3.2   Fenestration    

Choosing the proper windows, glazed doors, and skylights is one of the most important decisions for any high-performance project. The use of high performance fenestration can actually reduce energy consumption by decreasing the lighting and heating and cooling loads in nonresidential and high-rise residential buildings. The size, orientation, and types of fenestration products can dramatically affect overall energy performance. The National Fenestration Rating Council (NFRC) can help facility managers, specifiers, designers and others make informed purchasing and design decisions about fenestration products. To help select windows with the desired energy performance for institutional projects, NFRC developed a performance base calculation, the Component Modeling Approach, or CMA. The benefits of using CMA are discussed later in this chapter. 

3.2.1    Mandatory Measures    

The mandatory measures for windows, glazed doors, and skylights address the air-tightness of the units (air leakage), how their U-factor and Solar Heat Gain Coefficient (SHGC) are determined, as well as Visible Transmittance (VT). 

Any fenestration product or glazed door, other than field-fabricated fenestration products and field-fabricated glazed doors, may be installed only if the manufacturer has certified to the Energy Commission by using a default label, or if an independent certifying organization approved by the Energy Commission has certified that the product complies with all of the applicable requirements of this subsection.

3.2.2    Certification and Labeling    

The Administrative Regulations §10-111 and  §110.6 of the Standards require that fenestration products have labels that list the U-factor, SHGC, VT and the method used to determine those values. The label must also certify that the fenestration product meets the requirements for air leakage from §110.6(a)1 of the Standards. See Table 3-1 – Maximum Air Infiltration Rates of this chapter.

Minimum visible transmittance (VT) is now a prescriptive requirement for windows and skylights.  The NFRC 200 test method is only appropriate for flat clear glazing and does not cover curved glazing, or diffusing glazing.  For these special types of fenestration, including dome skylights, use ASTM E972 to rate the visible transmittance.  Manufacturer specification sheets and/or product data sheets are acceptable for verifying compliance to ASTM E972.  Tubular skylights use NFRC 200 or NFRC 203.

 VT for diffusing skylights, which are not covered by NFRC 200 or NFRC 203, are tested using ASTM E972.  Manufacturers, specifiers or the responsible party must include proof of VT testing using the E972 method by including a VT test report or a manufacture cut-sheet with all energy compliance documentation.

A.   Manufactured (Factory-Assembled) Fenestration Label Certificates

Each manufactured (factory-assembled) fenestration product must have a clearly visible temporary label attached to it, which is not to be removed before inspection by the enforcement agency. Manufactured fenestration products, are to be rated and labeled for U-factor, SHG and VT by the manufacturer.

The manufacturer can choose to have the fenestration product rated and labeled in accordance with NFRC Rating Procedure (NFRC 100 for U-factors and NFRC 200 for SHGC and VT) see Figure 3-2. If the manufactured fenestration product is rated using the NFRC Rating Procedure, it must also be permanently labeled in accordance with NFRC procedures.

Where possible, it is best to select a NFRC-rated fenestration product , and to do so before completing compliance documents, as this enables the use of NFRC-certified data for compliance purposes.

Figure 3-2 - NFRC Manufactured Label

B.   Default Temporary Label

Fenestration product manufacturers can choose to use default performance values 'listed in Tables in §110.6 of the Standards for U-factors and SHGC in lieu of testing. For fenestration products requiring a VT value, assume a value of 1.0 as specified in the Reference Nonresidential Appendix NA6.  If default values are used, the manufacturer must attach a temporary label to each window, see Figure 3-3, and manufacturer specification sheets or cut-sheets must be included with compliance documentation. A NRCC-ENV-05-E will be required to document the thermal performance if no default temporary labels are attached to the window units.

Although there is no exact format for the default temporary label, it must be large enough to be clearly visible from 4 ft, such that the enforcement agency field inspectors may read easily and it must include all information required by the regulations.

The minimum suggested label size is 4 in. x 4 in. and the label must have the words at the bottom of the label as noted in the example shown in Figure 3-3.

If the product claims the default U-factor for a thermal-break product, the manufacturer must certify that the thermal-break criteria, upon which the default value is based, are met by placing a check in the check box;

1.   Air space 7/16 in. or greater;

2.   For skylights, the label must indicate the product was rated with a Built-in Curb;

3.   Meets Thermal-Break Default Criteria.

Figure 3-3 – Sample Default Temporary Label

The person taking responsibility for fenestration compliance can choose to attach Default Temporary Labels to each fenestration product as described previously instead of providing a Default Label Certificate for each product line.

Alternatively, for diffusing skylight’s VT which is not covered by NFRC 200 or NFRC 203, a test report should be included using the ASTM E972 method or a manufacturer’s cut-sheet which specifically describes the method used to calculate the VT.  Manufactures, specifiers or the responsible party should include the report with their energy documentation.

