Within The Building Code the area that relates to thermal performance in new construction is called H1, and designers and contractors must prove compliance to local Councils that the new construction meets the minimum levels of H1.
An H1 calculation looks at a number of factors in the home, such as location, types of glazing used (single or double), percentage of glazing versus total wall area, skylights, and the system performance for walls, floors and ceilings. Therefore the level of insulation needed is actually determined by all the other factors, but must still meet basic minimums. For example a home with more than 30% glazing may need to increase the level of insulation to counter this. Also, a home that increases the level of ceiling insulation may reduce insulation in it’s walls, and vice versa. A home with single glazing would need to make significant upgrades elsewhere and is therefore no longer commonly used.