If any installment of compensation payable in accordance with the terms of an agreement approved by the commission without an award is not paid within fourteen days after it becomes due, as provided in § 42-9-230, or if any installment of compensation payable in accordance with the terms of an award by the commission is not paid within fourteen days after it becomes due, as provided in § 42-9-240, there shall be added to such unpaid installment an amount equal to ten per cent thereof, which shall be paid at the same time as, but in addition to, such installment, unless such nonpayment is excused by the commission after a showing by the employer that owing to conditions over which he had no control such installment could not be paid within the period prescribed for the payment.