insulation rules for energy efficient homes
If building a new home, the first rule you should consider about insulation is: Do it right the first time. Do not try to make insulation savings. A small extra initial investment can save you thousands of dollars in energy (in the future) and provide a much more comfortable home.
Insulation in Existing homes
Insulation rule: in new homes, bet on high levels of insulation.
Do not install just the minimum (even the official recommended insulation levels are often too low). To get higher energy efficiency and comfort, and to avoid higher energy bills, bet on high insulation levels...
Retrofit insulation can be disturbing and much more costly than installing the ideal levels of insulation the first time.
Retrofit insulation
Insulation rule: Consider adding more insulation in existing homes; insulation is always a good investment.
In existing homes with low levels of insulation, consider adding more insulation. To reduce its costs, install retrofit insulation (to walls, floors and other parts of the building) during other home repairing works.
What to insulate?
Insulation rule: insulate the whole building envelope (exterior walls, ceiling, foundations...) with a continuous insulation layer, without gaps and without giving room to thermal bridging. 
Do not let joists, pipes and other elements of the walls or other parts of the house uncovered. The overall insulation value of your home will diminish proportionately. It's important to insulate the whole home envelope with a continuous layer of insulation: the exterior walls, the attic floor, the foundation walls and floor...
New insulation technologies
Insulation rule: Pay attention to new technologies (involving walls, floors or the roof) and their implications on insulation.
New home framing techniques (engineered timber) or new wall technologies like ICF forms and SIP panels are outstanding examples of it.
New vs. traditional insulation Materials
Insulation rule: traditional insulation materials like batts and blowing fibers of fiberglass, rock wool or cellulose are not alone any longer.
Traditional insulation materials are cheap and effective in attic insulation or in cavity insulation, for instance.
But there are now other new insulation materials - namely insulation foams like molded and extruded polystyrene, rigid polyurethane and polyisocyanurate (polyso) - with higher efficiency, that should be considered in more demanding situations.
Make an insulation audit
Insulation rule: Assess the existing insulation levels.
If you don't know the exact insulation levels of your walls, or other parts of your house, you should carry out a careful DIY insulation audit (or even a professional audit).
See: Testing Insulation
Installation
Insulation rule on installation: be aware with DIY and incompetent professional insulation.
Low-quality installation is a common cause of lack of comfort and lower insulation value. Avoiding insulation mistakes and problems like compressed batts, voids, fluffing or settling is crucial.
See:
Professional Insulation
Insulation problems (cellulose)
Climate and R-values
Insulation rule: Different climates and rooms or parts of the house have differente insulation requirements.
The walls, the attic, the basement or the floor have different insulation requirements.
And different climates also require different insulation levels. Only in hot-tropical climates traditional insulation loses is effectiveness (only reflective insulation is truly important in such climates).
See, for recommendations:
Insulation values
Insulation and Climate zones
Sealing the windows, the ducts and...
Insulation rule: the ducts, the plumbing, the pipes in walls and uninsulated spaces should be insulated; the windows and doors should also be sealed.
Stopping air leaks in windows and doors (or around them) is crucial to the insulation value of the home. The walls, the attic or the basement are primary areas of insulation, but aren't all. Ducts, pipes, plumbing should also be insutated, in most cases.
See, for tips:
Advanced Windows
Ducts sealing and insulation
Pipe insulation
See also: