HUD proposed to waive certain requirements of the HUD Code for exterior wall vapor retarders, to utilize design alternatives to reduce condensation and moisture problems that have been experienced in the hot/humid climates of the Southeast Atlantic and Gulf Coast regions. Presently, the HUD Code does not specifically address the unique condensation control and vapor retarder installations for manufactured homes sited in these climate regions.

This waiver would permit manufactured home producers to place the vapor retarder on the exterior side of wall insulation in certain counties. Presently, by interpretation of the HUD Code, the vapor retarder is required on the living space side of exterior wall insulation. The HUD Code provides two exceptions to this rule, but they are not commonly used in established designs and construction practices of today's manufactured homes. Along with this waiver, manufacturers would be required to make other design changes and to add a climate map to the data plate. Also, a statement must be included on the data plate that requires the home to remain in the specified regions.

An MHI task force of suppliers and manufacturers reviewed HUD's proposed waiver. Through this review process, one special design requirement included in the waiver was questioned. This requirement was that the interior wallboard and interior finish must attain a combined vapor permeance of equal to or greater than 3.0 perms. The higher a perm rating, the easier water passed through a material. Several task force members indicated that they have perm ratings for individual materials that are typically installed for interior walls, but have not established data to verify the perm ratings for combined interior wallboard and wall coverings, such as wallpaper and paint.

Several MHI suppliers and manufacturer members are cooperating in a testing program for commonly used interior finished wallboard designs. This testing program is being conducted by the Manufactured Housing Research Alliance (MHRA). MHRA indicated that the testing of combined assemblies and evaluation of test results would take at least 60 days. Results should be available by the time you receive this issue of Modern Homes.

NIST STUDY ON VENTILATION IN MANUFACTURED HOMES

HUD released a National Institute of Standards and Technology (NIST) report entitled, "A Modeling Study of Ventilation in Manufactured Houses," in February 2000. The study amounted to ventilation simulations that were performed in a double-section manufactured home using a NIST multi-zone airflow model, called "CONTAM." This computer model includes the effects of exterior envelope leakage, interior partitions, forced-air distribution and associated duct leakage, exhaust fan operation and outdoor weather conditions. The model simulated ventilation activities in Albany, NY, Miami, FL, and Seattle, WA. The manufactured home modeled in this project was intended to represent construction consistent with the Federal Manufactured Home Construction and Safety Standards.

The study provides some useful information to manufacturers, in their continuing design of ventilation systems to meet the HUD Code. One obvious message is that crossover duct leakage needs to be minimized. The study looked at the effectiveness of both an air inlet to the furnace system and a whole-house exhaust fan near the main living room.

The objective of this investigation was to test the validity of the current HUD Code requirement for whole-house ventilation. That requirement specifies that each manufactured home should be capable of providing a minimum of 0.35 air changes per hour continuously or at an equivalent hourly average rate. The standard assumes that natural ventilation (either by infiltration or exfiltration) should be considered as providing 0.25 air changes per hour. The standard requires that the remaining ventilation capacity of 0.10 air changes per hour be provided in the manufactured home design

The notable study findings were as follows and validate current industry knowledge:

• Validity of the 0.25 air changes per hour assumption for natural ventilation--It was found that this assumption may be too high, but more importantly, it ignores variations due to weather and fan operation by the homeowner.
• Impact and effectiveness of an outdoor air inlet to the furnace--An outdoor intake duct to the furnace system is an effective way of raising air change rates, but it depends on the extent of the system use through manual switches and timers. There is an energy cost associated with the increased fan operation to provide higher ventilation rates.
• The impact and effectiveness of a whole-house exhaust fan--A whole-house exhaust fan, with or without passive inlet vents, provides an air change rate that exceeds the federal standards. However, the overall impact of the system depends on the fan-operating schedule. Passive inlet vents o not appear to be essential in the proper functioning of a supplemental ventilation system.
• Location of the whole-house exhaust fan--The effect of a whole-house fan did not depend much on its location. It was effective in both the main living room area or in a bathroom near the main living room.

For more information on the articles in Tech Topics, contact Mark Nunn at 703-558-0400 or mark@mfghome.org.

© 2000 by Manufactured Housing Institute. All rights reserved.