By Todd Collins
There are three primary components to creating a safe, high performance wall system: air tightness, thermal protection and vapor management. The key is to achieve low air infiltration and more insulation to improve comfort and reduce energy use. It is also important to avoid creating problems such as instability down the road due to moisture condensation. If air gets into the wall and the moisture it carries condenses, that moisture needs a way to exit.
One of the largest sources of energy loss is due to air leaking through walls, windows, doors, ceilings, and floors. These are called convective losses. In standard construction, it is estimated that if you add up all the little cracks and holes in the envelope, they often add up to a window size opening that is open 24/7, 365 days a year. The air that passes through carries the heat you pay to create in the winter and the cool you pay to create in the summer.
In addition, the air carries moisture with it from cooking, showering, plants, and even breathing. This is especially important in winter because the vapor in the air condenses as it passes through the wall assembly and potentially causes rot and mold in the walls. By making the walls as air tight as possible, you don't lose as much heating and cooling. And you don't let as much vapor into the walls --thereby reducing the risk of wall failure.
Super insulation, continuous thermal insulation and minimal thermal bridging
Walls are generally made of 2 x 4 or 2 x 6 framing. Those framing members are thermal bridges that transfer heat out in the winter and in during the summer. To reduce thermal bridging, it is important to super insulate and design with a continuous thermal layer.
Using "enough" insulation moves the dew point out to the exterior of the wall assembly and away from the framing members so that if air/vapor does get into the walls it will not condense on the framing members, create mold, and rot. "Enough" insulation depends upon climate, as does the location of the dew point. The dew point is the location within the wall assembly where vapor will condense on the surface of the wall assembly materials.
It is nearly impossible to completely air seal a home - especially when retrofitting an existing home. Some air will get through so it is critical to ensure the walls can dry out. Generally walls should dry to the exterior because it is dryer outside than inside. As mentioned above, we cook, shower, breath, etc. and sometimes we even mechanically humidify our homes. It is therefore important to have a vapor open air/weather barrier and vapor open continuous insulation layer somewhere within the wall assembly. Understanding the permeability of the building materials used in a wall assembly is important when creating a successful wall assembly.
By practicing these approaches to achieve a high performance wall system, we can improve the comfort of our homes, reduce our energy use, and create a safer wall assembly that is less likely to fail from rot and mold. It is important to note that an airtight building will also need mechanical ventilation to bring in fresh air. Energy Recovery Ventilation systems (ERVs) not only bring in fresh air, but also exchange the heat from the outgoing air stream to the incoming air stream. This effectively reduces the energy loss from fresh air ventilation.
Todd Collins is a CGBG board member. His company, AE Building Systems, provides products for high performance homes and buildings and helps architects, builders and home owners improve the energy efficiency of new and existing buildings. A resident of Golden, Todd enjoys spending quality time with his wife and children especially hiking, biking, skiing, and camping in our lovely state of Colorado.