Making a Sustainable Energy Efficient Wall

This blog accompanies 1-2 minute videos (1-6) of’s new educational videos on creating an energy efficient wall. They present Joseph Lstiburek, Ph.D., P.Eng, ASHRAE Fellow of, an international expert in building science. Videos are here.

energy efficient wall north american climatesNorth America is a continent with a variety of climates that offer architects and engineers the challenge of designing buildings that can accommodate changing temperatures, atmospheric pressures and moisture levels to fit the needs of a structure’s occupants.

When we walk into a building, we often overlook the basic premise of what that building is actually doing. A building, by its simplest definition, is an environmental separator that maintains a barrier between the interior and exterior climates of a space. (See 2:07 video The “Perfect” Wall Part 1 – Intro to Building Science)

The second law of thermodynamics explains how a building can maintain climate control through its wall insulation. According to Rudolph Clausius, differences in temperature, pressure and density tend to even out after a while. Hmmm.

Where’s the wall part come in?

As defined by the second law, energy is transferred to areas that have less energy. In the realm of building insulation – if you’re air conditioning your building, moisture will go from the outside of the insulated space to the inside and if you’re heating your building, moisture will go from the inside to the outside. Temperatures try to even out, simple as that. This concept becomes valuable when designing a building that can exist in all types of climates. A so-called ‘perfect’ energy-efficient wall should work, in fact, in all types of climates. (See 1:02 video The “Perfect” Wall Part 2 – 2nd Law Thermodynamics)

This ‘perfect’ wall consists of different layers that handle the results of climate. A water control layer, an air control layer, a vapor control layer and a thermal control layer must exist within this wall to optimize the efficiency of its insulation. The order in which these layers exist is extremely important. Water must be considered over all else, because it is difficult to control followed by air, vapor and then thermal transfer protection in the best place…the outside.

The moisture control layer of an energy efficient wall (usually a breathable housewrap-type material) is therefore the first layer outside of a wall. An energy efficient approach today, is to place the thermal control layer outside that, outside the framing. The cladding of the wall would rest on the outside of these control layers, with the structure (framing) of the wall being on the innermost side.

Outboard, continuous insulation is key to a sustainable a wall that can be optimized in all types of environments. (See 1:13 video The “Perfect” Wall Part 4 – The Environment)