Question 6.4: A window upgrade is considered for an apartment building fro...

To determine the energy savings due to the addition of insulation, we assume that the annual heating and cooling degree-days before and after the retrofit remained unchanged (this assumption is justified by the fact that the window contribution to the BLC is relatively small) and are, respectively, DD_{H} = 2,244°C-day/yr and DDC = 255°C-day/yr. The energy savings during heating (assume that the system efficiency is 1.0 for electric heating) are calculated to be:

ΔE=24.200 m^{2}*[(4.61-2.02)W/m^{2}°C]*27244°C.day/yr=27,897kWh/yr

If an EER (energy efficiency ratio) value of 8.0 is assumed for the AC system, the energy savings during cooling are estimated as follows:

ΔE=24.200 m^{2}*[(4.61-2.02)W/m^{2}°C]*2755°C.day/yr *1/8.0=396kWh/yr

Therefore the total energy savings due to upgrading the windows is 28,293 kWh which corresponds to about $2,829 when the electricity cost is $.10/kWh. The cost of replacing the windows is rather high (it is estimated to be $150/m^{2} for this project). The payback period of the window retrofit can be estimated to be:

Payback=\frac{200m^{2}*150\$/m^{2}}{28,293kWh/yr*0.10\$/kWh}=10.4 years

Therefore, the window upgrade is not cost-effective based solely on thermal performance. The investment on new windows may, however, be justifiable based on other factors such as increased comfort within the space.