C.   Field-Fabricated Fenestration

Field-fabricated fenestration is not the same as site-built fenestration. Field-fabricated fenestration is a very limited category of fenestration that is made at the construction site out of materials that were not previously formed or cut with the intention of being used to fabricate a fenestration product. No attached labeling is required for field-fabricated fenestration products, only a NRCC-ENV-05-E. Field-fabricated fenestration and field-fabricated exterior doors may be installed only if the compliance performance documentation has demonstrated compliance. The field inspector is responsible for ensuring field-fabricated fenestration meets the specific; U-factor, SHGC and VT as 'listed on the NRCC-ENV-05-E. Thermal break values do not apply to field-fabricated fenestration products. Further explanation of Field-Fabricated Fenestration as well as required performance values can be found in Section 3.2 of this chapter.

D.   Site-Built Label Certificates

The designer should select a U-factor, SHGC, and VT for the fenestration system that meets the design objectives and occupancy needs for the building. Site-built fenestration is field-assembled using specific factory-cut or factory-formed framing and glazing units that are manufactured with the intention of being assembled at the construction site or glazing contractor’s shop. Site-built certificates should be filed at the contractor’s project office during construction or in the building manager’s office, see the CMA sample on  Figures 3-4 and 3-4A and discussion of CMA in subsection F below.  Note: The red circles in the figures indicate the field inspector’s area to inspect and compare to the energy compliance submittal and building plans.

Figure 3-4 - NFRC - CMA Label Certificate Page 1

Figure 3-4 A - NFRC - CMA Label Certificate Page 2

1.   For site-built fenestration totaling 1,000 ft² or greater, the glazing contractor or specifier must generate a NFRC label certificate from either approach 'listed below: 

A.   A NFRC label certificate generated by the CMA computer program; or

B.   Default to the U-factor values from Table 110.6-A and the SHGC values from 110.6-B and for Visible transmittance values, use the method specified in NA6.

2.  For site-built fenestration totaling less than 1,000 ft² or any area of replacement of site-built fenestration which includes vertical windows, glazed doors, and skylights, compliance must be demostrated using any of the approaches 'listed below:

A.   NFRC Label Certificate generated by the CMA computer program; or

B.   Use the center-of-glass values from the manufactures product literature to determine the total U-factor, SHGC and VT. See Reference Nonresidential Appendix NA6 - the Alternative Default Fenestration Procedure; or

C.   Default to the U-factor values from Table 110.6-A and SHGC values from Table 110.6 B.  For VT values, use the method specified in NA6.

Note: NA6 calculations are based on center-of-glass (COG) values from the manufacturer. For example, when using a manufacturer’s SHGC center-of-glass specification of 0.27, the NA6 calculation results in an overall SGHC value of 0.312, which then may be rounded to 0.31. Rounding to the nearest hundredth decimal place is acceptable to determine the overall fenestration efficiency value with either the prescriptive or performance approach.

E.   Fenestration Certificate NRCC-ENV-05-E (formally FC-1)

For non-rated products and where no default label certificates are placed on the  fenestration product, use the Fenestration Certificate NRCC-ENV-05-E to document thermal performances of each fenestration product that results in a different U-factor, SHGC, and VT.  Alternatively, one certificate will suffice when all the windows are the same.  

The NRCC-ENV-05-E should indicate the total amount of non-NFRC rated fenestration products throughout the project. The locations and orientations where products are being installed should be indicated on the drawings and in a fenestration schedule that lists all fenestration products.

The NRCC-ENV-05-E should clearly identify the appropriate table or equation that is used to determine the default U-factor and SHGC and, if applicable, the center of glass SHGC_C used in calculating the SHGC_fen. Manufacturer’s documentation of these product characteristics which list the center-of-glass values must also be attached to the NRCC-ENV-05-E and located at the job site for verification.

F.   Component Modeling Approach (CMA) and Software Tool (CMAST)

NFRC has developed the CMA to make the rating process quicker and simpler and serve as an energy ratings certification program for windows and other fenestration products used in non-residential (commercial) projects. Launched in January 2008 specifically for California’s Title 24 Part 6, the CMA Software Tool, also called CMAST, allows users to assemble fenestration products in a virtual environment. CMAST draws data for NFRC-approved components from online libraries choosing from pre-approved glazing, frame and spacer components. CMA users are able to obtain preliminary ratings for various configurations of their designs. CMA is a fair, accurate and  credible method based on NFRC 100 and 200 program documents, which are verified by third-party rating procedures.

