In a wireless sensor-actuator network (WSAN), sensors send sensed values to actuators in a wireless channel. Messages sent by nodes might be lost due to collision and noise if multiple nodes simultaneously send the messages. If a message is detected to be lost, the source node retransmits the message to the destination nodes. It takes at least three rounds to detect and retransmit a lost message since the source node sends the message. The more number of messages are retransmitted, the more number of messages collide. We have to reduce the number of messages retransmitted to realize the efficient data transmission. We newly propose a redundant data transmission (RT) protocol where a sensed value is redundantly carried to the destination node by multiple messages. Even if a message is lost, the destination node can receive data in the lost message if the destination node receives other redundant messages. In addition, we have to reduce the energy consumption of a sensor node. A sensor node mainly consumes the energy to send and receive messages. Even if an event occurs, only some number of sensor nodes sensing the event send the sensed values to reduce the total energy consumption. We discuss an energy-efficient data transmission protocol. We evaluate the RT protocol compared with the CSMA protocol in terms of how much sensing data a node can receive in presence of messages loss.