R-value (insulation) contain no trapped CFCs or HFCs (e.g., half-pound low density foams). On certain brands, twenty-year tests have shown no shrinkage or reduction in insulating value. This has led to controversy as how to rate the insulation of these products. Many manufacturers will rate the R-value at the time of manufacture; critics argue that a more fair assessment would be its settled value. The foam industry adopted the LTTR (Long-Term Thermal Resistance) method, which rates the R-value based on a 15-year weighted average. However, the LTTR effectively provides only an eight-year aged R-value, short in the scale of a building that
History of construction History of construction The History of construction overlaps many other fields like structural engineering and relies on other branches of science like archaeology, history and architecture to investigate how the builders lived and recorded their accomplishments. Those fields allow us to analyse constructed buildings and other structures built since prehistory, the tools used and the different uses of building materials. History of building is evolving by different trends in time, marked by few key principles : durability of the materials used, the increasing of height and span, the degree of control exercised over the interior environment and finally the energy