Siding and wall cladding is the exterior material applied to the walls of a house or other building meant to shed water, protect the walls from the effects of weather, insulate, and is a key in the aesthetics of the structure. Some walls such as solid brickwork and masonry veneer are not covered with siding, but some buildings such as log buildings can have siding added. Siding may be formed of horizontal or vertical boards, shingles, or sheet materials. In all cases, avoiding wind and rain infiltration through the joints is a major challenge, met by overlapping, covering or sealing the joints, or by creating an interlocking joint such as a tongue and groove or rabbet. Since building materials expand and contract with changing temperature and humidity, it is not practical to make rigid joints between the siding elements so they often leak. Rainscreen construction is used to improve siding's ability to keep walls dry. Siding may be made of wood, metal, plastic (vinyl), masonry, or composite materials. It may be attached directly to the building structure (studs in the case of wood construction), or to an intermediate layer of wood (boards, planks, plywood, oriented strand board) called sheathing (or sheeting in some regions of the United States). An intermediate air/moisture barrier such as housewrap or felt paper may be applied to the sheathing or a modern sheathing material also serves as an air/moisture barrier.