Glazing that Complies with IECC (International Energy Conservation Code for Residential Buildings)

The IECC (International Energy Conservation Code) is one of the adopted codes of the U.S. Department of Energy. The Building Energy Codes Program is designed to support the building of energy codes, development of codes and generally leading states to adopt and implement building codes with the view to achieve the maximum practicable, cost-effective improvements in energy efficiency while providing safe, healthy buildings for occupants.

The International Energy Conservation Code (IECC) is one of the most widely adopted model energy codes in the US for residential buildings (ASHRAE 90.1 is for commercial buildings).

Residential buildings are classified as buildings that are three stories or less in height. These include single family, detached homes, duplex, triplex, fourplex and townhouses.

Each state may implement their own requirements for builds that might differ from the IECC. For example, New York has adopted a Stretch Code program which exceeds the requirements of the base code.

This building code sets out the maximum thermal insulation, air leakage and SHGC for glazing installations and structures, as looked at as part of a wider build.

We have detailed out some of the key areas of the IECC below as it applies to architectural glazing to help with your glazing specification.

Climate Specific Performance Requirements

The required U factors and SHGC for glazing (including skylights, windows and glass doors) are all classed as ‘climate specific requirements. This means that they change depending on the assigned climate zone of the building site.

To determine the ‘climate zone’ of the building you must look at two components of the location. There are eight temperature separated zones with three moisture focused clusters (A = Moist, B = Dry, C = Marine). This creates 24 separate zones however only 15 of these are in the US.

In addition to this, there are two additional climate specific designations;

‘warm-humid’ = counties defined as ‘warm-humid’ by ASHRAE
‘tropical climate zones’ = simplified or relaxed requirements can apply in some conditions

Figure R301.1

a colour map of the US showing the various climate zones under the IECC

These zones are detailed in Figure R301.1 with a further county break down in Table R301.1. This will help you to determine the expected thermal insulation and SHGC performance requirements for your glazing based on the building location.

There are exemptions and areas where you can use lower performing glazing, as long as the overall envelope complies with the code.

Insulation Requirements for Windows Under IECC

The below table provides the details of the basic thermal insulation requirements and solar protection requirements for glazing on your project based on its Climate Zone.

Extract from Table R402.1.2

Climate Zone	Fenestration U-Factor	Skylight U-Factor	Glazing SHGC
1	NR	0.75	0.25
2	0.40	0.65	0.25
3	0.35	0.55	0.25
4 (expect C (Marine) locations)	0.35	0.55	0.40
5 and 4C (Marine 4)	0.32	0.55	NR
6	0.32	0.55	NR
7 and 8	0.32	0.55	NR

There are no SHGC requirements in a Marine Zone (C).

In Climate Zones 1-3, skylights can be excluded from the SHGC requirements as long as the SHGC for the skylights does not exceed 0.30.

There are also alternative tables to refer to in R402.1.3 and R402.1.4 using slightly different types of values however, the overall performance is the same (just shown in a different way).

U-factor and SHGC for each fenestration product must be obtained from a label attached to the product certifying that the values were determined in accordance with NFRC procedures by an accredited, independent lab (R303.1.3) or the U factor must be taken from a limited default table (Table R303.1.3(1)).

Default table U factors are very high and also do not have a default value for triple glazing. It is therefore important that NFRC labels are used where possible.

Read our technical article regarding U factors and what can be achieved with architectural and minimal glazing here.

SHGC Values for Dynamic Glass

There are separate rules for Dynamic Glazing products as detailed within Section R402.3.2. Dynamic glazing is a technical glass solution that allows the SHGC of the glass to change by the application of energy through a coating within the glass. It can help to create more adaptive glass designs which are proven to reduce energy use within a building and create more comfortable spaces.

As dynamic glass has a changeable SHGC, you have to look at the ratio of the higher to lower SHGC. This must be greater than or equal to 2.4.

In addition, the dynamic glass must have an automatic control (either using Photochromic glass or including automatic control within the Electrochromic glass control).

