Effective Insulation is Two Parts
Regardless of which material and application is used, insulation is not fully effective unless it is properly installed and fully aligned with a contiguous air barrier. Insulation works because it incorporates air pockets that resist the flow of heat, or slows the conduction of heat. This resistance to heat flow is measured by the R-value of the material. 

Most insulation  (with the exception of spray foam and rigid foam board) does not stop air flow.   Most cost effective insulation solutions require a separate air barrier or "skin" to stop the flow of air. For the air barrier itself to be effective, it must be contiguous and continuous across the entire building envelope, with all holes and cracks fully sealed, and it must be perfectly aligned with the insulation. 

You need both parts of the jacket to make it work the best.  The same goes for the insulation around the house.