Found these 2 diagrams on a Google image search. I think they do a good job of outlining the difference between a positively performing attic vs. a negative situation.
the space within the roof area of a building. Where insulation is located.
Sometimes attics are used for extra storage space. In order for IDEAL to properly assess and then upgrade the insulation in your attic obstructions such as this plywood must be removed. There could be issues with the ceiling vapor barrier, electrical (such as pot lights and wiring penetrations) and penetrations (such as framing voids) that must be addressed. It also restricts the amount of new insulation that can be installed.
Grave warning from the Winnipeg Fire Department about the potential risk of deteriorated Cellulose attic insulation. CLICK HERE
This video has a good overview of some of the principles and terminology involved in home Insulation.
Moisture in an attic can build over time. It can take quite a while before it makes its way down through the ceiling and onto a light fixture (for example). Take a look into your attic every once in a while to ensure there is no moisture. One of the first signs when you open the hatch and look up is the underside of the roof. If you have a wet attic roof you have a problem that must be addressed right away. If things seem to be Ok but you can still identify black staining on the wooden sheathing of your roof (the roof deck composed of wood slats, plywood or OSB) you may have an indication of a prior wet attic roof.
Melted poly on top of an electrical boxes (light fixtures, outlets, ceiling fans) is a sign of trouble. Have an electrician inspect the installation to ensure it is safe. You can help by ensuring the light fixture below has the appropriate bulb and wattage to help reduce the chances of over-heating. This overheated fixture is also a source of warm moist air into the attic and should be sealed.
Soffits are designed to allow air intake into the attic space. If they are blocked for any reason the intake ventilation must be created through Roof intake vents. These are vents installed on the top of the roof but down near the Eave edges. They must be installed on both sides of the roof to balance. This is now common practice in New construction.
Attic batt insulation should not be block the edges of the attic space and hence restrict air intake flow from the soffit areas. The attic batt insulation should be pulled back to allow an air space above, preferably with the aid of a cardboard stop (installed at time of construction) or retro-fitted with a styrofoam Air Chute.
Bare spots in the attic insulation can cause temperature variations within a room below but are also a prime culprit for Ice Damming on the roof above. Excessive heat transfers through the ceiling and up to the roof above. This localized heat can cause the under-side of the snow pack on the roof to melt only to later freeze. This cycle builds resulting in an Ice Dam. Ice Dams damage shingles and can lead to leaks. Under-insulated attics are often easy to address and can be a quick and cost effective improvement to your attic insulation problems.