Due to its leachability, Japanese authorities deem disodium octaborate tetrahydrate (DOT) unsuitable for use as a preservative treatment for sill plates. Although the sill plates are not in direct ground contact, and are usually protected from the elements by siding, some authorities suggest that the preservative will diffuse into the damp concrete sill, or that condensation or occasional flooding will compromise the treatment. Due to a lack of data on the extent of boron loss under these circumstances, a test was designed to investigate borate losses from DOT-treated sill plates. This project set up sill plates on wet sills under conditions of high humidity/light condensation, with and without a sill gasket, and with and without water spray (simulating extreme condensation) or occasional flooding.
End-matched samples of DOT-treated 105mm squares were prepared and analyzed for the penetration and retention of preservative. Each sample was placed on a concrete sill contained in polyethylene containers and kept at high humidity (100% relative humidity) with light condensation occurring. A controlled water spray was applied to two sets of test samples. In another set the samples were individually flooded in a separate container for 24 hours once each month. Weighing the samples and analyzing the water for borate content enabled monitoring of moisture uptake and loss of borate over a six-month period. Following completion of the six-month test, the moisture content, preservative penetration, and preservative retention of the test samples were again determined.
The test with high humidity and light condensation alone gave conditions which resulted in sill plates comparable to the wettest sill plates observed in Japanese houses. The data showed that negligible borate leaching occurred from DOT-treated sill plates stored under these conditions. The effect of placing a gasket between the sill plate and the sill could therefore not be determined because of the negligible loss. This work confirmed that significant borate loss only occurred when DOT treated sill plates were washed with large quantities of liquid water, either by spraying or flooding. Spraying resulted in sill plate moisture contents which were double those found in service in older houses in Japan. Modern house construction has dramatically reduced crawlspace moisture contents. Consequently, wetting to this extent would not occur during normal modern house construction and service. The presence of a sill gasket did not stop loss of borate following spraying indicating that the water was dripping off and not passing into the concrete. 2000-2615