It is not always economical to provide the highest-quality insulation in a building envelope. The insulation of a building should be guided by the following general principles:
1. The higher the heating and cooling degree-days, the more insulation that is needed.
2. The roof needs the highest quantity of insulation, followed by the floor, then the walls.
3. Better insulation is needed when the space temperature is warmer in the winter or colder in the summer.
4. U-values for buildings that are both heated and cooled should be lower than if only one process is done.
These guidelines represent a reasonable target for energy efficiency and should be considered a minimum standard.
Any facility that exceeds these by a noticeable margin may be providing an uneconomical excess of insulation that will not have an acceptable payback on the investment. Consequently, any variance beyond these standards should be justified by a thorough economic analysis based on years of simple payback. The ceiling insulation is easiest to install, so it is almost always a good prospect. Wall insulation must be blown in from above or through holes in the fenestration, and floor insulation must be installed carefully, with vapor and weather barriers. Both are often justifiable but have a longer payback than augmenting the ceiling insulation.
__________
Excerpt from Energy Conservation Projects.