We have sliding patio doors in our dining. We are having very cold air coming in through the bottom of the frame (underneath the actual base.) There is a difference in temperature of between 2 and 4 degrees depending on how cold it is outside in the air. If I put the thermometer at the base on the floor it reads about 20 degrees colder than the room temperature. The house is almost five years old. We just moved in in May and did not notice it particularly as the temperatures were so mild, but now it is an acute problem.
Answer:
Wherever you have windows you have heat loss and patio doors therefore, are areas of major heat loss. The drastic temperature difference you're recording probably has a lot to do with what's undrneath this part of the house. If the basement is not insulated, if there is a lot of air leakage, if heat in the basement is directed away from this area, etc. these measures will make the area cold and can easily give you 20F degrees difference from the floor to six or seven feet up.
