Adding ceiling insulation will save energy. For instance, a home with #1500 ft^2# is located in a city that needs heating system (outside temperature is 40 degrees Fahrenheit and inside temperature is 70 degrees Fahrenheit) for eight months. Let's assume, a new material #R=40 (ft^2*F*hr/(BTU))# is applied instead of old ceiling material (R value 11). This house will save nearly 3000 BTU per hour under new condition. It is nearly 500 US dollars per year (saving).
And also now this house reduces #CO2# emitted into the atmosphere if it is using natural gas. If it uses electricity from a nearby thermal power plant, this situation helps reducing other gases (such as NOx and SO2) emitted .
Reference: Masters, G. M. and Ela, W. P. (2008). Introduction to Environmental Engineering and Science. Pearson Education, Upper Saddle River, New Jersey, USA.