In countries where excessive heat is a problem the use of foil is prevalent, for example parts of Australia and the Pacific islands. This is because foil a great radiant heat barrier. Basically meaning it will reflect away the radiant heat from the sun, and prevent your building from heating up excessively.
Unfortunately in New Zealand our national average temperature is around 12.5°C, this means excessive heat not too much of an issue for us. Normally we are trying to heat our homes to a healthy and comfortable temperature, at least 18°C is the World Health Organisation (WHO) recommendation.
The heat we are trying to prevent from escaping our homes is generally not radiant heat, but heat transferring through conduction and convection. The main way to reduce this type of heat transfer is by using bulk insulation such as Pink Batts insulation.
So you can see why the answer is not so simple. Foil does get used as insulation, but for New Zealand conditions the use of bulk insulation is far more effective.