I'm getting condensation on all my dual pane wood casements when the outside temp. is 20F degrees or lower. Otherwise the windows stay dry. The humidity in the house is 35% to 38%. This two story house was built in 2000 and I recently insulated the attic with closed cell foam and cellulose, but I've had this problem since I moved in this house 6 years ago.
Is this a sign of too much humidity or just poor performing windows? I don't think that my humidity level was too high or is it ?

This house is equipped with a high efficiency gas furnace that does not draw fresh air from the outside. Gas stove and electric dryer. All bathrooms have functioning exhaust fans that are used after showers.