The best time to perform a DER for a major building system is when repairs or renovations are needed for that building system. For example, a re-roofing project (made necessary by the existing roof condition) is likely to represent the best opportunity to provide a super-insulated roof and bring the attic into conditioned space. If homeowners desire to remodel and finish a basement space, DER methods can provide guidance for achieving high levels of performance while avoiding typical finished basement problems. The cost for superinsulation of exterior walls (for an existing home) is typically lowest when siding and trim are being replaced. In fact, even when facing the expense of re-painting an exterior, it is worth considering whether those resources might be directed toward a DER for the exterior wall that will essentially eliminate the need for painting for many years into the future.

One of the early participants in the National Grid Deep Energy Retrofit Pilot program observed that significant expenditures to replace (as in siding or windows) or renovate (such as a basement or attic) a building component essentially prevents that building component from receiving further performance improvement for the life of the component. In other words, once windows are replaced, a roof is re-roofed, or siding is re-moved and reinstalled on an exterior wall, it is very unlikely that these systems will be upgraded for a long time. Certainly, once the renovation or replacement has been implemented, the most cost effective opportunity for high performance retrofit is past.

This is why the DER program is designed to capture opportunities presented by typical home maintenance activities such as re-roofing, re-siding and remodeling or finishing basement and attic spaces. When resources are already allocated for repairs or renovations to a major building system to meet non-energy-related needs, the benefits of DER can be achieved at a relatively small increment.