The attic. That dirty, hot, dusty place that you probably never go except perhaps to toss the Christmas decorations until next year.
It deserves more respect. No other part of your home does more to hold in the heat of your home, for the simple reason that heat rises. It should hold out the heat in the summer, too, because that attic becomes a veritable oven under the scorching summer sun.
So learning about your attic is a major part of ensuring your home is as energy efficient as possible.