When adding to or altering an existing home, compliance credit can be taken for upgrading existing features by using the performance approach when the existing features are verified by a qualified HERS rater prior to registration of the certificate of compliance (CF1R).

The performance approach provides a means to trade off against features that may not meet the prescriptive requirements, such as exceeding the allowed maximum glass area, by demonstrating that the project (proposed design) achieves the same level of efficiency as it would if it were built to the prescriptive requirements (standard design). The standard design is a hypothetical building with prescriptive requirements from Table 150.1-A that sets the target energy budget for the proposed project.

The Existing + Addition + Alteration approach gives further credit for upgrading existing features. It does this by modifying the standard design for an altered building feature to match the requirements specified in Section 150.2, particularly Table . The greater the efficiency of the altered building feature is relative to the existing energy efficiency, the greater the compliance credit will be. Third-party verification of the features prior to the construction is required to achieve the maximum compliance credit.

The proposed design is calculated using the actual energy efficiency values of the existing unaltered components of the existing building, and the proposed values of the altered components, plus the proposed addition’s features. Each building component must be modeled with one of the following classifications to determine the standard design:

“Existing” – building components that remain unchanged (e.g., exterior walls in the existing portion of the building that will not be altered) but must be verified.

“Altered” –existing building components proposed to be changed (e.g., added roof insulation, or a furnace that is being replaced).

“New” – building components that do not exist prior to the construction work (e.g., new walls added to create the addition). This includes building components in a previously unconditioned space being converted to conditioned space.

All of these building components determine how the standard design is calculated. Existing features are modeled the same in both the proposed and standard designs. New features are modeled in the standard design according to prescriptive requirements in Table 150.1‐A. Altered features are modeled in the standard design according to Table 150.2-D.

There are two columns in Table 150.2-D. One column defines how the standard design is calculated for altered components when the existing features are not verified by a HERS rater. The other column indicates how the standard design is calculated when the existing features are verified by a HERS rater prior to construction.

For the building to comply, the proposed design (proposed project details as modeled) must be equal to or less than the standard design. When a feature in the proposed design is better than the standard design, it receives a compliance credit that can be used to trade against less efficient features. For example, without third-party verification, windows to be altered are assumed to have 0.40 U-factor and 0.35 solar heat gain coefficient (SHGC). With HERS verification, if the existing windows are single pane metal framed, they are assumed to have 1.28 U-factor and 0.80 SHGC, resulting in substantial potential compliance credit if the new windows meet current prescriptive requirements of 0.30 U-factor and 0.23 SHGC.

Example:

Consider the house in Figure G-1 in climate zone 12. The shaded area is the addition. Some windows and walls are removed to build the addition. These are ignored. The existing home has the following features:

Single‐pane metal framed windows

2x4 R‐0 walls, and R‐19 attic insulation

AFUE 75 furnace

Figure G-1 – The Proposed Addition and Alterations