If both the higher and lower SHGC values of the dynamic glass are compliant with Table R402.1.2 then you do not need to do this.

Using an Area Weighted U Factor for Compliance

Section R402.3.1 allows you to use the calculated area weighted average U-factor of all fenestration products in the building to satisfy the fenestration U-factor requirements set by Table R402.1.2 or R402.1.4.

If all fenestration products (window, door or skylight) do not meet the specific value in the tables above, the designer can still achieve overall compliance by showing that the weighted average of all products is equal to or less than the specified value.

This option permits the use of some windows that have values lower than the prescriptive general requirement, as long as these lower performing windows are offset by windows with values better than the requirement.

When applying this area-weighted option, it is important to remember that the term “fenestration” includes windows, skylights, doors with glazing and opaque doors, all of which would be included in the average calculation.

However, you cannot exempt more than 15 square feet (1.4 m²) of glazed fenestration per dwelling unit from the U-factor and SHGC requirements in Section R402.1.2 (requirement of R402.3.3).

This might be a suitable solution to compliance if you are using systems or glazing solutions that do not carry an NFRC label (such as bespoke structural glass arrangements). You can use the default table for the U factor for the specific non-labelled bespoke construction and then refer to Section R402.3.1 to calculate the average U factor across all glazing which is an alternative way to meet the thermal insulation criteria.

Using An Area Weighted SHGC Calculation for Compliance

Similar to the above, you can use an area-weighted average calculation of the SHGC for your glazing if individual items do not comply with the values in the table above for some elements. This is detailed in Section R402.3.2.

You can only use an area-weighted calculation if the fenestration products are over 50% glazed.

When calculating the area-weighted SHGC all dynamic glazing must be considered separately.

Specifics for Tropical Zone (R401.2.1)

If a residential building within the Tropical Zone is located at an elevation of below 2400 ft (731.5m) above sea level then you can reach compliance by meeting specific conditions, rather than the full IECC as other residential builds.

This is because these areas (between the Tropic of Cancer and the Tropic of Capricorn) have already developed inexpensive building techniques over the years to tackle the high temperatures present to create low cost, comfortable homes. These areas – also known as ‘The Tropics’ - include buildings in Mexico, Costa Rica and areas of the Caribbean (including The Bahamas, Dominican Republic, Barbados and St Lucia).

It would therefore be counterproductive to push these buildings in tropical climates to adopt mainland construction practices to achieve what they already do.

Traditional construction in these tropical climates focuses on greatly reducing or eliminating the need for air conditioning by making a living space that is comfortable without space conditioning (due to the high temperature climate, heating is seldom needed). Traditional methods for solar water heating, ventilation and shading are already in wide practice.

Specific Conditions for Tropical Zone Residential Builds

Less than half of the occupied space is air conditioned.
The occupied space is not heated.
The service water heating is provided by at least 80% renewable sources (such as solar, wind or other renewable energy).
Glazing in conditioned spaces has a solar heat gain coefficient of less than or equal to 0.40 or has an overhang with a projection factor equal to or greater than 0.30.
Permanently installed lighting is in accordance with Section R404.
The exterior roof surface complies with one of the options in Table C402.3 or the roof/ceiling has insulation with an R-value of R-15 or greater. If present, attics above the insulation are vented and attics below the insulation are unvented.
Roof surfaces have a minimum slope of 1/4 inch per foot of run. The finished roof does not have water accumulation areas.
Openable windows / glass doors (fenestration) provide ventilation area equal to at least 14% of the floor area in each room. Or equivalent ventilation is provided by a ventilation fan.
Bedrooms with exterior walls facing two different directions have operable fenestration on exterior walls facing two directions (for cross ventilation).
Interior doors to bedrooms are capable of being secured in the open position.
A ceiling fan or ceiling fan rough-in is provided for bedrooms and the largest space that is not used as a bedroom.

Proving Compliance with IECC

Once you have understood the thermal performance requirements for your windows and glazing under IECC, you then need to prove that the building complies with the code.

This can be done in a few different ways depending on the overall design and performance of the individual elements of your build.

