Crawlspaces are very common under additons. They can create comfort problems in the rooms above them. People used to have the thought process of letting them breath and allow the cold air in the winter to come into them. This can make the room above freezing. Currently, sealing off the crawlspace, insulating the walls, and allowing heat into the crawlspace, will make the room up above warmer. Call an expert to determine what method would be best for your crawlspace to ensure that the rooms above are comfortable for you and your family!