Here we have two identical windows on the same wall in an old New Orleans bathroom. A spider has built a web on one window (top), but not the other (bottom). Do you wonder why?
It's all about air leaks and food. Spiders don't choose their home sites like we do. They don't check Zillow for nearby home prices, or get school ratings, or wonder what the zip code will do for their social standing.
What spiders want is a cozy, protected spot with a steady food supply, which means little insects. What spiders know is that insects enter a house where there are air leaks. If there are no air leaks, there will be fewer insects.
Why do you care? If you want to control the temperature and humidity and comfort levels inside your home, and most of us do, sealing up air leaks is critical.
The scientific way to determine air leaks involves a home energy audit with a blower door test, infrared camera and other scientific means.
But spider webs also have a tale to tell. If you spot spider webs in your home, they did not end up there accidentally. They were placed there by a genius spider bent on survival. And thus you have evidence of a leak. Your move.