The main options are:

Prescriptive = following the required U factors within table R402.1.2.

Simulated Performance (Section 405) = an alternative route to compliance using building performance simulation software. The calculation can be used to show that the predicted annual energy use of a proposed design is less than or equal to that of the same home if it had been built to meet the prescriptive criteria in Sections R402 and R403 of the ICC.

This route to compliance can provide a greater level of flexibility in terms of design and material specification for bespoke home designs as it does not require every element of the envelope to perform to the code compliance levels. As long as the plans meet the overall code requirements, individual components of the home may fall below required code compliance levels.

Energy Rating Index (ERI) (Section 406) = an alternative route to proving compliance. Designers must meet the requirements of specific areas within Sections 401, 402, 403 and 404 (as indicated in Table 406.2) AND the proposed design must be shown to have an Energy Rating Index of less than or equal to the maximum ERI based on the building’s climate zone (as indicated in Table 406.5).

Total U Factor vs Area (Section R402.1.5) = an alternative route to proving compliance. Here, you take the sum of the U-factor of all materials and times it by the area of the building (assembly area). This UA figure must then be less than or equal to the total UA of the same sizes building using the required U-factors within Table R402.1.2. The calculation must be done in line with the ASHRAE Handbook of Fundamentals.

This alternative allows one portion of the building to make up for another. It recognizes that there may be reasons for less insulation in some parts of the building, which can be compensated for by more insulation in other parts of the residence. The key concept is that the overall building thermal flow (UA) meets the code and can allow for greater levels of design flexibility for bespoke home designs.

Hard Limits on U Factors and SHGC

When using any form of alternative weighted calculation for compliance (as above in Section R402.1.5 or R405), there are prescribed ‘hard limits’ for both U factors and SHGC values.

These are listed in Section R402.5:

Climate Zone	Vertical Fenestration U Factor	Skylight U factor	SHGC
1-3	NR	NR	0.50
4 and 5	0.48	0.75	NR
6-8	0.40	0.75	NR

Air Tightness Requirements for Houses in IECC

Section R402.4 showcases the requirements for air tightness to reduce air leakage from house builds. This includes the air tightness of windows and exterior doors.

A key part of ensuring that windows and doors are airtight is the installation. All fenestration installation must be undertaken by trained professionals to ensure a weather tight finish. This is why IQ undertake the installation on all our projects.

R402.4.3 details the exact air infiltration valued allowed for windows and doors. These values must be tested in accordance with NFRC 400 or AAMA/WDMA/CSA 101/I.S.2/A440.

Opening Type	Max Air Filtration
Windows, skylights and sliding glass doors	0.3 cfm per square foot (1.5 L/s/m2)
Swing doors	0.5 cfm per square foot (2.6 L/s/m2)

The Keller minimal windows sliding door system has been tested for air infiltration under AAMA/WDMA/CSA 101/I.S.2/A440 receiving a classification of A3 for air infiltration. This equates to a maximum air leakage of 0.1 cfm per square foot (5 L/s/m2). This far surpasses the requirements of air tightness under IECC for sliding doors.
- This test was performed on a specimen of over 16ft tall and 10ft wide (5m x 3.1m).
The Sieger Legacy TH+ Window and Door system has been tested for air infiltration under AAMA/WDMA/CSA 101/I.S.2/A440

Requirements for air tightness do not apply to site-built windows like structural glazing and the Invisio fixing system for structural glass.

Exemptions and Specifics

There are specific types of extension and build that are detailed within the building code with some exemptions and specific considerations.

Sunrooms

A sunroom is defined by the IECC as a:

Single storey structure
Where the glazing area is greater than 40% of the gross exterior wall or roof area.
It must have a separate heating or cooling system from the rest of the house.
It must be thermally separated from the rest of the house

Although there are different performance allowances for the ceiling and walls of a sunroom, the glazing of a sunroom must comply with the performance requirements of the code.

Windborne Debris Regions

Windborne Debris Regions are areas within hurricane-prone regions that require some additional protection to their glazing.

