Exterior Walls Best Practices
Check out photos from HERS rating projects in Western Massachusetts, with detailed descriptions.
To make an efficient wall system, we must consider the key components of the wall: insulation, framing, and glazing (windows).
A code built wall is fine, but by designing windows with intention, and reducing energy loss through the framing (‘thermal bridging’), we can have a great wall that has big impact on overall energy performance.
Principals of an energy efficient wall:
Clearly defined air barrier - Detail it on drawings, including the transitions to adjacent components (ie. attic, rim/band, foundation). Typically, the primary air barrier is the sealed sheathing.
For best performance, add continuous insulation to stop energy loss through the frame - exterior rigid board, double-stud walls, are examples (See images!). Ideally 30% of overall R-value is on the exterior, which is a measure of insulation’s ability to resist heat traveling through it. More is better. If you go too low, you can have durability issues due to dew point location and condensation risk.
Window shading can be as simple as extended eaves, porches, overhangs etc - In the summer, when the sun is high, we want shade. In the winter, we want heat gain. Lower U-values are better; they are a measure of how much heat is lost or gained through the window or door (inverse of R-value). A higher SHGC (solar heat gain coefficient), heat gained from the sun's energy through windows, is better. A clever designer will explore SHGC based on orientation and passive solar opportunities.
Check out these many approaches to energy efficient wall systems: