In response to the scarcity of construction materials following the war, the FPL intensified its study of structural panels, erecting a test unit to evaluate how different constructions performed over time. When the unit was broken down and assessed in the mid-1970s, researchers observed the panels were still in very good condition and had suffered no significant loss of structural strength.
In addition to the work of the FPL, architect Frank Lloyd Wright was also an early pioneer of the “structural sandwich” approach, utilising the panels in his Usonian homes. The panels featured a core of three overlapping softwood layers faced with thick tar paper. This modular design helped to reduce construction time and expense on the project, supporting Lloyd Wright’s Usonian vision for homes which blended high-quality architecture with a stripped down design, removing features such as attics and garages to minimise costs.
Despite the obvious advantages of these early systems, both the FPL prototypes and the Usonian Homes were characterised by poor insulation performance. In 1952, architect Alden B. Dow addressed this with a new panel design using a polystyrene foam insulation core manufactured by his family’s company, Dow Chemicals, and in the process creating the first true Structural Insulated Panel. Unlike its sandwich panel predecessors, the first Dow SIP homes were observed to be “draft free, easy to heat in winter and easy to keep cool in the summer”.
In addition to its thermal performance benefits, the insulation layer in Dow’s SIPs also helped to align the sheets of plywood in a parallel plane. This created a robust structure without the need for rafters and studs. The strength of this construction was clearly demonstrated when one of Dow’s first test properties survived the impact of a runaway car, suffering only damage to the four panels the car passed through whilst entering and exiting the building.
Whilst Dow’s homes demonstrated the energy saving benefits of SIPs, the low cost of both energy and labour limited early uptake. However, following the rapid increase in energy prices after the 1973 oil crisis, the construction method firmly took root, first in the US and Canada, and then across Europe. The development of the Passivhaus Standard in the early 1990s further supported this growth, with the fast, simple erection process and outstanding fabric performance making SIPs a natural choice for the fabric based standard.
In addition to their original use as a whole building system, SIPs are now increasingly being installed in external cladding applications. We will explore this approach in detail in the next blog in this series.