 Architects and others can use this tool to:

1.   Help design energy-efficient windows, curtain wall systems and skylights for high performance building projects;

2.   Determine whether a product meets a project’s specifications and local/state building energy codes;

3.   Model different fenestration designs to compare energy performance.

Once the user is satisfied with the product, he or she creates a Bid Report containing the data for all fenestration products to be reviewed. This report can serve as an initial indication that the products comply with the project’s energy-related requirements and building energy codes. The physical windows are then built, either on-site or in a factory. The final products are reviewed and are rated by an NFRC-Approved Calculation Entity (ACE) then a license agreement is signed with NFRC.

At this point, NFRC issues a CMA Label Certificate for the project. This Label Certificate, unlike NFRC’s residential window label that is applied to individual units, is a single document that lists the certified fenestration ratings at the NFRC standard testing size for the entire building project. Once approved, the CMA Label Certificate is available online immediately. This single certificate serves as code compliance documentation for fenestration energy performance, and the certified products may be applied to future projects without repeating the certification process.

Benefits of CMA/CMAST

CMA provides facility managers, specifiers, building owners and design teams with a simple method for designing and certifying the energy performance of fenestration systems for their buildings. However, there are several additional advantages gained by using the CMA:

1.   CMA’s online tool, CMAST, has the ability to output a file with values for use in building energy analysis software programs, such as Energy Plus and DOE-2.

2.   The program can export detailed information for angular-dependent SHGC and VT values, seamlessly transferring the data to the analytical software.

3.   A 2010 study¹ conducted in California demonstrated that fenestration modeled with the CMA program can provide an increase in compliance margins by as much as 11.7 percent over the Energy Commission’s default calculation methods.

4.   CMA can help demonstrate above-code performance which is useful for environmental rating programs such as LEED™ or local green building programs.

Use of the CMA can help lead to a more efficient building, and also enable cost-savings due to more accurate fenestration performances and potential energy benefits from above-code utility incentives. Further details are available at www.NFRC.org/ .

G.   Air Leakage

Manufactured and site-built fenestration such as doors and windows must be tested and shown to have infiltration rates not exceeding the values shown in Table 3-1. For field-fabricated products or an exterior door, the Standards require that the unit be caulked, gasketed, weather-stripped or otherwise sealed. Unframed glass doors and fire doors are the two exceptions to these air leakage requirements. Field-fabricated fenestration and field-fabricated exterior doors are not required to comply with Table 3-1.

Table 3-1  – Maximum Air Infiltration Rates

3.2.3    Dynamic Glazing    

A.   Chromatic Glazing

Chromatic type fenestration has the ability to change its performance properties, allowing the occupant to control manually or automatically their environment by tinting or darkening a window with the flip of a switch or by raising and lowering a shade positioned between panes of glass. Some windows and doors change their performance automatically in response to a control or environmental signal. These high-performance windows sometimes referred to as “smart windows,” provide a variety of benefits, including reduced energy costs due to controlled daylighting and unwanted heat gain or heat loss. While still a relatively new technology, they are expected to grow substantially in the coming years. Look for the NFRC certified Dynamic Glazing Label to compare and contrast the energy performance for these new products. See the example of a NFRC Dynamic Glazing Label in Figure 3-5 below. Its unique rating identifiers help consumers understand the “dynamics” of the product, and allow comparison with other similar fenestration products.

Figure 3-5 - Dynamic Glazing NFRC Label

The label references the following information:

1.   U-factor measures the rate of heat loss through a product. Therefore, the lower the U-factor, the lower the amount of heat loss. In cold climates where heating bills are a concern, choosing products with lower U-factors will reduce the amount of heat that escapes from inside the house.

2.   The Solar Heat Gain Coefficient (SHGC) measures the rate of heat gain from solar energy passing through a product. Therefore, the lower the SHGC, the less amount of solar heat gain. In hot climates where air conditioning bills are a concern, choosing products with a lower SHGC will reduce the amount of heat that comes in from the outside.

3.   Visible Transmittance (VT) measures the amount of light that comes through a product. The higher the VT rating, the more light is allowed through a window or door.

4.   The Variable Arrow – If the product can operate at intermediate states, a dual directional arrow, (↔), with the word “Variable” underneath will appear on the label. Some dynamic glazing is able to adjust to intermediate states allowing for a performance level between the endpoints. The low value rating is displayed to the left (in the Closed position) and the high value rating is displayed to the right (in the Open position). This lets the consumer know the best and worst case performance of the product at a glance, as well as what the default or de-energized performance level will be.

5.   Air Leakage (AL) is a measurement of heat loss and gain by infiltration through cracks in the window assembly which affects occupant comfort. The lower the AL, the less air will pass through cracks in the window assembly.

