normal map, mask different areas?

ok, so i have made a metal door with a glass window in $$anonymous$$gh poly and created a normal map w$$anonymous$$ch 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 t$$anonymous$$s might be inefficient? if t$$anonymous$$s 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 ac$$anonymous$$eve t$$anonymous$$s please

Is there an existing shader to do t$$anonymous$$s?