The file interleave is usually retained through most ENVI processes, but there are some (such as Band Math and CTC) that always output to BSQ due to the nature of how the algorithm works. It would be useful if a conversion was applied prior to outputting the file but unfortunately this is not the case. So if you know some of the processes are going to convert the data to BSQ, then Iwould run those processes together, and then run the interleave conversion on the result of that process.