rman backup process consuming all available space in directory

the directory that holds the rman backup files shows almost 100% utilization while the process is running, after the rman process has finished the space consumed is 57%. The first thing the rman process does is to remove all the files in the directory (the directory only contains rman backup files), we have a cron script that is monitoring the useage and we can see that it keeps growing until it consumes 100% of the space and then the directory usage returns to 57% utilized when the rman process finishes... there are no errors in the log file.

does rman use the directory for temporary files, if so how would we go about specifying a different directory for these temp files ???