels. This was evident in several of the homes visited this year. As a result of this field research, BAIHP has designed a study that will be initiated in the summer of 2004 to evaluate the moisture flow characteristics of crawl space conditions.

Figure 41 The downstream exit for the water draining across the site via the crawl space. Note flow pattern away from house.

Figure 42 HUD Code required perforations in skirting may not allow adequate volumes of ventilation, creating higher than usual vapor pressure difference across the floor assembly even though the ground cover and belly board are in good condition.

WSU Field Visits to Problem Manufactured Homes

In offering technical support to owners of over 100,000 homes built since 1990, the BAIHP staff in the Northwest answers questions from homeowners, manufacturers, retailers and others. In The 5th budget period, staff from Washington, Oregon and Idaho responded to over 90 phone calls and conducted 19 field visits. The number of field visits to problem homes has significantly decreased over the history of the program, in large part because of manufacturers’ and installers’ increased adoption of the NEEM Super Good Cents/Energy Star (SGC/E-Star) specifications which include duct air tightness specifications (duct leakage is a major contributor to pressure and air flow related moisture problems), and the requirement that manufactured home installers be certified in Washington and Oregon.

BAIHP staff participated in quarterly meetings of the Washington State Manufactured Housing Technical Working Group, which coordinates the certification of manufactured housing
set-up crews.

While butyl duct tape is no longer allowed under current NEEM SGC/E-Star specifications, a consistent issue in the field continues to be excessive duct leakage, due in large part to failures of duct tape. These findings were brought to the attention of the NFPA-501 Manufactured Housing Standards Committee, resulting in a successful proposal to revise the duct sealing specifications to eliminate the use of duct tape in favor of better performing mastic and fiberglass mesh in the NFPA-501 standard. See a summary of supporting research findings in BAIHP Duct Data Compilation.

• Manufacturers Participating in Building Science Research

Blue Sky Foundation

Blue Sky Foundation, in coordination with FSEC, conducted an evaluation of energy efficiency and the moisture damage potential in 16 North Carolina homes in the summer of 2001. Blue Sky foundation proposed that the energy and moisture evaluation focus on the building envelope integrity, HVAC duct systems, and the moisture impact of unvented space heaters. All of the homes in the study were manufactured models located in Carteret and Craven counties, each located on the North Carolina coast. Field teams gathered additional energy and moisture information from homeowners.

Only three of the 15 tested homes recorded moisture and/or mildew problems. Because of the small sample size, the results are mostly anecdotal and would need to be evaluated within a larger data set. Planning for this is underway. Data from the summer field program as well as the final report are now on the BAIHP website (www.baihp.org) under Publications.

Cavalier Homes

BAIHP visited one Cavalier Home in Florida for a moisture damage investigation in response to home owner complaints of persistent air flow problems and floor damage. BAIHP made recommendations to correct the installation of the duct system and supply registers, repair the rodent barrier to make it air tight, do site work to reduce flooding under house, place a ground cover if site work done, increase crawl space venting, and replace damaged flooring with plywood.

Fleetwood Homes

During the 5 th budget period, BAIHP continued to support Fleetwood’s service department making six visits to moisture damaged homes in Florida (4), Texas (1), and West Virginia (1).

Figure 43 Testing Results from Fleetwood Homes Plant in Alma, Georgia illustrate that tape sealed ducts can result in total duct leakage under Qn=<6%. This initial tightness, however, is often eroded by adhesive failure.

Figure 44 Wall assembly used in moisture transmission experiment.

Six Fleetwood homes, all in Florida, were tested for moisture and mold damage from April 2002 through March 2003, the 4 th budget period. All of the homes had damaged flooring due in part to a lack of ground cover and poor crawlspace ventilation. Damage to the floor in one home was exacerbated by a plumbing leak. Only one home had moisture damage to the wallboard material, and this home showed a history of thermostat settings below 72 F. A report for each home was submitted to Fleetwood for corrective measures. One additional high bill complaint in Cobb, Georgia was investigated during this reporting period.

In 2002, four Fleetwood factories in Southern Georgia were visited to investigate possible causes of moisture related building failures found in homes installed in hot, humid climates. The factories were located in Douglas, Alma, Pearson, and Willacootche. (Figure 43.)

Homes of Merit

In 2002, researchers performed multiple diagnostic tests on a home located in Marathon, Florida that was experiencing “mold problems.” Researchers determined that the mechanical system was significantly oversized and that the home was operating under negative pressure when the system was operational. The home’s owner exacerbated humidity problems by leaving the fan in the “on” mode. On-site relative humidity readings showed that indoor and outdoor relative humidity were the same, approximately 70%.

Palm Harbor Homes

(See also, Palm Harbor Homes in Section I, Technical Support).

Palm Harbor Homes, James Hardie®, and FSEC performed two separate drywall assembly tests to determine the cause of some moisture damage occurring in homes sheathed with Hardipanel. Hobo dataloggers recorded temperature and relative humidity measurements inside the assembled panels on eight different wall panel configurations. (Figure 44)

Results determined that the unprimed, unwrapped sheathing performed best. The painted drywall assemblies allowed the greatest moisture movement - or wall assembly drying. (Table 23) The vinyl-covered drywall held moisture longest, recording the slowest drying time. Adding perforations to the vinyl reduced the drying time.

In 2002, two Palm Harbor homes with comfort problems were tested in Ocala and Okahumpka, Florida and one high bill complaint was investigated in Odessa, Florida. Duct leakage testing and infrared imaging revealed a duct disconnect near the attic crossover in the Ocala home. Inspections with the IR camera found no insulation problems in the Odessa home. Ductblaster and blower door tests revealed airtight duct and envelope systems. Other than an oversized air conditioning system, there were no obvious reasons for the high bills.

Southern Energy Homes

(See also, Southern Energy Homes in Section I, Technical Assistance.)

During Year 2001, 12 homes were field tested in the Houma, Louisiana area. Some of the homes had new moisture damage. Others were rechecks of previous moisture problems already repaired by SEH personnel. FSEC inspectors reported improper repairs and recommended additional dealer and staff training. An additional five homes were field tested in Houma during the 4th reporting period, with another home in Mississippi and one in Alabama also field tested.

During the 5 th budget period, BAIHP visited two Southern Energy Homes in Texas (1) and Kentucky (1).