Air leakage: For every cubic foot of air that leaks out, a new cubic foot of air from the outside leaks in at the bottom of your home, causing drafts and uncomfortable spaces. Air leaks through pipes and wire holes, duct chases, the seam at the top of walls between drywall and through framing and many other places.
Ducts: In winter when you heat the air and that air is blown in ducts situated in a cold attic, these ducts will cool the air your heating before they even reach the rooms your heating.
Air leaks around the chimney: Since the building code requires a space between the wood and the chimney, this creates a highway for air leaks from the basement to your attic and out.
Attic hatch or scuttle: The attic access is often leaky since it's the largest whole in the top of the house, and thus a big gap in the insulation.
Can lights: Can lights are very often installed without having the wholes and joints sealed, which lets air leak freely around the bulbs and through the housing to the attic. The more the light is on and heats up, the more the airflow increases. Moreover, this is a big fire risk because can lights are not fire rated to have insulation up against them, so they are gaps in the insulation.
Whole house fan in the ceiling: The louvers leak air you paid to heat. An airtight insulated box built around the fan and accessible when needed is a good solution to this problem.
Bathroom fans vented into the attic: Bath fans dumping into the attic bring humidity and moisture which feed mold and rot.
Ice damming: When the heat gets into an attic that is badly insulated, snow on the roof melts and runs down to the eave where it freezes, blocking water from flowing off the roof. Water then backs up under the shingles and leaks into the house causing major water damage.
Radiant summer heat: In the summer, the sun can heat the shingles up to a temperature of 71 degrees Celsius. This heat radiates into the attic and down through the ceiling, warming the air your paying to cool.
Not enough insulation: There must always be enough insulation to keep your ceilings under the attic above 18 degrees Celsius in winter and below 24 degrees Celsius in summer. Fiberglass batts are poor insulation because of gaps, spaces, thermal bridging, and wind washing.
