How come there is any doubt at all that this obvious statement is true?
Whatever scepticism there may be, the following article compiles evidence that is beyond doubt:
These statements have been proven by practical experience with highly insulated new buildings as well as older buildings retrofitted with heat insulation. In all cases, this evidence is based on accurate scientific methods. The measurement results are so clear that they can be understood by anyone and they correspond exactly with the state of knowledge in building physics, the scientific discipline dealing with such matters. The following articles illustrates practical examples that provide comprehensible evidence for the effectiveness of thermal insulation – and that have already been presented in scientific publications.
Evidence no.1 - Insulation works - Temperature measurements at a highly insulated wall
Evidence no.2 - Insulation saves energy - Heating energy use in a well insulated new building
Evidence no.3 - Insulation increases comfort - Outdoor thermography
Evidence no.4 - Insulation prevents structural damage - Measurements in a retrofit
Challenging connection details and their impact on the heating demand
Comparison of in-situ measurements and hygrothermal simulations of four different interior insulation systems
A detailed study on the cost-effectiveness of heat insulation is included in the publication “Bewertung energetischer Anforderungen im Lichte steigender Energiepreise für die EnEV und die KfW-Förderung” (available in German only) elaborated by the Passive House Institute. (free download).