To receive chromatic glazing credit the following must be met:

1.   Optional Prescriptive - Default to Table 150.1-A U-factor and SHGC;

2.   Performance Approach Compliance - maximum credit allowance for best rating;

3.   Automatic controls must be used to receive best rating values or

4.   NFRC Dynamic Glazing Compliance Label is required; otherwise, Default to
Table 110.6-A and 110.6-B values.

3.2.4    Window Films    

Developed in the early 1950’s, window films are mostly made of polyester substrate that is durable, tough, and highly flexible. It absorbs little moisture and has both high and low temperature resistances. Polyester film offers crystal clarity and can be pre-treated to accept different types of coatings for energy control and long term performance.  Window films are made with a special scratch resistant coating on one side and with a mounting adhesive layer on the other side. The adhesive is normally applied to the interior surface (room side) of the glass, unless a film is specifically designed to go on the exterior window surface.

Polyester film can be metalized and easily laminated to other layers of polyester film. It can be tinted or dyed, or metalized through vacuum coating, sputtering, or reactive deposition to produce an array of colored and spectrally selective films either clear or in color, often the color comes from the metallic coating rather than from tinting or dying.

There are three basic categories of window films:

1.   Clear (Non-Reflective)

2.   Tinted or dyed (Non-Reflective)

3.   Metalized (Reflective), which can be metalized though vacuum coating, sputtering, or reactive deposition, and may be clear or colored.

Clear films are used as safety or security films and to reduce ultraviolet (UV) light which contributes to fading of finished surfaces and furnishings.  However, they are not normally used for solar control or energy savings.

Tinted or dyed window films reduce both heat and light transmission, mostly through increased absorptance and can be used in applications where the primary benefit desired is glare control with energy savings as secondary benefit.

Metalized (reflective) films are the preferred window film in most energy saving applications, since they reduce transmission primarily through reflectance, and are manufactured to selectively reflect heat more than visible light.

Look for the NFRC certified- Attachment Ratings Energy- Performance Label which helps consumers understands the contrast in energy performance of Window Films. An example of a Window Film Energy Performance Label is shown in Figure 3-6 given below.

Figure 3-6 - Window Film Energy Performance Label

A.   Window Film Compliance

To receive window film credit the following must be met:

1.   The Performance Approach must be used;

2.   Only use the Alteration to Existing building compliance method;

a.   NFRC Window Film Energy Performance Label is required for each different film; otherwise, use the Default Table 110.6-A and 110.6-B values;

b.   Window Films to be used shall have a 10 year warranty or better

B.   Window Prescriptive Requirements

There are four aspects of the envelope component approach for windows:

1.   Maximum total area plus west-facing

2.   Maximum U-factor

3.   Maximum Relative Solar Heat Gain (RSHG)

4.   Minimum visible transmittance (VT)

3.2.5    Window Area    

Under the envelope component approach, the total window area may not exceed 40 percent of the gross wall area (encompassing total conditioned space) for the building. Likewise, the west-facing window area may not exceed 40 percent of the west gross wall area (encompassing total conditioned space for the building). This maximum area requirement will affect those buildings with very large glass areas, such as high-rise offices, automobile showrooms or airport terminals.

Optionally, the maximum area may be determined by multiplying the length of the display perimeter (see definition below in this section) by 6 ft in height and use the larger of the product of that multiplication or 40 percent of gross exterior wall area.

Glazing in a demising wall does not count toward the total building allowance. There is no limit to the amount of glazing allowed in demising walls, but it must meet the prescriptive U-factor, relative solar heat gain (RSHG), and visible light transmission (VT) requirements for the climate zone.

As a practical matter, window area is generally taken from the rough opening dimensions. To the extent this opening is slightly larger than the frame; the rough opening area will be slightly larger than the formally-defined window area.

For glazed doors, also use the rough opening area, except where the door glass area is less than 50 percent of the door, in which case the glazing area may be either the entire door area, or the glass area plus two inches added to all four sides of the glass (to represent the “window frame”) for a window in a door. Calculate the window area from the rough opening dimensions and divide by the gross exterior wall area, which does not include demising walls. Glazing area in demising walls has no limit and any glazing in demising walls is not counted as part of the exterior wall/window ratio.

Display perimeter is the length of an exterior wall in a Group B; Group F, Division 1; or Group M occupancy that immediately abuts a public sidewalk, measured at the sidewalk level for each story that abuts a public sidewalk. This generally refers to retail display windows, although other occupancies such as offices can also have a display perimeter. Public sidewalks are accessible to the public at large (no obstructions, limits to access, or intervening non-public spaces). The display perimeter is used for a special calculation of window area (§140.3(a)5A). Demising walls are not counted as part of the display perimeter.

