I remember once, several years ago, I had a cordyline that I decided to give some winter protection to, on the onset of a potentially hard winter.
I used pipe lagging, which was a felt material with a polythene backing. Trouble was, it trapped in moisture which also attracted things like woodlice and earwigs, plus it even started to produce some aerial roots, I think if I had left it on much longer it would have started to rot.
Since then I've always uses 'horticultural fleece', which provides good insulation, also allowing good air circulation.
The type of fleece bags that I use have a draw string to keep it in place, so that you can cover the crown of the plant or use it in sheet form to cover the stems loosely.