Many insulation studies have come to the rule of thumb consideration that can be called
The 5% Rule of Missing Insulation.
Basically stated, if a given area, say a wall or ceiling in a room in a home, is missing 5% of insulation, that missing 5% will cause a more than 50% loss in R-value for that area. To repeat:
More than 50% loss in R-value for that area.
That is substantial!
Let's say you are in newer construction, and there because the homeowners have called about cold rooms. If Mighty Mo is tagging along doing a thermal image sweep of the house to check on the insulation, and the high wall in a cathedral ceiling room is found to have missing insulation, would the room feel cooler?
The answer is YES!
This is no mystery. This is not rocket science. Enough missing insulation, let's say 5% or more of a given area, results in a very uncomfortable room.
Turning on the heat in winter, and letting it influence a room, and then viewing a wall that has a uniform line where it is obvious the insulation seems to end, a thermal camera can tell a lot!
That line on the wall is 9' high. It appears to me the rolls of insulation used in that wall were 9' long.
Perhaps the installers did not have a tall enough ladder that day to insulate higher and were going to get back to this wall after lunch? The insulation at the edge of the ceiling isn't so great either.
I think what we are looking at is the attic cavity of the townhouse next door, above a room in that house that does not have a cathedral ceiling. The insulation high on the wall in this subject house looks to be missing altogether. Since it is a shared wall it is not as cold as it would be if this was the exterior wall of the room. You can see that the cooler temperature is toward the edge of the roof, closer to the outdoors.
My recommendation: thermal cameras not only reveal information, they are very instructive in that they define it. The problem in this room is immediately evident. This is definitive information! It is derived without any invasion to the wall at all. This area could not be seen from the attic space of the house next door because the area seen here is between the interior wall of this room and the fire wall between the two houses. A thermal camera is the only way to clearly define this problem.