In general, any orientation within 45° of true north, east, south or west will be assigned to that orientation. The orientation can be determined from an accurate site plan. Figure 3- 7demonstrates how surface orientations are determined and what to do if the surface is oriented exactly at 45° of a cardinal orientation. For example, an east-facing surface cannot face exactly northeast, but it can face exactly southeast. If the surface were facing exactly northeast, it would be considered north-facing.

Figure 3-7 – Four Surface Orientations

3.2.6    Window U-factor    

Fenestration products are required to use default U-factors and solar heat gain coefficients (see Tables 110.6-A and B of the standards) or have their performance characteristics certified by NFRC.  However, each window must meet the prescriptively required maximum U-factor criteria (see

Table 3-2 below) for each climate zone. For nonresidential buildings, the U-factor criterion is 0.36 Btu/h-°F-ft² for fixed windows, 0.46 operable windows, 0.41, for curtain wall or store front and 0.45 for glazed doors. In general, most NFRC-rated double-glazed windows with a low-e coating and a thermally broken frame will comply with the U-factor criterion; however, other window constructions may also comply. See www.NFRC.org, Certified Product Directory database or use Equation NA6-1in Reference Nonresidential Appendix NA6.

Table 3-2 – Window Prescriptive Requirements U-factors

3.2.7    Window Relative Solar Heat Gain (RSHG)    

Each window or skylight must meet the prescriptively required relative solar heat gain (RSHG) (see Table 3-2 above). In the prescriptive compliance approach, the relative solar heat gain incorporates the shading benefits from overhang.

 In the performance approach, the solar heat gain coefficient for the standard design (the reference building for comparison) is set to match the prescriptive RSHG requirements, and the overhang effects are modeled directly by the performance approach.

Note: (Exception §140.3(a)5C). The relative solar heat gain criteria also depends on the window-wall ratio, becoming more stringent with larger window areas.

Also note that the RSHG limitation includes the benefit of  window overhangs. In order to use the benefit of an overhang to meet prescriptive requirements, it must extend beyond both each side of the window jamb by a distance equal to the overhang projection (§140.3(a)5Cii). This would occur naturally with a continuous eave overhang but may require special attention in some designs. See subsection 3.2.9 for more information on RSHG.

3.2.8    Visible Light Transmittance (VT)    

New for the 2013 Standards is a requirement for visible light transmittance. Fenestration must meet the climate zone specific prescriptive requirement of having an area-weighted average VT of 0.42 or greater for fixed windows, 0.32 or greater for operable windows, 0.46 or greater for curtain walls and 0.17 or greater for glazed doors.  Products with spectrally selective “low-e” coatings (also known as single, double or triple silver low-e) are available to meet this requirement.  For a more detailed discussion of VT see subsection 2.3.11, Determining Visible Transmittance (VT).

A combination of high VT glazing in the upper part of a window (clerestory) and lower VT glazing at the lower part of the window (view window) can be used, as long as the area-weighted average meets the prescriptive requirement. This allows daylight to enter the space through the high VT glazing making a better daylighting design.

The standards also allow a slight variance if the window-to-wall ratio (WWR) is greater than 40%.  For this case, assume 0.40 for the WWR in the equation below, or if the glazing can comply with the prescriptive requirements if the area-weighted average VT meets the following minimum
requirement:

Visible Light Transmittance Equation:   VT ≥ 0.11 / WWR

In this equation VT is the visible transmittance of the framed window, and WWR is the gross window-to-wall ratio.

The graph below shows the allowed area weighted average min. VT’s by gross WWR for four types of windows.

Figure 3-8 – Area Weighted Average Minimum VT by Gross Window to Wall Ratio

The average VT requirements apply separately to chromogenic (dynamic or color changing) glazing and non-chromogenic glazing.  For chromogenic glazing, higher ranges of VT can be used to meet the prescriptive requirements. However, all glazing that is not chromogenic must separately meet the area-weighted VT prescriptive requirements.

3.2.9    Skylight Prescriptive Envelope Requirements     

As with windows, there are four aspects of the envelope component approach for skylights:

•      Maximum area

•      Maximum U-factor

•      Maximum solar heat gain coefficient

•      Minimum Visible Transmittance (VT)

Table 3-3 - Skylight Requirements (Area-weighted Performance Rating)

Excerpt from Standards Tables 140.3-B and 140.3-C, Skylight Roof Ratio, SRR

3.2.10  Skylight Area    

The area limit for skylights is 5 percent of the gross exterior roof area or skylight roof ratio (SRR). The limit increases to 10 percent for buildings with an atrium over 55 ft high (see Reference Joint Appendix JA1 definition). The 55 feet height is also the height threshold at which the California Building Code requires a mechanical smoke-control system for  atriums (CBC Sec. 909). This means that the 10 percent skylight allowance is not allowed for atriums unless they also meet this smoke control requirement. All skylights must meet the maximum U-factor criteria.

