Re: overwintering and moisture
Benny, don't use insulating tubes on your bananas. They are not suited for bananas if the tubes touches the pseudostems, and so are only good for most woody stemmed plants like grapes, young fruit trees, to give them a good head start and protection against deer, rodents and other pests. For bananas, you would be better off with several layers of cheapo burlap and a plastic sheet over them.
IT IS THE COLD WET SOIL THAT IS THE BIGGEST KILLER OF BANANAS IN CALIFORNIA and efforts should be concentrated in keeping the ground relatively dry during the winter, and not the protection of pseudostems and other above ground parts. If you have protected the base of your plants by diverting water away, using a simple black plastic cover around the base, then you can proceed to protecting the above ground parts to have a head start.