Placement of vapor diffusion retarders interior to wall insulation is standard practice in cold climates where moisture content is typically higher inside the home than out. Having a vapor retarder on the side of the wall with the highest average moisture content retards wetting and encourages drying of the cavity. In hot, humid regions where moisture content is more often higher outside, most building scientists advise against interior vapor retarders. The Manufactured Housing Construction and Safety Standards (MHCSS) (24 CFR Part 3280) were drafted at a time when heating issues dominated and cooling manufactured homes was the exception. This resulted in the HUD-code requirement for "Condensation control and installation of vapor retarders" (MHCSS 3280.504) that call for vapor retarders to be "installed on the living space side of the wall" for all regions.

The dynamics of moisture movement inside wall cavities are in fact quite complex such that individual measures are often insufficient to cause or to prevent moisture problems. Vinyl-covered wall board, which has a particularly effective water vapor retarder on the interior surface, has been used for years in manufactured housing located in hot, humid climates and have been implicated as the main cause of some building failures. Recently, however, the number of homes experiencing moisture-related problems has risen precipitously, and the mandated use of an interior vapor retarder is likely to be the major culprit.

HUD responded to requests for changes in the standards by issuing a proposed regulatory waiver, outlined in the Federal Register of March 30, 2000 (65 FR 17110). This proposed waiver would permit the optional placement of the vapor retarder on the outside of exterior wall insulation in the hot, humid climate regions of the Gulf and Atlantic Coasts with the caveat that the interior wall board and finish must attain a combined perm rating of 3.0 or greater. However, little data was available to verify that typical interior wall coverings had sufficient vapor transmittance to satisfy the 3.0 perm requirement. To resolve this concern, MHRA conducted permanence testing on the range of typical interior wall coverings to identify those materials would be acceptable under the proposed regulation. A MHRA project committee, chaired by Frank Walter of the Manufactured Housing Institute, was formed to measure the permanence values of typical wall covering assemblies. A group of manufacturers and suppliers provided both guidance and materials for the testing.

Because interior wall coverings are often painted and otherwise finished in the plant, manufacturers were solicited as the primary source to supply finished interior wall covering samples for testing. Fifteen samples were submitted to MHRA, and from those 8 samples representing the range of common interior wall materials were selected for testing. Excluded from the consideration for testing were wall coverings that were typically used for less than 50 square feet of the interior wall surface area or that covered less than 40% of the wall surface such as tub surrounds, backsplashes, tile highlights, wall mounted furniture and wainscoting. Also excluded from testing were seldom used materials such as 5/8" wall board and ¼" hardboard and surfaces already known to have low permeability such as vinyl surfaces and vapor barrier paints (although one vinyl surfaced wall boardmaterial was included in the testing as a reference value). Table 1 describes the materials selected for the permeance testing.

A testing contractor was selected from several candidates based on pricing, testing capacity and how soon the contractor could complete the tests. All testing candidates were accredited by the National Institute of Standards and Technology to conduct the ASTM E 96-95 Test Method for Water Vapor Transmission of Materials - the test procedure specified in the HUD standards. The testing contract was awarded to Integrex Testing Systems, an independent testing laboratory and a subsidiary of Owens Corning.

The ASTM method calls for three replicates and one control sample to be run for each individual material. This was done in each case with the following exceptions (see Table 1):

• Only three intact samples of constructions 1A and 1B (5/16" wall board material with embossed paper) were provided to MHRA while four samples are required for the test. Since the only difference between materials 1A and 1B was the printed pattern on the paper laminate, these samples were combined and treated as a single material.
  
  
• Sample 11, lauan paneling with clear gloss finish, was tested over coated wall board (see sample 8) as is typical construction using this material.

Results

Wall samples were tested that represent the range of products typically used in manufactured homes installed in the hot, humid Gulf Coast states. The results from the permeance testing show that only the vinyl-coated wall board failed to reach the 3.0 perm rating limit set by HUD in the proposed waiver. The results from the ASTM E96-95 testing are summarized in the table below:

Notes:

1. Samples 1A and 1B are identical except for the pattern printed on the paper lamination. A total of four samples, two of each pattern, were tested and the results averaged.
   
   
2. Materials not included for water vapor permanence testing:
   
   a. Interior wall coverings of less than 50 square feet or typically covering less than 40% of the wall surface such as: tub surrounds, backsplashes, tile highlights, wall mounted furniture and wainscoting.
   
   b. Seldom used materials such as 5/8" gypsum wall board and ¼" hardboard.
   
   c. With the exception of sample 5, surfaces assumed to be of high permeability such as vinyl coated surfaces and low permeability paints.