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Moisture in new homes built in the late 1940's and early 1950's was a significant problem. Few moisture problems in new homes have been reported to us in the past 20 years or so. Home size, family size, life style, thermal protection, double and triple glazing, and vapor barriers all contribute to the turnabout.
Moisture conditions in homes were looked into in two studies. In one study, 17 homes in four states were probed in mid-March 1977, when it was expected the moisture content of wood framing members would be at their high or near high due to any moisture condensation in walls. All moisture content levels were found to be well within the acceptable and expected range of equilibrium moisture contents (EMC) for wood framing members (6 to 14 percent) and well below the decay level in wood (20 percent or greater). There was no evidence of past or present water damage in any of these homes.
In another study, data were collected for 16 homes in north-central Utah, southern Alabama, northern Ohio, and central Maryland. Measurements were made in each home for approximately 1 week in the summer and 1 week in the winter. All homes had well ventilated attics. Some had attic vapor barriers and others had none. Calculations showed the average absolute humidities in winter in attics for homes with a ceiling vapor barrier were some 19 percent higher than for attics without a ceiling vapor barrier.
Much more extensive measurements probably would show no consequential difference in relative humidity in well ventilated attics with homes having a ceiling vapor barrier or those without. In practice, the millions of house years of experience with well ventilated and insulated attics, having no ceiling vapor barrier and no condensation problems, indicates that extremes of temperature, ventilation and lifestyle rarely occur simultaneously and to an extent necessary to produce attic condensation sufficient to cause moisture damage.
framing members, home building, moisture content, relative humidity, vapor barrier, water vapor condensation
President, NAHB Research Foundation, Inc., Rockville, Md.