Building Envelope basically refers to the materials that make up your house. The floors, walls, windows, doors, attic, basement, etc. I'm also going to include home orientation and exterior items that impact your energy/water usage in this category.
As you should know by now (if you've become a Mapawatt Blog fan!), you use the greatest amount of energy in your home heating it in the winter and cooling it in the summer. Your home's building envelope has a huge impact on how much heat or AC you need to add to live comfortably!
Except for a few days out of the year (unless you live in San Diego) Mother Nature is constantly battling with the interior of your home. She wants your home to adhere to her temperature policies, while your home is fighting to keep itself comfortable for you (hopefully you aren't making it work too hard). She wants icicles in your bedroom in February and a sweltering desert in July. Think of your home's envelope as its body armor against Mother Nature's efforts to dominate it. The tighter the envelope, the less chance her "temperature arrows" will get through (you can tell I'm a fan of bad analogies).
Your overall goal is to prevent your conditioned air (which refers to any air that is heated or cooled by you) to be affected by Mother Nature. This means insulating your walls and attic, installing efficient windows and door, and preventing air from breaking in or escaping out of your house.
So continue reading this section to make sure your house stays nice and cozy no matter how frightful the weather outside is!