Simeon,
People get all worked up about insulation, but its simple really.
You picked up on some good points in your comments. Thre must be an air gap to make reflective foil keep the R value. But the most important part of the reflective foil is that it reflects the radiant heat.
Wall batts are made to be stiff so they stay upright. if you start using R3.0 etc in walls, then as you say they will sag - not a good option.
This is why reflective foil is needed:
Research has shown that when heat flows down such as heat gain through the ceiling in summer, and losing heat through the floor in winter:
Up to 93% is transferred by radiation
Up to 7% is transferred by conduction
0% is transferred by convection
When heat flows up, such as heat loss
through the ceiling
45 - 50% is transferred by radiation
5 - 7% is transferred by conduction
45% is transferred by convection
When heat flows sideways such as through
the walls of a house:
65 - 80% is transferred by radiation
5 - 7% is transferred by conduction
15 - 28% is transferred by convection
Get your insulation, thermal mass and cross flow ventilation right and you wont need a heater or A/C in most of ou big country!