Do the three approaches produce different results?
Yes, they can. Performance approaches require a higher degree of detail so that an individual building can be designed to exactly meet the IECC requirements. Prescriptive approaches tend to be somewhat conservative and use worst-case default assumptions so the prescriptive packages will apply to all buildings. Although the prescriptive approach may result in a more energy-efficient building because of its conservative assumptions, this situation is not always the case. The prescriptive approach generally does not account for many of the features that affect energy use, such as the effect window orientation and external shading may have on solar heat gain. Trade-off approaches fall somewhere between the prescriptive and performance approaches in flexibility and complexity.
