Anton TenWolde

Moisture Control Strategies for the Building Envelope

C ONTROL IS NECESSARY to avoid moisture-related problems in building envelopes and human health. Moisture damage is one of the most important factors affecting the service life of a building. High moisture levels can lead to decay of wood, corrosion of metal, and discoloration. However, decay and damage to the structure are by no means the only possible consequences of excessive moisture. Mold and mildew growth in or on the exterior envelope may also affect indoor air quality. Presence of mold often indicates an environment that is favorable for decay in wood. The moisture and humidity conditions conducive to mold and mildew growth are generally less severe than those needed for decay and structural damage. While decay of wood is generally believed to initiate at fiber saturation, mold and mildew can grow at surface moisture conditions below fiber saturation. To avoid mold growth, the International Energy Agency Annex XIV (1991) ad-

Accuracy of Wood Resistance Sensors for Measurement of Humidity

This work was undertaken to evaluate the accuracy of wood resistance sensors for measurement of relative humidity and to identify sources of error in this use. Relative humidity can be expressed as a function of the logarithm of the sensors electrical resistance and of its temperature. We found that single-point calibration of each sensor compensates for most between-sensor variation, although care must be exercised during calibration. With careful calibration readings, error in relative humidity readings made with these sensors can be limited to ± 10% relative humidity under most conditions. The literature indicates that a lower degree of error than this is anticipated when electrical resistance is used to estimate moisture content measurements. Our data suggest that sorption hysteresis and sensor memory are significant contributors to this (± 10%) relative humidity error.

How variability in OSB mechanical properties affects biological durability testing

Summary Loss in bending strength of wood has been shown to be a more sensitive measure of decay than is weight loss. Using modulus of rupture as the decay criterion is problematic for oriented strandboard (OSB) because of variation in mechanical properties due to particle orientation and size. Moreover, the small specimen size required for such tests increases the variance in mechanical properties. This study compared the variance in bending strength of ASTM D1037 standard-sized specimens and small specimens from two samples of commercial OSB. The small specimens were found to have a significantly higher level of variance in bending strength than the standard-sized specimens. A simple method of sorting the specimens based on strand orientation on the tensile surface significantly reduced the level of variance measured. The effects of differing levels of variance on the size, design and limitations of the experimental study are presented.