capn, bees are more cold hardy than we give them credit for. my thoughts are:
first and foremost:
1. strong healthy colonies and strong queen going into winter, mite (relatively mite free) and disease free.
2. honey/pollen stores. honey stores according to your area that the bees will need during winter months. if not provide a shim to feed winter patties or candy board on top.
if we don't have the above, all the insulation on the planet will not save them or get them through winter.
what i do knowing i have the above:
1. provide a winter wind break.
2. wrap with something called a 'colony quilt' made by b and b honey farm, houston, minnesota
3. provide top and bottom entrances, and sometimes i will leave the entrance reducer totally off the bottom board.
4. polystyrene (2 inches) underneath the outer cover and on top the inner cover.
moisture is a killer in our hives, like perry said, cold and wet, and also you can over insulate. moisture will drip down on them from the inner cover. skip the polyiso board to insulate, IMHO, this is not necessary. top and bottom entrances for ventilation in our very cold climates, also, the top entrance allows them to leave the hive on warmer winter days, the bottom does and will get clogged up from dead bees, snow and ice. my top entrances are 3-4 inch long notches cut out of each on my inner covers. also, my HO, do not use moisture absorbing material on top of the inner cover. it does collect moisture, however, it collects a great deal and in freeze and thaw winter months, moisture is released from this material on warmer days or during thaws and will rain down on your bees.