I have a three year old modular house. we have noticed some dark marks on the wall boards so we pulled off the sheetrock to look. we noticed there is lots of black marks on the wires and the insulation to the outlet boxes etc. the wireing is white romex. the wire does not feel rough or melted but it looks like tar on the wire and it is sticking to the paper insulation. what causes this and is it dangerous and in need of changeing? the room is a bedroom.