As
@
bjstare
said, your enemy is the air movemen. My guess it's coming between the sill plate and the foundation. No easy way to seal that up in a completed house. I have same issue in my house, water always freezes. Easy solution...leave it dripping when it gets cold out. Water won't freeze if it's moving. Just one drop every 10 seconds is enough to prevent it from freezing. That's what I ended up doing, didn't want to rip into finished walls. I was able to slide some pipe insulation onto the pipe as far as I could, but it didn't do anything. The moment the pipe goes through the bottom plate on the wall (as seen from my basement), it's all over. Real solution would require some destruction unfortunately.