External walls of timber-framed buildings get weather protection by a masonry veneer, a siding, or a rendering. In all cases, the external sheathing of the wall is usually covered by a breathable (i.e., airtight and liquid tight, but vapor open) membrane to protect the timber construction from wind-driven rain. Such a covering is also necessary to reduce the moisture in the wall due to high vapor transmission. Sheathings made of

• PUR/PDMI-bonded high-performance wood-based panels, such as oriented strand boards (OSB) or medium-density fiber-boards (MDF), or
• hydraulic-bonded low-density (800 kg/m³) calcium silicate boards (CSB) stop airflow through the wall--they are water repellent and allow vapor transportation to escape from the interior. In these cases, the robustness of the design does not need a breathable membrane covering the whole sheathing but depends on good planning of all details as well as on good workmanship.

Authors: Helmut B.R. Marquardt, Dr. Ing.; Georg-W. Mainka, Dr. Ing., P.E.
Citation: Thermal Performance of the Exterior Envelopes of Buildings VIII
Keywords: December, Florida, 2001