Emission reduction due to energy efficient building refurbishment

The change (reduction) of $CO_2e$ emissions due to an energetic building refurbishment can be estimated as follows:

$\Delta E = A_H \cdot ( \alpha_{old} \cdot \beta_{old} - \alpha_{new} \cdot \beta_{new})$

$\Delta E$: change of $CO_2e$-emissions due to refurbishment in $t/a$
$A_H$: heated building area in $m^2$
$\alpha$: energy consumption index of the building in $kWh/(a\cdot m²$)
$\beta$: $CO_2e$ emission index of the used energy source in $t/kWh$

• $\alpha_{old}$ (before refurbishment) is either known (e.g. by the energy certificate) or can be estimated by the age of the building:
  https://www.bmwi-energiewende.de/EWD/Redaktion/Newsletter/2015/4/Meldung/infografik-energiekosten-gebaeude.html

• $\alpha_{new}$ (after refurbishment) KfW-Effizienzhaus 55 standard: $\leq 35\ kWh/(a\cdot m^2)$

• $\beta$: only changes if the heating system or energy source is replaced. Typical values depending on energy source:
  https://www.heizspiegel.de/heizkosten-verstehen/brennstoffe-energietraeger-im-vergleich/#c94193