Although French door installation is similar for all materials, upkeep for fiberglass is virtually nonexistent while maintenance for steel requires rust prevention over time. Steel French doors also become damaged more easily than fiberglass French doors, and repairing the scratches or dents can be difficult or impossible.
Understanding the particular application of French doors is important when deciding on materials. External French doors, such as patio installations, are susceptible to climate. Fiberglass has the best performance in these circumstances. It is impervious to heat or cold, whereas steel is a poor insulator. Even with the addition of insulating cores, steel still feels hot or cold to the touch. Steel may form condensation on the inside in extremes of temperature or damp weather. Steel doors consist of thin plates sandwiched over an internal space and can sustain damage in the event of an attempted break-in.
Internal French doors do not experience most of the issues related to external doors. In these installations, considerations relating to climate and home security do not apply. When deciding on materials for internal doors, expense and looks are the primary considerations. Steel doors are less expensive than fiberglass, and come in surface coats that match paint schemes. Fiberglass is available in wood-like finishes that are almost indistinguishable from actual wood, but they cost much less than wooden doors.