Ducks - Keeping Them Warm!!!!!!!

I have two 15 old welsh harlequin ducks and was wondering if they will need any extra measures to keep them warm both in the day and at night during the winter.
I currently have their house closed all day otherwise they seem to get confused and wont got to bed in the evening.
There duck house is about 2 foot off the ground and the floor is covered in about 1-2 inches of hemcore,
Should I add any further bedding during the the cold nights and will they be ok outside all day even during the cold cold winter days??