Can anyone give me some advice on installing a bay window where there is currently a standard casement window. I can obtain the bay and its considerably cheaper if I install it. The bay will come with a plastic roof on the top. Can someone advise on what I need to install on the underside of the roof i.e on the room side at the top. I presume some form of extension is required to meet the new window. Should I insulate above this?. I'm a pretty experienced DIYer.