One of the primary ways that rain is driven into walls is by air pressure differentials. In a word - wind. During the fall of 1998 I brought samples of 15 pound felt, Tyvek, Rwrap, Typar, Amowrap, Pinkwrap, and Barricade into the University's Building Materials Laboratory to see how these popular housewraps stand up to pressure-driven water. Perforated wraps leaked like sieves. As promised in the November 1998 issue of JLC, I have extended the study to look at the ability of housewraps to block the capillary flow of water.
Siding provides discontinuous coverage.
True, overlaps and seams are slender ports, but narrow breaks are large enough to pass significant amounts of water. Capillary suction is a strong force. Even under conditions of light or no wind pressure, water can be sucked through seams, cracks, joints and upward behind the overlaps of horizontal siding. Add to that fact, most wood siding is not back primed, so its backside sucks water up like a sponge. The face of wood siding is usually painted.