I have a bay window which extends out about one foot from the house. The bottom of it is not insulated. It looks like I could install 1.5" thick foam insulation. 1) Is that the right approach? 2) Would I need to cover the insulation say with exterior plywood or something else?
Yes it's the right approach, you could even install more if you extend the trim down. It should be covered also exterior plywood or nicer stuff if visible. I see stuff like this all the time even in new houses I work on for some reason most bay windows have little or nothing on the tops and bottoms of them and builders rarely do anything about it
We typically add a 2" piece of XPS (R10) and then bend up some aluminum to dress up the edges... You'll notice a big difference in the temp of the seat board, as well as condensation if that is an issue for you.