On windy days, houses in the Prairies often develop a small frost line on the rug about an inch away from the wall on the north or west side of the house. Here the wind is succeeding in lowering the temperature of the most poorly insulated part of the house -- the floor boards. In most construction the floor boards extend right outside just under the siding, with little or often no insulation. Wood, not being a good insulator, allows the cold to come in along the sub-flooring and lower the temperature of the rug just inside the wall. Add sufficient humidity in the house and you have frost and discoloration. Some possible solutions: if there is an actual cold air draft here, pull back the rug and caulk it shut; lower the humidity in the house; landscape to deflect the wind from this area of the house; insulate the house on the outside if possible; and remove a bit of the insulation from the header area of the basement just below the frost areas to allow a planned heat loss which will counteract the cold wind action and keep the rug warm.