Designing An Energy Efficient Home in the Pacific Northwest
Building an energy efficient home comes down to the following factors; proper air sealing, high performance windows, reducing thermal bridging, using the right type of insulation, mechanical ventilation, and strategically locating your heating/AC systems.
The Control Layers
In a building, we need to be controlling the following four factors in order of importance:
This is the bulk water that comes in the form of rain, snow, sleet, etc. If the structure isn’t managing this properly, this will immediately result in problems. Leaks and water infiltration can absolutely destroy buildings; whether it’s causing mold and rot in wood components, rust in steel components, or simply ruining interior finish materials.
Controlling air movement in a home is by far the most important thing we need to be doing apart from controlling bulk water. Air leaks in a home are vectors for heat loss/heat gain, moisture, and mold. If we have air leaks bringing in the cold, these are prime spots where water vapor can condense. We need to be mechanically ventilating our homes and bringing in fresh, filtered, and tempered air on our own terms.
Water vapor is always diffusing through building materials, and will move from the warm side of a wall to the cold side of a wall. The greater the temperature difference, the greater the vapor drive. This means that we need to be very careful about placing impermeable materials on exterior walls, and making sure we are preventing condensation from occurring on the inside of the wall cavity by moving the dew point outwards as best as we can, while ensuring that the walls can easily dry.
Finally, controlling heat flow is important to keeping a home comfortable and energy usage low. This thermal control layer is also known as insulation, and the more there is, the better. If we can stop heat flow through our walls, roofs, and floors, we can reduce the need for mechanical heating and cooling, which make up the majority of the energy consumption in our homes. However, if we haven’t properly controlled the previous three factors, the thermal layer can actually do more damage than good since it slows the drying process.
Air Tightness & Air Sealing
According to Energy Star, air leaks are responsible for between 25% to 40% of energy loss in a building. When we refer to air tightness, we are really referring to the amount of air changes in a home per hour at 50 Pascals, or “ACH50”. This is measured using a blower door test, where all of the doors, windows, and openings in a home are closed except one, and a calibrated negative pressure fan is sealed around this opening to measure the amount of air leaks.