Some basic insight about the cost optimal opaque thermal insulation in buildings without overheating or cooling (e.g. transalpine Europe)

Panel: 5. Buildings and construction technologies and systems

This is a peer-reviewed paper.

Author:
Dirk Van Orshoven, Independent energy engineer, Belgium

Abstract

In the pursuit of nearly zero energy buildings, it appears a widely held belief that technological progress and cost reductions due to large-scale application will allow to achieve in an economic manner new buildings (or new building elements) with an extremely low energy demand (similar to so-called passive houses). This often results in very ambitious roadmaps, including scenarios for deep renovation of the building stock.

This hypothesis seems to materialize for major aspects such as heat generators (highly efficient condensing boilers), efficient ventilation systems and good envelope air tightness. However, cost optimal calculations performed in all EU member states usually result in economic thermal transmittances much higher than those typically found in passive houses (e.g. by a factor of 2).

The paper first derives the simple analytical formulas that provide a clear understanding of the different factors that influence the cost optimal insulation thickness. They are applicable to buildings that are not subject to meaningful overheating and/or do not need active cooling (such as most dwellings in transalpine Europe). They can be useful for setting requirements in public regulations.

The paper then illustrates in a graphical manner the influence on the economic optimum of several variables, such as the initial minimum cost of insulation, the marginal cost of extra thickness, the energy price and the upgrading of an already semi-insulated component. Among other things, the analysis shows that when components are (initially or during renovation) insulated, the full cost optimal insulation level should be achieved at once, because later retrofitting of suboptimal insulation usually becomes uneconomical.

Finally, the paper illustrates that it seems unlikely that typical passive house insulation levels will ever be effective from a strictly economic point of view, even if energy prices were to double or triple compared to current levels.