Code Guides


When the IECC model code was released in 1998, it addressed the need for energy conservation and improved human comfort in our new and existing buildings. Although it is often adopted carte blanche by local authorities, along with other ICC codes such as the IEBC, its implementation on historic buildings is often poorly considered. This summary offers initial guidance on the use of the IECC. The 2021 version of the code is used as a reference.

Purpose and Intent of the Energy Code

  • Provide baseline/minimum efficiency and improve building performance
  • Improve performance without impacting the safety or durability of the building
  • Improve existing buildings as much as feasible
  • Ensures that buildings are cost-effective to operate, comfortable, and resilient
  • Renovating or rehabbing for energy efficiency can be cost-effective but is limited

Multiple Compliance Paths

  • The code provides flexibility for existing buildings.
  • The Prescriptive Path is the most restrictive but easiest path for compliance. Essentially meeting defined targets.
  • UA-Trade-off path (Prescriptive) is less restrictive, for example, trading envelope insulation values between assemblies. Consider this path for major envelope renovation.
  • The Performance Path is the least restrictive. Must meet mandatory requirements. Show that your design is better than a minimally code-compliant design.

Guiding Provisions

(2018) C101.3 The IECC shall not abridge the safety, health, or environmental requirements of other codes.

(2018) C501.2 Code shall not require removal, alteration, or abandonment of, nor prevent continued use of an existing building or system.

(2018) C501.6 Provisions of this code shall not be mandatory for historic buildings, given documentation from a qualified design and/or preservation professional demonstrating that compliance would change the historic form, fabric, or function of the building.

(2018) C505.1 Spaces undergoing a change in occupancy that would increase demand for either fossil fuel or electrical energy shall comply with this code. Calculations, simulations/models, or field testing can be used.

Existing Building Additions (C502)

(2018) C502.1 Modified portions or systems of a building shall be brought into compliance with current codes without requiring changes to unmodified systems.

Additions can comply with the code as part of the existing building or standalone. Standalone is typically easier to meet compliance requirements. 

Prescriptive or Performance Paths are common. 

Existing Building Alterations (C503)

  • (2018) C402.1 Building envelope requirements based on a prescriptive basis per the compliance path selected in
  • (2018) C401.2. Other compliance paths may allow alternative criteria.
  • Alterations have specific exemptions, but generally, a modified portion of the building must meet the code without requiring updates to unmodified portions. 
  • Repairs are specific alterations that are not required to comply with energy code requirements.
  • Prescriptive is most common (single-system renovations) 
  • UA Trade-off (Prescriptive) is beneficial if modifying multiple envelope and lighting systems. UA Trade-off can be 110% of prescriptive UA. (a little more fl exibility)
  • Performance Path compliance requires total energy cost to be no more than 110% of reference design energy cost. (also, provide a 10% buffer…since it is an existing building) 

Existing Building Repairs (C504)

  • Glass-only replacement 
  • Roof repairs (patching membrane or shingle replacement). 
  • Replacing doors will not require the addition of a vestibule. 
  • Repair of lighting where only the bulb/ ballast is replaced.
  • Work that does not require a building permit also falls under “repairs.”

Existing Building Change of Use (C505)

  • If space is converted and will increase energy consumption.
    • Either must fully comply with the prescriptive code, or
    • Comply with a modified performance path (proposed energy costs must be less than 110% of the reference design’s energy costs). This is usually the most cost-effective, as you can trade high-cost measures for lower-cost ones.
  • If converted space does not increase energy consumption, then code is not triggered.

Building Components

  • The following offers general information on some of the individual building components and systems in the code.

Exterior Wall

  • Use a parge coating as a base layer on the inside of the masonry wall.
  • Use an interior air barrier on masonry walls when possible (liquid vapor seal (semi-permeable fluid-applied membrane, closed-cell foam insulation). This can be integrated into furring/ framing.
  • If appropriate, utilize wall furring to create an insulated cavity
  • Install a “smart” air barrier over wall framing and insulation and under interior finish (e.g., drywall) 
  • If you open a wall, you must ensure that the renovated wall has the same or better U-factor. 
  • Closed-cell spray foam can assist as an air barrier.
  • Masonry walls - if there is a furring cavity (1.5” or less) does not trigger insulation requirements. A 2x4 or larger cavity triggers full R-Value compliance with code.


  • Use light-colored/reflective roofing membranes when possible.
  • Use spectrally-selective shingles or metal panels for high-sloped roofs.
  • Limited option: green roofing (requires additional structural analysis/updates)
  • Deck top insulation - If you remove the existing membrane, insulation is added to the maximum able, given constraints (e.g., parapet heights, roof curbs, access hatches, and flashing heights/membrane attachment requirements)
  • Under deck insulation - If rafters are exposed, their cavity must be filled. Get as close to R-30 as possible. Must maintain venting conditions. If unvented attic. Sealing at the drywall level is very important to avoid moisture damage.
    **Note: if the ceiling inside does not qualify as an air barrier (e.g., tile ceiling), the insulation under the deck will not qualify as a “thermal
    barrier.” In this case, insulation must be added to the top of the deck.
  • Attic floor insulation - roof deck work may also require attic baffle vents to prevent wind washing. But not required to add insulation.


  • Applying storm windows or film does not trigger energy code compliance. Neither is required to meet U-Value/SHGC requirements.
  • Replacement windows must comply individually or as an area-weighted average (Table C402.1.2). Proper flashing and air-sealing should be done correctly. Air sealing is where the main energy savings occur.
  • Replacing the entire fenestration unit triggers compliance for that unit only.
  • Historic windows are often difficult to approve replacement windows.
  • Historic windows refurbishment is an option (falls under repairs).

Air Leakage Testing

  • Not required for existing buildings, but exceedingly helpful
    • Improves insulation effectiveness
    • Prevent draughts, moisture, and pollutant migration in & out of the building
  • Air leakage pathways can reveal breaks in fi re spread barriers.
    • Combine with smoke testing to locate penetrations
    • Infra-red cameras can find outdoor air leaks but generally not interior leaks.

HVAC Replacement (C403)

  • Conduct load and sizing calculations based on anticipated use.
    • Ex. (2018) R402.4.1.2 The building or dwelling unit shall be tested and verified as having an air leakage rate not exceeding five air changes per hour in Climate Zones 1 and 2 and three air changes per hour in Climate Zones 3 through 8.
  • Must have required applicable features (energy recovery, economizers, controls).
  • The duct system is separate from the HVAC system.
  • Replacing a component of HVAC (fan, coil, damper) falls under repair, and the energy code is not triggered.

Lighting Alterations

  • If more than 10% of fixtures in the space are renovated, must meet energy code requirements (maximum lighting power density, lighting occupancy, time, and daylight controls, as applicable).
  • If less than 10% of fixtures are renovated, it must not increase energy use in space.
  • Swapping bulbs or ballasts falls under repair, and the energy code is not triggered. 


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