Note an atrium is defined in Reference Joint Appendix JA1 as:

There are two ways that skylights can be mounted into a roof system, either flush-mounted or curb-mounted. In order to create a positive water flow around them, skylights are often mounted on "curbs" set above the roof plane. These curbs, rising 6 to 12 inches (15 to 30 centimeters) above the roof, create additional heat loss surfaces, right where the warmest air of the building tends to collect.

Skylight area of unit skylights is the area of the rough opening of a skylight. The rough framed opening is used in the NFRC U-factor ratings (NFRC U-factor ratings for manufactured skylights with integrated curbs include glazing, framing, and the curb) procedure; it is also the basis of the default U-factors in Reference Appendix NA6. For skylights, the U-factor represents the heat loss per unit of rough framed opening (the denominator). However, the heat loss (the numerator) includes losses through the glazing, the frame, and the part of the curb that is integral with the skylight and included in the skylight test. Portions of roof that serve as curbs that mount the skylight above the level of the roof (see Figure 3-9) are part of the opaque building envelope.

Site-built monumental or architectural skylights that are equipped with integral built-in or site-built curbs (not part of the roof construction) are often used for atrium roofs, malls, and other applications that need large skylights and are treated differently. In such cases the skylight area is the surface area of the glazing and frame/curb (not the area of the rough framed opening), regardless of the geometry of the skylight (i.e., could be flat pyramid, bubble, barrel vault, or other three-dimensional shape). For special cases such as clerestory, rooftop monitor or tubular skylights, see Chapter 5 of this 'manual.

Figure 3-9 – Skylight Area

When skylights are specified, the designer must also show the Skylit Daylight Zones on the building plans. When the total installed general lighting power in the Skylit Daylight Zones in a room is greater than 120W, the general lighting in these Daylit Zones is controlled by automatic daylighting controls.  See Chapter 5 of this 'manual for a detailed discussion of the Daylight Zones.

3.2.11  Skylight U-factor    

For skylights, the U-factor and solar heat gain coefficient (SHGC) criteria are different, depending on whether the skylight glazing material is plastic or glass. For glass skylights, the U-factor criteria depend on whether or not the skylight is intended to be mounted on a curb. It is assumed that all plastic skylights are mounted on a curb. See Standards Tables 140.3-B, 140.3-C, and 140.3-D for U-factor requirements. As discussed above, the U-factor for skylights includes heat losses through the glazing, the frame and the integral curb (when one exists). In many cases, an NFRC rating does not exist for projecting plastic skylights. In this case, the designer can make use of the default fenestration U-factors in Standards Table 110.6-A.

If a glass skylight is installed and it is not possible to determine whether the skylight was rated with a curb, compliance shall be determined by assuming that the skylight must meet the requirements for skylights with a curb. All plastic skylight types are assumed to meet the requirements for plastic skylights with a curb.

3.2.12  Skylight SHGC    

Skylights are regulated only for SHGC, not RSHG, because skylights cannot have overhangs. The SHGC criteria vary with the skylight to roof ratio (SRR). The SHGC requirements apply to all skylight to roof ratios. See Tables 140.3-B, 140.3-C, and 140.3-D for SHGC requirements in the Standards. The designer can make use of default solar heat gain coefficients in Standards Table 110.6-Bor use the Nonresidential Reference Appendix NA6 if all site-built fenestration (skylights and vertical fenestration) is less than 1,000 ft².

3.2.13  Daylighting Prescriptive Requirements for Skylights in Large Enclosed Spaces      

Appropriately-sized skylight systems, when combined with daylighting controls, can dramatically reduce the energy consumption of a building. Daylighting control requirements under skylights are discussed in Chapter 5 of this 'manual. With too little skylight area, insufficient light is available to turn off electric lighting; with too much skylight area, solar gains and heat losses through skylights negate the lighting savings with heating and cooling loads.

Figure 3-10 – Present Value Savings of Skylight 50,000 ft² Warehouse in Sacramento

Skylights and automatic daylighting controls are most cost-effective in large open spaces and are prescriptively required in enclosed spaces (rooms) that are larger than 5,000 ft², directly under a roof and have ceiling heights greater than 15 ft., and have lighting power densities greater than 0.5 W/ft². The standards require a total of at least 75% of the floor area be within Skylit Daylit Zones or Primary Sidelit Zones.