Hurricane-prone regions are areas vulnerable to hurricanes, defined as the U.S. Atlantic Ocean and Gulf of Mexico coasts where the ultimate design wind speed, V_ult, is greater than 115 miles per hour (51 m/s), including Hawaii, Puerto Rico, Guam, Virgin Islands and America Samoa.

Windborne debris regions are within these areas and are either:

Within 1 mile (1.61 km) of the coastal mean high-water line where the ultimate design wind speed, V_ult, is 130 mph (58 m/s) or greater.
In areas where the ultimate design wind speed, V_ult, is 140 mph (63.6 m/s) or greater; or Hawaii.

Under Section R301.2.1.2 any exterior glazing on buildings in these regions require additional protection. You can use glazing systems or separate shutters that have been tested to the Large Missile Test of ASTM E1886 and ASTM E1996 to provide this protection.

The Keller minimal windows sliding door system has been tested to this standard for use in windborne debris areas and High Velocity Hurricane Zones (HVHZ).

Or, you can use bespoke wooden structural panels to provide this protection as long as they meet the requirements of R301.2.1.2 and Table R301.2.1.2.

Explore our Hurricane Resistant Glazing options here.

Extensions and Renovations

Section 502 details the rules for building works to existing buildings. Any new constructions (like extensions) or new materials should adhere to the IECC requirements for thermal performance as above. This also applies to change of use alterations.

Renovation projects are exempt from the air tightness on site testing requirements of R402.4.1.2.

There are exemptions for historical buildings in order to protect the historic design and architecture of the existing building.

Specifying Glazing that Complies with IECC Building Code

The best way to ensure that the glazing you are specifying will comply with your local building code is to work with the experts in architectural glazing. IQ Glass have showcased our expertise in architectural glazing across the globe and have unique and comprehensive insights into the specific requirements for architectural glazing to bespoke houses in the US.

Get in touch with the team at IQ Glass with your project requirements and let us assist you in your design and glazing specification.

Click here to send us your drawings or set up a Teams call with one of our experts.

Glossary / Index

Figure R301.1 = the climate zone map of the US.

Table R301.1 = a table version of the climate zones for each state, county and territory.

R303.1.3 = requirement that all fenestration U factors must be determined by NFRC 100 and labelled.

Table R3603.1.3(1) = if the product is not labelled, this table shows the U factor that you must assume in absence of proper certification.

R301.2.1.2 and included Table R301.2.1.2 = additional protection required to glazing in ‘windborne debris regions’.

R401.2.1 = separate requirements for houses in tropical climate zones

R402.1.2 and included Table R402.1.2 = the requirements of thermal insulation for the external envelope. Includes all exterior elements of a home including fenestration, skylights, walls and roofs.

R402.1.4 and included Table R402.1.4 = a table of compliance using U factors instead of R values (an alternative to Table R402.1.2).

R402.1.5 = an alternative route to compliance using UA (sum of U-factor times assembly area)

R402.3 = additional requirements for thermal insulation specifically related to the ‘fenestration’ (including opaque doors as well as windows, glass doors and skylights).

R402.3.1 = rules for the areaweighted U-factor calculation (combining all U- factors for fenestration together to prove overall compliance).

R402.3.2 = rules for the area-weighted average SHGC calculations.

R402.3.3 = requirements for the maximum amount of glazing that can be exempted from the requirements of Table R402.1.2.

R402.4 = requirements for air tightness (air leakage)

R402.4.3 = specific requirements for the air tightness of windows, skylights, swing doors and sliding glass doors.

R402.5 = hard limits on U factors and SHGC when area-weighted average calculations are used including the UA route (R402.1.5) or a simulated energy performance calculation (R405).

R405 = an alternative compliance method based on a simulated energy performance calculation.

R406.2 and included Table R406.2 = an alternative compliance method based on an ERI analysis (Energy Rating Index).

R406.5 and included Table R406.5 = the maximum ERI for buildings in specific climate zones in order to comply with the code using the R406.2 method.