Performance of the energy efficiency of cooperative communications in WSNs (wireless sensor networks), assuming flat fading channels, has been well investigated. However, wireless channels are characterised by frequency-selective fading when wireless networks operate in indoor environments. Analysis of the energy efficiency for cooperation in WSNs subject to frequency-selective channels, practically has not been addressed. Therefore, this paper studies the energy efficiency of DF (decode and forward) cooperative diversity technique in WSNs operating over a frequency-selective fading channel, that is composed of a primary channel and a number of secondary channels. Our work indicates that DF cooperation can be used to improve the energy efficiency in WSNs operating in indoor environments. Significant energy savings can be achieved from cooperation in WSNs with frequency-selective fading channels, compared to WSNs without cooperation. Finally, we have developed a basic cooperation strategy of determining the number of partner nodes for energy-efficient multi-node DF cooperation.