SIPs – The History of a Truly Modern Method of Construction

“Little pig, little pig, let me come in…”

British children learn to fear alternative construction methods at an early age, courtesy of the tale of the three little pigs. Whilst hungry wolves aren’t exactly a concern for most UK homebuyers, this shared cultural experience may still go some way towards explaining our continued love of brick construction. However, in this age of tough building energy regulations, it makes sense to explore modern alternatives which are specifically designed to deliver excellent building fabric performance. Chief amongst these are Structural Insulated Panels (SIPs).

What are SIPs?

SIPs feature a timber facing, typically either OSB or particleboard, bonded either side of an insulated core. This highly insulated design can allow them to achieve exceptional thermal performance. For example, 172 mm SIPs with a rigid urethane insulation core can achieve U-values of 0.16 W/m^2.K or better without the need for additional insulation. Their factory engineered joints can also help to significantly limit air-leakage from the building envelope.

The panels can be cut to each building’s unique specifications whilst still in the factory. This provides designers with a high degree of creative freedom and also helps to keep waste and adjustments to a minimum on-site. The panels slot together snugly like pieces in a jigsaw, and their lightweight design means they can be quickly installed by a small team of operatives. In addition, once the building shell is formed and a breather membrane applied, the panels are temporarily weatherproof, allowing internal fit-out to begin.

Whilst SIPs are a relatively recent addition to the UK market, they have a long and well-established history, beginning across the Atlantic Ocean.

History

The story of SIPs begins at the Forest Products Laboratory (FPL) in Madison, Wisconsin. The FPL was formed by the US Government to identify new timber technologies which could help to preserve the country’s limited resources. The FPL began experimenting with what they described as ‘stress-skin’ or ‘structural sandwich’ panels in 1935. The initial panels were formed from a honeycombed paper core with a number of facing materials including plywood, Douglas-fir, and aluminium. The FPL researchers quickly recognised the benefits of this approach, acknowledging:

“The panels are strong, stiff, light and economical of raw materials. They can be made by processes that lend themselves to mass production from materials that are produced in large volume”

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.

A new study has shown that by specifying SIPs in external cladding systems, over traditional approaches, it is possible to reduce the thickness of wall constructions, creating more lettable space within the same footprint. The report can be viewed in full here.

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