Brief Overview of 2015 IECC Compliance Options

The 2015 IECC has 3 main compliance options: Prescriptive; Performance; and Energy

Rating Index. 

1. Prescriptive Path

This is the primary means of compliance with the IECC, and it incorporates three 

different means of calculating the efficiency of the thermal envelope: R-value, U-factor, or 

Total UA. Each path starts with the same reasonable model home with common thermal 

envelope components, and adds varying degrees of flexibility (and complexity). All three paths 

also require compliance with a number of specific requirements and mandatory backstops.

R-value. Compliance is based on meeting a recipe-like table of requirements for specific 

thermal envelope components, along with a more detailed list of requirements for materials 

and systems. This is the simplest compliance option, but also the least flexible.

• Building components must meet the R-values and fenestration U-factors in the 

prescriptive table (R402.1.2). 

• Other requirements and exceptions apply, including requirements specific to heating, 

cooling, and water heating systems, ducts, pipe insulation, mechanical ventilation, air 

tightness, and lighting. The code also lists specific air barrier criteria, insulation 

installation requirements, etc.

• Certain mandatory backstops apply (e.g. air leakage and testing, duct testing, 

fenestration trade-off maximums, etc.)

U-Factor Alternative. This is similar to R-value compliance, except that the building 

complies when an area-weighted average of the whole assembly (such as wall assembly, 

fenestration, etc.) achieves a U-factor less than or equal to the assembly values listed in the 

Equivalent U-factors Table (R402.1.4).
• As with the R-value option, the other specific requirements and mandatory backstops 

apply. This option is more flexible because it permits trading within assemblies. 

Different parts of the ceiling can have different levels of insulation, so long as the Ufactor of all ceiling area is less than the value in Table R402.1.4.

Total UA Alternative. Compliance is achieved if the total proposed building thermal 

envelope UA (sum of U-factor times assembly area) is less than or equal to the total UA 

resulting from the U-factor table, based on the same assembly areas as the proposed building. 

This is different from a Uo calculation because the Total UA is a modeled comparison between 

two buildings with the same assembly areas, rather than a single number that is applied to all 

buildings, regardless of the glazing area percentage. As an example, in the Total UA, glazing 

area is the same on both sides of the equation, so there are no built-in “credits” or “penalties” 

for different glazing area percentages.

• Assembly areas are the same for base case and proposed home. Assembly areas are 

multiplied by U-factors, then added together for a total UA number. For example:

o (Wall area x IECC U-factor) + (ceiling area x IECC U-factor) + (fenestration area x

code U-factor), etc. = Base case UA

o Wall area x proposed U-factor + ceiling area x proposed U-factor + fenestration 

area x proposed U-factor, etc. = Proposed UA. The proposed UA must be ≤ the 

base case UA.

• U.S. DOE’s REScheck is a very commonly-used code compliance software that calculates 

Total UA. 
• As with the R-value and U-factor options, other requirements specific to installation and 

construction apply, such as air barrier criteria, insulation installation requirements, etc.

• Fenestration SHGC requirements must also be met, and certain mandatory backstops 

apply (air leakage and testing, duct testing, fenestration trade-off maximums, etc.).

2. Performance Path

The Simulated Performance 

Alternative is another outgrowth in 

Section R405 begins with a similar 

model home, but permits a wider 

range of trade-offs. In addition to 

trade-offs among thermal envelope 

components, the performance path 

includes the ability to trade among air 

leakage and duct tightness, etc. The 

proposed design must be shown to 

have an annual energy cost less than 

or equal to the annual energy cost of 

the IECC standard reference design 

(which is a model home based on the 

U-factor table). This is more flexible 
than the prescriptive or Total UA paths because it includes a broader range of components and 

systems in the calculation.

• Fenestration area assumption is set at 15%, unless the proposed design is less than 15% 

-- in which case, the standard reference and proposed design use the same glazing area 

assumption. Thus a proposed home with 12% glazing is compared against a standard 

reference home with 12% glazing. Again, this eliminates the built-in “credit” for 

reduction in glazing area.

• Like the prescriptive path, certain mandatory backstops apply (air leakage and testing, 

duct testing, fenestration maximums, etc.).

• DOE’s REScheck also contains a truncated version of the performance path.

• Note: The performance path does not award “credit” for heating, cooling, or water 

heating equipment more efficient than the mandatory federal minimum efficiency 

levels, and the performance path does not include appliances or other plug loads. 

3. Energy Rating Index (New in 2015 IECC)

This is the most complex of the options, but it also offers the most flexibility. 

• The home undergoes an analysis to determine an energy rating score, or index number. 

The baseline standard reference home generally reflects the 2006 IECC requirements, 

Scored from 0-100 or more; buildings comparable to a 2006 IECC baseline earn a score 

of approximately 100, and buildings with no net purchased energy use achieve a score 

of 0. 

• Calculation is based on a methodology called “Normalized Modified End-Use Loads” for 

all energy used in the building, including heating and cooling equipment, water heating, 

lighting, and certain appliances. 

• The ERI is designed to mirror the Home Energy Rating Index

developed by RESNET, but like other IECC compliance 

options, it contains a few key backstops:

o Building thermal envelope shall be at least as 

efficient as the prescriptive tables in the 2009 IECC

o All mandatory requirements of the 2015 IECC shall 

be met

o Maximum ERI scores by climate zone are listed in 

IECC Table R406.4.
Notes:

• The IECC does not control the size or configuration of the house. It establishes a 

reasonable baseline home, and all compliance options are outgrowths from that 

baseline home, based on the assembly areas.

• When the prescriptive or performance path is used to achieve compliance with thermal 

envelope R-values or U-factors, a list of other specific requirements applies, including:

o Fenestration SHGC, 

o requirements related to heating, cooling, and water heating systems, 

o duct systems, including duct testing, 

o air tightness and testing, 

o pipe insulation, 

o mechanical ventilation, 

o lighting

• The ERI option has its own set of assumptions and a different baseline, and its own 

backstops, but it still requires compliance with the mandatory measures of the IECC. 

• This overview is not intended to be a complete summary of all code requirements, but a 

simplified list of a few key elements in the IECC’s various compliance paths.