Glass – Windows and glass doors are the largest single source of heat gain in most houses. Houses with extensive
glass exposures are particularly difficult to control due to the thermal performance of glass and high peak-to- part load
Vaulted Ceilings – Since heat rises, vaulted or two-story ceiling make proper air mixing difficult and are particularly
problematic when adjacent to living spaces such as a catwalks or lofts.
Open Returns – Many commercial buildings have air handlers located above a drop acoustical ceiling without a
ducted return. This arrangement places the space above the drop ceiling under a significant negative pressure,
possibly resulting in unwanted outside air (unfiltered and humid) to being drawn into the conditioned space.
Canned Lights – Although sealed canned lights are available, many are not and often contribute to a significant
"stack" effect, particularly in homes with ventilated attics, ultimately resulting in unfiltered warm humid air being drawn
into the living space.
FROGs – Without a separate AC system or zoned controls, and due to the thermal envelope consisting mostly of
roof and knee walls, FROGs often experience large temperature swings.
Stack Effect – Also called the Chimney effect, naturally ventilated attics (during hot weather) often draw air out of the
living space through access opening, canned lights, and other ceiling penetrations. This puts the living space under
negative pressure and the air is replaced with unfiltered warm humid air from the outside.
Knee Walls – Knee walls are the walls between the living space and an attic space. As with an overhead attic,
temperatures can exceed 120º during hot weather, making temperature control difficult unless properly insulated.