The definitions of Daylit Zones are contained in §130.1(d) and applies to the circuiting of daylighting controls near windows and skylights.  These definitions are applied to the calculation of Daylit Zones areas to show compliance with these minimum daylighting requirements. However, the application of these daylit definitions for purposes of complying with the 75% floor area requirements do not need to account for the presence of partitions, stacks or racks. 

The rationale for these relaxed definitions are that the design of the envelope may be developed before there is any knowledge of the location of the partial height partitions or shelves as is often the case for core and shell buildings.  Thus, the architectural Daylit Zone requirement of 75% of the space area indicates the possibility of the architectural space being mostly daylit.  By not accounting for partial ceiling height partitions and racks, the standards are consistent in addressing architectural daylit areas regardless of whether the design is core and the shell or a complete design development including interior design.  This approach does not require the addition of extra skylights or windows if racks and partial height partitions are added later.

The Daylit Zone and controls specifications in §130.1(d) describe which luminaires are controlled and this specification must consider the daylit obstructing effects of tall racks, shelves and partitions.  There is a greater likelihood that the electrical design will occur later than the architectural design and, thus, greater planning for these obstructions can be built into the lighting circuiting design.

The demanding lighting control needs in auditoriums, churches, museums and movie theaters, and the need to minimize heat gains through the roofs of refrigerated warehouses, render these few occupancies exempt from the skylight requirement. Gymnasiums do not qualify for this exemption unless there is a stage or there is a determination that this space will be used to hold theatrical events.

Since skylights with controls reduce electric lighting, they are mandatory on all nonresidential occupancies that meet the above criteria, whether the space is conditioned or unconditioned.  See further discussion in subsection 3.2.6 B. Controls.

In qualifying high ceiling large buildings, the core of many of these spaces will be daylighted with skylights.  Skylighting 75% of the floor area is achieved by evenly spacing skylights across the roof of the building.  A space can be fully skylighted by having skylights spaced so that the edges of the skylights are not further apart than 1.4 times the ceiling height.  Thus, in a space having a ceiling height of 20 feet, the space will be fully skylighted if the skylights are spaced so there is no more than 28 feet of opaque ceiling between the skylights.

The total skylight area on the roof of a building is prescriptively limited to a maximum of 5% of the gross roof area (§140.3(a)6.  Studies have found that energy savings can be optimized if the product of the Visible Transmittance (VT) of the skylight and the skylight to daylit area ratio is greater than 2% (this accounts for a light well factor of 75% and a skylight dirt depreciation factor of 85%)¹.  If one fully daylights the space with skylights and the skylights meet a prescriptive requirement of 49% VT, then approximately a minimum skylight area that is at least 3% of the roof area is typically needed to optimize energy cost savings (See Figure 3-11).

Figure 3-11 – Skylighting Savings by Skylight-to-Floor Ratio and Building Type
 in San Bernardino, CA (Climate Zone 10)

3.2.14  Skylight Characteristics     

Skylights installed to comply with the minimum skylight area for large enclosed spaces shall meet the requirements in §140.3(a)6 and §140.3(c):

1.   Have a glazing material or diffuser that has a measured haze value greater than 90 percent, tested according to ASTM D1003 (notwithstanding its scope) or other test method approved by the Energy Commission.

2.   If the space is conditioned, meet the requirements in §140.3(a)6.

In general, the standards require the use of double-glazed skylights. When the skylights are above unconditioned spaces, there is no limitation placed on the maximum skylight area or its U-factor or SHGC.

If the space is unconditioned, single-glazed skylights will comply with the code requirements as long as they are sufficiently diffusing (i.e. the glazing or diffuser material has a haze rating greater 90 percent) and their visible transmittance is above the VT requirements in Table 140.3-B or C. Products that have such a rating include prismatic diffusers, laminated glass with diffusing interlayer’s, pigmented plastics, etc. The purpose of this requirement is to assure the light is diffused over all sun angles.  Note, any unconditioned space that later becomes conditioned must meet the new construction envelope requirements.  Therefore, if the space may become conditioned in the future, it is recommended that the envelope meet the conditioned envelope thermal requirements. 

Other methods of diffusion that result in sufficient diffusion of light over the course of the entire year would also be acceptable in lieu of using diffusing glazing. Acceptable alternatives are baffles or reflecting surfaces that ensure direct beam light is reflected off of a diffuse surface prior to entering the space over all sun angles encountered during the course of a year. This alternative method of diffusion would have to be documented by the designer and approved by the code authority in your jurisdiction.

3.2.15  Controls    

Electric lighting in the Skylight Daylit Zones shall meet the mandatory control requirements in §130.1(d). See Chapter 5 for more information about lighting control requirements and for more information about daylighting control requirements.

