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. When painted wood absorbs water, it takes a long time to dry.
Paint holds water in.
As a result, wet siding is held tightly against housewrapped sheathing in most homes. I am leery of extending lab results into predictable field performance. However, lab testing can help clarify our understanding of field-use potentials. We imitated this lay-up in our experiment. Saturating the siding was an attempt to mimic what happens to untreated wood siding or wood siding that has not been backprimed when it is exposed to a period of rain.