Windows and Doors under the National Green Building Standard

A look at minimum performance requirements and available points for industry products within the draft NAHB/ICC standard

The National Association of Home Builders published its National Green Building Guidelines in 2005.  It is now working with the International Code Council, through its NAHB Research Center, to develop the National Green Building Standard. Based on the National Green Building Guidelines, the standard is expected to be approved by the American National Standards Institute in spring 2008.  In the standard development process, the guidelines have been enhanced to include residential remodeling, multifamily building, and lot and site development, and reflect advancements in the International Residential Codes.

 
The new NAHB/ICC standard requires builders to include sustainable features in seven categories: energy, water and resource efficiency; lot and site development; indoor environmental quality and homeowner education. Table 1 shows how the point system works in the existing guidelines, but a new higher emerald level is being added to the bronze, silver and gold certification levels when the new standard gets issued.
 
NAHB Green Building Guidelines Point System
 
Level of Certification
Category
Bronze
Silver
Gold
Lot Design, Preparation & Development
8
10
12
Resource Efficiency
44
60
77
Energy Efficiency
37
62
100
Water Efficiency
6
13
19
Indoor Environmental Quality
32
54
72
Operation, Maintenance & Homeowner Education
7
7
9
Global Impact
3
5
6
Additional Points From Sections of Your Choice
100
100
100
Table 1--Not all green building progams are the same, but many use a scorecard approach with builders required to follow some procedures and then given credits or points for taking certain steps or choosing particular types of products. Under NAHB's initial green building guidelines, builders could achieve three levels of green certification—bronze, silver and gold. At all levels, there are a minimum number of points required for each of the document's seven "guiding principles" or categories to assure that all aspects of green building are addressed and that there is a balanced, whole-systems approach. After reaching the thresholds, an additional 100 points must be achieved by implementing any of the remaining line items. The new NAHB/ICC green standard will add a fourth "emerald" level of certification.
 
 
ENERGY PERFORMANCE REQUIREMENTS
The most specific requirements for window and door products appear in the section of the National Green Building Standard documents covering energy efficiency.  The standard features two compliance paths to achieve its energy efficiency goals—one performance-based and one prescriptive. The same energy performance requirements for fenestration products are mandatory for both approaches, however. 
 
Section 701.4.6 of the second draft says that windows, exterior doors, skylights, and tubular daylighting devices must have NFRC-certified U-factors and SHGCs in accordance with Energy Star or equivalent, or those figures shown in Table 2. There is an exception for decorative fenestration elements up to 15 square feet or 10 per of the total glazing area, whichever is less. 
 
NGBS Fenestration Specifications
 
 
 
Windows & Exterior Doors
Skylights & TDDs
 
Climate Zones
Level
U-factor
SHGC
U-factor
SHGC
 
1 and 2
Mandatory
0.65
0.40
0.75
0.40
 
 
4-8 addl points
0.45
0.30
0.55
0.35
 
 
10-12 addl points
0.45
0.25
0.50
0.35
 
3
Mandatory
0.40
0.40
0.75
0.40
 
 
4-8 addl points
0.35
0.30
0.55
0.35
 
 
10-12 addl points
0.35
0.25
0.50
0.35
 
4 to 8
Mandatory
0.35
Any
0.60
Any
 
 
4-8 addl points
0.30
Any
0.55
Any
 
 
10-12 addl points
0.25
Any
0.50
Any
 
Table 2--Performance numbers referenced in second draft of National Green Buiding Standard
 
Under the performance-based approach, builders might be encouraged to use higher performing windows and doors, as they can earn additional points by exceeding the requirements of the International Energy Conservation Code.  The prescriptive approach in the document also outlines two sets of “enhanced fenestration specifications” that can earn builders additional points, also shown in Table 2.

SOLAR DESIGN
Under the Renewable Energy section, the use of “sun-tempered design” is worth five points. Covering building orientation, sizing of glazing, and design of overhangs, the requirements for sun-tempered design in Section 704.3.1.1 include the following:

►The long side (or one side if of equal length) of the building must face within 20º of true south;

►Vertical glazing area is between 5 and 7 percent of gross conditioned

►Floor Area on the south face and windows have an SHGC of .40 or below.

