If a person is sentenced for two or more separate offenses, the sentencing judge may order the second or further sentence to begin at the expiration of the first or succeeding sentence. If a person is sentenced for escape under section 719.4 or for a crime committed while confined in a detention facility or penal institution, the sentencing judge shall order the sentence to begin at the expiration of any existing sentence. If the person is presently in the custody of the director of the Iowa department of corrections, the sentence shall be served at the facility or institution in which the person is already confined unless the person is transferred by the director. Except as otherwise provided in section 903A.7, if consecutive sentences are specified in the order of commitment, the several terms shall be construed as one continuous term of imprisonment.