Thermal Boundaries are the parts of the home that physically separate the conditioned space of the home from the unconditioned spaces. This is where insulation should be applied and where you get the most efficient use of insulation. An example of this is a home with an attached garage.
The wall that separates the living space and the garage is part of the thermal boundary, whereas the wall that separates the garage and outside the garage is not.
Breaks in the thermal boundary affect the effectiveness of the insulation. There have been many studies throughout the U.S. that suggest breaks in thermal boundaries reduce the effectiveness of insulation by as much as forty percent. In other words, for every dollar you spent on the insulation, you only receive the benefit of sixty cents. In attics the most common breaks are gaps in the insulation.
Especially around vent stacks, electrical lines, recess lights, chimney chases, stairwells into attics, drop ceilings in bathrooms, just to mention a few. It is very cost effective to fill in these gaps, primarily due to it's access.
For walls, it is a different story. If you live in a home that does not have insulation in the walls it is cost effective to have insulation blown in. It is necessary for you to ask the contractor on the techniques they will use to prohibit breaks and settling of the insulation.
It would be nice if the contractor uses thermal imaging after the job is done to verify there are no breaks in the insulation. To avoid settling, most contractors use dense packing as there method.
For homes that have some insulation in the walls it is usually not cost effective to have insulation installed. The most common breaks in the wall insulation are electrical outlets and switches, plumbing, braces, fire stops and the rough openings around windows and doors. The problem here is the cost to correct this verses the benefit you will receive.