Abstract: An improved curtain wall or wall system for building construction is provided which employs a unitary thermal barrier or thermal break vertical mullion. The novel unitary vertical thermal barrier mullion comprises metal exterior and interior vertical members uniquely joined together, and an insulating medium, or member further joining and separating the metal members. The members are so constructed that a fail-safe condition is provided which prohibits complete separation of the metal members should the insulating member or medium subsequently lose its structural integrity. The shape of the mullion may be varied without changing the construction providing the fail-safe condition.