Energy payback of added thermal insulation

The graph shows cumulative heat which escapes through a wall during years. Central European climate with assumed 3500 kelvin days in a year (it is a time integral of temperature difference inside-outside, taken over one heating season).

A common old house with a solid brick wall of overall thickness of 0.5 m is shown by a red line; such a wall has about U = 1.4 W/(m2K) (denoted as its numeric value u in the graph). 0.33 m of added thermal insulation of common properties brings the wall to the passive standard, as shown by red crosses. The initial input of primary energy lost due to producing the insulation material is but 100 kWh/m2, an amount which is gained back in the course of the second (!) year after application. Even a 0.5 m of insulation would return its ‘gray energy’ within two years...

The graph shows clearly that even a wall made according to the recommendation as included in the current standard (blue line), with U = 0.25 W/(m2K) should be further insulated as soon as possible: energy paybacks of the two alternative insulation thicknesses are 7 and 10 years (0.5 m added, blue stars). Within a century the added insulation saves ten times the amount of energy dissipated for its production! Clearly, the current Czech standard is very outdated, nothing short of the passive standard should be obligatory. For climate protection, this is a condition sine qua non.

The above computation has been made assuming an unrealistically large primary energy dissipation for the production of the insulation material, 300 kWh/m3. Half of that holds probably for EPS with 15 kg/m3, two thirds of that for mineral wool. So the real energy paybacks of the applied material may be even twice shorter!

Simply put, even a uncommonly huge insulation layers, over 0.5 m thick, are entirely reasonable, even if not made of straw. Using them is a no-regret measure. Any doubts if such a generosity is not wasteful should be forgotten. Half a meter of insulation should become normal whenever there is no serious space limitation.

Of course, if any natural material can be used, whose application means carbon sequestration from the very beginning and which has the same insulation properties like four decimetres of grey EPS, the decision to apply it immediately would be even more adequate. Especially if the wall surface is to be improved in any case.

Jenik Hollan, and