International Energy Conservation Code-IECC 2000/2003

Regulatory Requirements

The U-factor and solar heat gain coefficient (SHGC) of fenestration products shall be determined in accordance with National Fenestration Rating Council Standards NFRC 100 and NFRC 200 respectively by an accredited, independent laboratory, then labeled and certified by the manufacturer. Products not meeting these requirements shall be assigned product values based on defaults values. The default values typically do not give full credit for energy efficient performance.

Air leakage requirements are set at 0.3 cfm/sf for windows and sliding doors (cubic feet per minute per square foot of window area) and 0.5 cfm/sf for swinging doors. Fenestration systems must be tested in accordance with AAMA/WDMA 101/I.S.2.

Residential Buildings

The IECC has two compliance paths for residential buildings: A Simplified Prescriptive path and a Component Approach.

Simplified Prescriptive Requirements

•  Fenestration (glazing) area cannot exceed 15% of the gross wall area for one and two family dwellings.

•  Opaque exterior doors cannot exceed a maximum U-factor of 0.35; however one door is exempt from the requirement.

•  The code has a table that defines 17 climate zones based on Heating Degree Days with minimum fenestration performance U-factors and SHGC requirements. These requirements can be simplified as follows:

Note that envelope performance criteria change for each climate zone.

NR – No Requirement.

Component Approach

This method for code compliance utilizes the same seventeen climate zones, but does not restrict fenestration area to 15% of the gross wall area; rather, it provides tables for 8%, 12%, 15%, 18%, 20% and 25% fenestration area. In addition,

•  The requirement remains for an SHGC = 0.40 for all climate zones with heating degree-days below 3,500.

•  U-factor requirements become more restrictive as the fenestration area increases. For example, if a residence only has 12% fenestration area, the U-factor requirements only go down to U < 0.40; while a residence with 25% fenestration area can require a U-factor < 0.25.

•  Up to one percent of the window area is also exempt from meeting the fenestration U-factor requirement

For the first time, window replacement and room additions were also addressed in an energy code. The 2000 IECC established the following table of prescriptive requirements for these products:

Performance Approach

This method allows the designer to model the building using prescriptive method compliant components creating a "standard" design and an associated energy budget.  The designer then may model the building again trading off components against one another as long as the proposed design's energy budget is less than the standard design's energy budget.  Some of the fenestration limitations on this approach are:

• Standard design window area = 18% of the proposed design conditioned floor area
• Standard design U factor maximums are prescribed (table 402.2.1.2 in Chapter 4)
• Skylights are not included in the standard design computation
• Standard design SHGC < 0.4 if HDD < 3,500
• Standard Design SHGC < 0.68 if HDD > 3,500
• Standard and proposed design interior shading factors must be equal (0.7 summer, 0.9 winter)

Commercial Buildings

The IECC has two compliance paths for commercial buildings. One is simply a reference to ASHRAE 90.1-99. The other is “Design By Acceptable Practice.”

Building envelope requirements are subdivided into seventeen climate zones and into four fenestration area tables including:

•  Fenestration area 10% or less than above grade wall area;

•  Fenestration area between 10% and 25% of the above grade wall area;

•  Fenestration area between 25% and 40% of the above grade wall area; and

•  Fenestration area between 40% and 50% of the above grade wall area.

U-factors for fenestration products range as low as 0.40 and SHGC requirements as low as 0.30. Credit is given for overhangs that help somewhat to minimize the strict SHGC requirements. Energy use in commercial buildings is typically dominated by cooling loads so lower SHGC fenestration products ratings are desirable.