This is a great article on the opinions behind which is better single or double layer insulation. The common assertion is that a double layer of insulation on a boiler system is better, but this was not always the case. The specification of single layers of insulation were common practice through the mid 80’s. The question really comes down to a cost benefit analysis. The double layer standard is more costly when considering the labor. “If this change has its roots in an energy-saving decision, then one layer will perform as well as two layers at the same overall thickness, assuming the insulation is installed correctly.”