I just bought a new house that was built in the 40's. We have a crawl space and every winter I close the crawl space vents so the pipes will not freeze. Every time I do this, we will get an odor that comes in the house after we run the washing machine or if we take showers. I've checked in the crawl space and cannot figure out what is causing this? It's a musty, wet, old water smell, sometimes a little sewer gas smell.

Can someone answer what the problem might be?

Thanks

This is what happens to crawl sopaces when they are not vented. You need air to flow in this space to keep it dry and freee of mold. The fixtures should be insulated rather than trying to insulate the entire crawl space. I would add vents and spray the entire crawl to the point of dripping with a mix of 2 parts water 1 part fresh bleach or mildewcide. Check all drain pipes for possible sewer leakage too.