Tesa 9100 Series Thermal Insulation The Law of Diminishing Returns The Law of Diminishing Returns as is applies to the 9100 Series. 80% of the insulation occurs with the first 10mm of insulation thickness using 91000 Series. A thickness of 15mm provides an increase of insulation by a further 13% to give a total of 93%. Each additional increase in insulation thickness provides less and less insulation up to the point where insulation will no longer be affected by material thickness. For example, an additional 5mm of insulation on 15mm to make a total of 20mm thickness will only increase the insulation capacity by only 1.8%. A further additional 5mm increase in thickness would have an even lesser impact, namely 1%. This is the basis of the “Law of Diminishing Returns” upon which many of the consultants have used 10mm thickness 9100 Series in place of 25mm fibreglass. Another factor taken into consideration was the tendency for fibreglass to be squashed down on duct corner and the area surrounding the pin mountings. This loss of thickness (and consequent loss of insulation value) would make the fibreglass less effective than would have been the case if it were lying on a flat surface without any distortion to its structure.