►Vertical glazing area is less than 2 percent of gross conditioned floor

►Area on the west face, less than 4 percent on the east face and less than 8 percent on the north face.

►Glazing must be Energy Star compliant or equivalent.

►Where installed, skylights must be installed with shades and insulated wells, all glazing must meet Energy Star specifications  Horizontal skylights must be less than 0.5 percent of finished ceiling area, and sloped skylights on slopes facing within 45º of true south, east or west must be less than 1.5 percent of finished ceiling area.

►Overhangs or adjustable canopies or awnings or trellises must be used to provide shading on south facing glass for the appropriate climate zone. The document spells out different overhang depths, depending on the vertical distance between the bottom of the overhang and the top of the window sill, and the different climate.
 
In addition to designing a home to benefit from solar heat, the NGBS award points for “passive cooling design features (Section 704.3.1.3).  Three points can be earned by taking two or more of a number of steps, including exterior shading on east and west windows through the use of moveable awnings or louvers, covered porches, vine-covered trellises or attached or detached structures, such as sheds or garages.  Other strategies include overhangs designed to provide shading on south-facing glazing and placement of windows and/or venting skylights to facilitate cross ventilation.
 
Other strategies to optimize a home’s use of the sun to reduce energy demand are rewarded as well.  Automated solar protection that provides shading for windows earns one point (704.3.1.2).  Two points are awarded for the use of tubular daylighting devices or low-E insulating glass skylights in rooms without windows (704.2.4).
 
RECYCLED/RENEWABLE CONTENT
The NGBS (604.1) awards one point if two “minor components” feature 25 to 50 percent recycled content, two points for two minor components with 50 to 75 percent recycled content and three points when recycled content is greater than 75 percent in two minor components.  For “major components,” the potential points for recycled content double. The draft standard does not say whether windows or doors would be classified as major or minor components of a building, however.
 
The standard also awards builders points for the use of building materials “derived from renewable renewable resources.” Up to eight points can be earned for using “biobased” products, including certified solid wood, engineered wood, bamboo, straw, and other natural fibers.  The points awarded are determined by the percentage of the projected building material cost (606.1).
 
Whether windows and/or doors could qualify is not spelled out, but NAHB’s draft document also award four points to builders using a minimum of two certified wood-based products in major elements of a home. NAHB recognizes a number of wood certification programs besides the Forest Stewardship Council, including the Sustainable Forestry Initiative, the American Tree Farm System and the Canadian Standards Association’s Sustainable Forest Management System Standards (606.2).
 
EXTRA CREDIT
Windows and doors can earn points in at least two other ways under the NGBS.  Section 601.7 awards points for using building materials or assemblies that do not require site applied finishes.  In this section, it specifically cites “window, skylight, and door assemblies not requiring paint or stain on exterior and/or interior surfaces.”
If 90 percent of the installed window or door assemblies meet this criteria, a builder can earn up to 5 points.  A builder can earn 2 points if 50 percent of windows and doors satisfy this criteria.
 
Section 610.1 awards points for purchases from product manufacturer with operations and business practices that incorporate environmental management system concepts. The standard says the production facility should be ISO 14001 certified or equivalent and offers one point for every 1 percent of the estimated total materials cost coming from such production facilities, with a maximum of 10 points possible
 
CONSTRUCTION PRACTICES
In its section on reducing air leakage, the NAHB document specifically requires that “caulking, gasketing, adhesive flashing tape, foam sealant, or weatherstripping is installed forming a complete air barrier” around windows and doors (Section 701.4.5.3).
.
The NAHB standard also awards practices that provide “enhanced durability and reduced maintenance.” In section 602.1, it awards three points for an exterior door assembly, including sidelites, that is covered and protected from the effects of precipitation and solar radiation. This can be done through the use of a porch roof or awning, extending the roof overhang or recessing the exterior door.  Additional covered door assemblies can earn an additional point.
 
The same durability section of the document (602.12) also awards six points when a builder shows flashing details in a home’s plans and flashing is installed around exterior fenestrations, skylights and doors and other applicable areas, such as roof valleys and various intersections.  To earn these points, a drip cap must also be provided above windows and doors that are not covered by a roof overhang or protected some other way.