Buffalo's "Windshirt" is not a waterproof. It is very lightweight windproof top, originally designed for fell-runners who needed an un-insulated top providing only light protection from the elements, but able to get rid of sweat effectively.
Generally used with a non-cotton thermal underlayer for high energy activities or when the weather is warmer, it noticeably raises the insulation value of the thermal base layer. Because it packs down small enough to fit into a pocket, it is also extremely useful as an extra layer to take onto the hill when wearing one of the Buffalo shirts. By putting the "Windshirt" over a Buffalo shirt the wind-proofness is increased along with resistance to rain.