Commissioning is the process of testing out that the retrofit completed is achieving the target which the retrofit or Building Information Model anticipated. This is a very important part of the process of achieving energy efficiency. In large commercial buildings, the sophisticated controls may require some employee training to operate. I have learned of instances where the manual override was left on permanently because employees did not understand the controls. Therefore, a system which could potentially function to conserve significant electricity savings will not be reflected in actual energy bills. A commissioning agent would be able to identify this problem. After a residential retrofit, the comparison of before and after bills, modified to account for a heating degree day differential, will indicate the extent to which the retrofit is meeting the target. If not, there may be additional measures to consider and possibly a few areas missed. If there is no indication of a change after,s ay, attic insulation is installed, a follow up visit from an auditor with a thermal imaging camera would be a good idea.