Convective Heat Loss, a sneaky villian

Strategies to increase building energy efficiency typically focus on increased efficiency of the HVAC system and increased thermal resistance of the envelope and fenestration. An oft- overlooked, but vital, path to efficiency is reduction of air-leakage.

Heat transfer in buildings occurs in three forms: conduction, convection and radiation. Of these, conduction and convection transfer most of the heat. Conduction occurs when one body transfers its heat to another with which it is in contact–in this case, the transfer between interior and exterior air. Convection occurs when a heated body physically transports itself to another location. In this case air physically moving through leaks in the wall. Thermal insulation prevents conduction but does little to prevent convective heat losses.

The results of my research at Michigan State University for the US Department of Energy on Residential Energy Retrofit showed the tremendous impact leaks had on energy efficiency. The research revealed that in an average American home, the aggregation of these leaks is an equivalent of a 1 square foot hole in the wall!

How can air-leakage be controlled? The solution is surprisingly simple. First, an energy auditor must measure and detect air leakage. They may use several tests to determine this. The blower-door test is a common method used in small and medium sized buildings. Once detected, they may be removed by either using an air barrier or by caulking and sealing them. Additionally, the use of foam insulation in place of fiberglass acts as an air barrier by itself.

The US Department of Energy’s Guide on Air-Sealing has some really good information to how to effectively air seal a building.

Air-tightening the thermal envelope is one of the most overlooked strategies for energy efficiency, yet is one of the most important and cost effective.