Tsongas, G. and Olson, J.
1995
Proceedings of the DOE/ORNL/ASHRAE/BETEC Conference, Thermal Performance of the Exterior Envelopes of Buildings VI, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Orlando, Florida, pp. 207-218
fungi decay, exterior vapor retard, plywood, manufactured home
| The first known cases of extensive decay in the wall ftaming members of hundreds, if not thousands, of nwm~actured homes involve the Tii State Homes in Wisconsin, Minnesota, and Mich-igan (the company has gone bankrupt). About 6,000 such homes were built in the 1970s. The decay has been attributed to high levlels of indoor moisture, but recentfield test and computer modeling results indicate that the primary cause is the presence of an exterior vapor retarder (EVR) that trapped moisture within the wall cavity. The low-permeability retarder (building paper) was on the outside of the plywood sheathing behind hardboard lap siding. The presence of the zvpor retarder caused the plyuvod to get much wetter than normal during the winter and spring, and it reduced the rate ofdrying of the plywood. Thus the wood zaas still quite wet in the late spring and early summer, when temperatures were high enough to promote the growth of decay fiingi. The result was severe and extensive rotting of the plywood sheathing that occurred aver the last 20 years. But the decay progressed slowly, such that it was first noticed only about eight years ago. Moreover, the wet wall conditions led to the growth of substantial mold that seriously impacted the health of many of the occupants.
An inspection of 15 Tri State homes in Wisconsin that had siding removed (completely rewtoved in 11 of the cases) revealed that 14 had plywood decay and 12 of the cases were sewre enough that the plywood could be torn apart by hand. Ten of tlw 11 homes with siding completely removed had decay present. In addition, decay was noted infive of six houses with siding removed only in two localized areas, those homes urre not inspected by the
Thus, overall, 19 of 21 homes inspected had plywood decay. It is conjectured that most, ~ not all, of the Tri State homes either already hazle experienced or eventually will experience severe plywood decay. Plywood delamination also ztas observed in 8 of 10 of the cases.
Many of the walls were unusually wet during winter and early spring, and plywood moisture contents above 60% (the meter limit) zww measured during late June and early July, when the plywood in a conventionally constructed wall is considerably drier. The plywood moisture contents measured in the Tri State homes during that early summer, as well as during the previous winter and early spring, um higher Own the highest values mea- sured in any of three Pacific Northwest wall moisturefield stud- ies. While it has long been noted that the outside layers of a wall should be less permeable than the inside layers, these results dra- matically emphasize the catastrophic results that can occur when the rule is notfollowed.
A comparison of sheathing and siding moisture levels for walls with and without an EVR also was undertaken using the MOIST computer model developed at a national laboratory. The modeling resultsfurther reinforce thefield inspectionfinding that the EVR is the cause of the structural damage.
Details of thefield tests and the computer modeling are pre-sented, along with conclusions and recommendations. Practical lessons learnedfrom this case study are presented to avoid similar problems in thefuture with presently available building products that act as an exterior vapor retarder.
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