What percentage of heat is lost via the different mechanisms in a typical house?

Answer 1

The majority of heat loss from a structure is through the buildings's walls. However if cracks are not sealed, airflow can increase heat loss by up to 30%. Other sources can be

Basements
Floors
Windows and doors
Attic

Air leakage can be traced to poor building construction, improper placement of the vapour barrier, windows and doors with poor insulation value and poor sealing characteristics. Other methods of improving heat loss effieciency are upgrading furnaces to a higher effieciency, and/or using direct cent or sealed combustion appliances.

The best way to seal against air leaks is to apply caulkind to cracks at openings. Sealant can be added around.

Windows (The most effective area)
Doors
Baseplactes and headers
foundations
electrical outlets and ceiling lights.

Sorry I don't have actual percentage values for you but its all pretty relative.

Answer 2

Specific numbers are almost impossible to come up with, without seeing the house. It all depends on the types and amounts of insulation, types of windows and doors, how many windows and doors, types of window treatments, etc.

In a typical home most of the heat is lost through the attic. Heat rises, so extra attic insulation is always a good bet.

Answer 3

I remember reading that about 17% of the heat loss is through windows. Don't know about the rest.