normal map, mask different areas?

ok, so i have made a metal door with a glass window in high poly and created a normal map which is appplied to just one poly. i know how to texture the metal door, but i want the window to be reflective. Maybe have a cube map. how do i make the window bit "glassy"

i presume i use an alpha to mask of the area, but this might be inefficient? if this is the only part of the texture that requires the glassyness. Im quiet new so just putting suggestions forward. what would be the best way to achieve this please

Is there an existing shader to do this?