Building Envelope
The building envelope is what separates the interior of the house from the exterior. A properly constructed envelope consists of an air tight pressure boundary aligned with a well installed thermal boundary. Most homes have significant air leaks and badly installed insulation. This results in cold, drafty homes with poor air quality. A blower door can easily measure the amount of air leakage in a house. Visual inspection and an infrared camera can identify insulation problems. It is not well know that a small 3/8" gap in an insulated bay can reduce its R-value by 40%. That's almost half of your insulation value lost due to commonly found poor installation quality.The insulation layer and the airtight boundary must be adjacent to each other. Insulation works by trapping and reducing convection or air movement. Heat loss from your home is accelerated by large temperature differentials. In other words, if the inside of your home is 70 degrees. And the outdoor temperature is 35 degrees. And you have no insulation in your wall then there is a large difference in temperature from one side of the wall to the other. Insulation creates minute temperature differences within the wall that retard heat transfer.
With that said, air moving through insulation will reduce its effectiveness. Any gaps between the pressure boundary and the insulation will negatively impact the ability of the insulation to do its job; leading to heat loss and wasted energy. In other words the envelope needs to be airtight, and the insulation needs to be installed right up against that air tight boundary. In the attic we often see gaps between the insulation and the underlying ceiling sheetrock due to the insulation being laid over electrical wiring. The air gaps get heated up by the warm living space below and the now warmed air rises up through the insulation and is lost out the attic vents. You have essentially paid to heat the air outside of your house.
Air sealing will reduce infiltration, improve air quality, eliminate drafts, save energy, and reduce humidity and excess dust in the house.
