The design of exterior wall assemblies has become increasingly focused on performance for greater energy efficiency and resistance to air and water penetration. The 2015 International Energy Conservation Code and ASHRAE 90.1 now require insulation that is continuous in wall assemblies (i.e. not interrupted by studs, framing, etc.) in all eight climate zones within the United States. This push for continuous insulation in exterior wall assemblies is also reflected in green building standards and the desires of building owners to reduce energy costs. The response by many architects has often been to use rigid foam board insulation located over the exterior structural face and behind exterior cladding. While this may be a common choice, it is not the only one. In fact, closer review suggests that using field-applied spray foam insulation, of a medium density, may be a better choice for many situations. This has been found to be true when looked at in terms of design flexibility, performance, construction efficiency, and cost.

Comparison of Foam Insulation Types

Plastic foam insulation has been in use for decades on a range of applications from coffee cups to the Space Shuttle. In buildings, it has been used in the building envelope for insulating walls, roofs, and floors and some specialty applications as well. In many cases, it carries a higher resistance to heat transfer (i.e. R-value) per inch of thickness than other insulating materials such fibrous materials that are blown or installed in blankets/batts. While similar in many ways, there are notable differences across the common types of plastic foam insulation used in buildings, some of which are discussed as follows.

Rigid Foam Board Insulation

These factory-produced, flat sheets of insulation come in a range of standard sizes with common widths of 2 or 4 feet and common lengths of 8, 10, or 12 feet. The most common, un-faced products are manufactured from polystyrene, which is a petroleum-based plastic made from the styrene monomer. Generically, polystyrene is a light-weight material with about 95 percent air trapped in the foam to produce its insulation properties. In practice, there are two types of rigid polystyrene foam plastic insulation boards available—expanded (EPS foam) and extruded (XPS foam). Although both types of insulation are comprised of polystyrene, the two types of manufacturing processes produce finished products with very different performance properties.