The basement of my Chicago frame house (built 1895) was finished 20 years ago. They put a ceiling fan in the bathroom down there but didn't vent it. The fan is three feet from an exterior wall. The wall is six feet of limestone foundation topped with what appears to be a 4" wide x 9" tall sill. The sill has notches that the joists rest in. The fan sits in the ceiling between two of those joists. This means any duct work would have to go straight through that sill. Is it a bad idea to drill through the sill to vent the fan? I was surprised to find no mildew/mold/rot in the joists despite all the moisture that's been pushed up there over the years. The hope is, properly venting the fan will keep the basement dryer. Just not sure it's worth messing with that sill. Maybe there's a better way to vent a basement fan?