I wish there was a way to write this without sounding like my typical rant about the qualities of fiberglass insulation, but alas that would have to be the fault of fiberglass insulation.
I am not sure why, but it seems builders (insulation contractors) like to change from fluffy types of insulation they install in flat areas to batt type insulation in sloped areas.
For example let’s say the master bedroom has sloped ceilings while the rest of the rooms have flat ceilings. These sloped ceiling are often created with scissor trusses that allow for attic space while at the same time create the sloped ceiling—like the one in the following picture with a nice ceiling fan.
When fiberglass batts are used in ceilings it can be very difficult (actually impossible) to install them such that there will be no gaps where air in the attic could find its way around the batts or that air leaking from the interior can find its way into the attic. Modern houses should be sealed such that this cannot happen, but too often sealing is not any more properly done than the installation of the fiberglass batts.
Imagine square edge batts installed on the slope of the ceiling. When they get to the top, unless the ends are somehow cut to a taper, there will be a triangular gap left with little to no insulation. This next picture is looking down between the ends of the batts with the drywall visible below.
With infrared thermometer one can see how warm the ceiling is in comparison to the attic.
This cap ran about 15 feet across the bedroom ceiling.
This is new construction and it is very important that the thermal barrier be consistent and continuous—especially when the insulation is fiberglass.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board
