Complex sentence: Such vestibules consist of merely a set of inner doors and a set of outer doors, the intent being to reduce air infiltration to the building by having only one set of doors open at any given time.
Split into simpler sentences: Such vestibules consist of merely a set of inner doors and a set of outer doors. The intent of such vestibules is to reduce air infiltration to the building by having only one set of doors open at any given time.