i have 10mm packaging polystyrene sheets well known auction site glued to the inside floor/walls of my cupboard.and as much foam and bubble wrap jammed into the underfloor/cavity type thing,as the cupboard is above the stairs in the the box room coldest room in an old house.i had thought to about running an extra airy exta perlite soil as last winter pots took an age between waterings.
good luck n keep er lit.