External wall insulation (EWI) is a continuous insulation layer applied to the outside of building walls, creating a thermal envelope that eliminates thermal bridging through structural elements. EWI systems improve building energy performance while protecting the building structure from weather exposure and temperature fluctuations.
System Components
EWI consists of rigid foam boards (typically R-3.5 to R-6.5 per inch), adhesive, mechanical fasteners, and a weather-resistant finish or cladding. Common materials include expanded polystyrene (EPS), extruded polystyrene (XPS), and polyurethane foam. System thickness ranges from 2 to 6 inches, achieving wall assembly R-values of R-15 to R-40. Typical cost is $8-15 per square foot installed.
Performance Advantages
EWI eliminates thermal bridging, improving overall wall performance by 25-35 percent compared to cavity insulation alone. It protects existing structure from weather and temperature cycling, extending building life. EWI can reduce heating loads by 20-30 percent. Disadvantages include higher upfront cost and need for professional installation, though retrofit applications often justify investment through reduced energy consumption.