Everyone knows if you want to stay warmer when it is really cold, your sweater will become much more effective if you wear a windbreaker over it. This is because the windbreaker decreases air movement into the materials that are keeping you warm---making the sweater more functional.
A while back I did a post about the importance of encapsulating fiberglass insulation to make it more effective. It is much the same thing as the sweater scenario.
At a recent inspection I was randomly scanning house walls with my Infrared camera and found a good example of what non-encapsulated insulation looks like--an all too often occurance.
This first picture is of the wall above the stairwell and behind this wall is attic space over the living room and dining room. 
That wall is insulated as can be seen in this next picture.
If you look closely at the picture, in the distance you can see where the fiberglass sticks out into the attic space a bit. These areas coincide with the darker areas of the thermal image.
Clearly the fiberglass is not in permanent contact with the drywall surface creating a space that cold air can find its way into. This condition increases both heating and cooling loads seasonally and could be improved by encapsulation of the insulation or use of insulations that are themselves better air barriers.
Repairs do not have to be difficult, but supports my contention that all fiberglass batt type insulation should be encapsulated.
Spray foam or spray cellulose would be better options for these types of wall structures.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board