Figure 3-12 – Daylit Area under Skylights

Figure 3-13– Daylit Area Tradeoff between Skylights and Windows

Figure 3-14 – Daylit Area under Rooftop Monitors

Figure 3-15 – Daylit Area under Rooftop Monitors

3.2.16  Determining Fenestration U-factors    

The U-factor for a fenestration product describes the rate of heat flow through the entire unit, not just the glass or plastic glazing material. The U-factor includes the heat flow effects of the glass, the frame, and the edge-of-glass conditions (there also may be spacers, sealants and other elements that affect heat conduction). For skylights mounted on a curb that is part of the roof construction, the total heat flow considered in determining the U-factor includes losses through the frame, glazing and other components, but not through the curb that is part of the roof construction.

Standards Tables 140.3-B, 140.3-C, and 140.3-D, lists skylight product that includes a curb, and the effects of this curb are included in the product U-factor rating. This curb included in the product rating is separate from the curb that is a part of the roof construction. For projecting windows (greenhouse windows), the total heat flow includes the side panels, base and roof of the projecting window assembly. However, the area used to determine the U-factor for skylights and projecting windows is the rough-framed opening. Using the rough-framed opening eases the process of making load calculations and verifying compliance, since the rough-framed opening is easier to calculate than the actual surface area of the projecting window or skylight.

Reference Joint Appendix JA1 lists many of the terms and product characteristics that relate to fenestration U-factors. In particular, see the definitions for window, skylight, window area, skylight area, site-built fenestration, and field-fabricated fenestration.Table 3-4 shows acceptable procedures for determining fenestration U-factors for four classes of fenestration: manufactured windows, manufactured skylights, site-built fenestration, and field-fabricated fenestration.

Table 3-4 – Acceptable Methods for Determining U-factors

The preferred methods for determining fenestration U-factor are those in NFRC 100 for manufactured windows and for site-built fenestration. For manufactured windows, the default U-factors in Standards Table 110.6-A (reproduced in Table 3-6 below) must be used if NFRC-determined U-factors are not available. These U-factors represent the high side of the range of possible values, thereby encouraging designers to obtain ratings through NFRC procedures, when they are available.

NFRC U-factors are becoming more common for skylights; increasingly, more manufacturers are getting NFRC labels for their skylights, including tubular skylights (which includes U-factor), and SHGC. If NFRC data is not available, the Alternative Default U-factor equation from Reference Nonresidential Appendix NA6, Equation NA6-1 may be used for skylights. This equation is derived from NFRC-100 and represent average typical values, as opposed to the values published in Table 110.6-A in the Standards that are on the high side of the range of typical values.

The recommended method for determining the U-factor of site-built fenestration systems (curtain walls and storefront systems) is the NFRC 100 procedure. This requires that site-build fenestration, including curtain walls, go through the NFRC process for obtaining label certificates for site-built products. If the building has less than 1,000 ft² of site-built fenestration area, which includes windows, glazed doors, and skylights, then U-factors used for compliance for site-built products may instead be calculated from Equation NA6-1 from the Reference Nonresidential Appendix NA6, or Standards default values from Table 110.6-A.

For buildings with more than 1,000 ft² of site-built fenestration area, there are two compliance choices with regard to U-factor and labeling of site-built fenestration:

Go through the NFRC process and obtain a label certificate. This is the option described in §10-111(a)1A

Provide a default label certificate using the default U-factors from Standards Table 110.6-A. This option results in very conservative U-factors.

3.2.17  Field-Fabricated Fenestration Product or Exterior Door    

Field-fabricated fenestration is fenestration assembled on site that does not qualify as site-built fenestration. It includes windows where wood frames are constructed from raw materials at the building site, salvaged windows that do not have an NFRC label or rating, and other similar fenestration items.

For field-fabricated fenestration, the U-factor and Solar Heat Gain Coefficient are default values that can be found in Table 3-5 and Table 3-6 below. Values are determined by frame type, fenestration type and glazing composition.

Exterior doors are doors through an exterior partition. They may be opaque or have glazed area that is less than or equal to one-half of the door area. U-factors for opaque exterior doors are 'listed in Reference Joint Appendix JA4, Table 4.5.1. Doors with glazing for more than one-half of the door area are treated as fenestration products and must meet all requirements and ratings associated with fenestration.

When a door has glazing of less than one-half the door area, the portion of the door with fenestration must be treated as part of the envelope fenestration independent of the remainder of the door area.

A field-fabricated product may become a site-built product if all the requirements for receiving a label certificate required of site-built products are met.

Table 3-5– Default Fenestration Product U-factors

Table 3-6– Default Solar Heat Gain Coefficient (SHGC)

3.2.18  Determining Relative Solar Heat Gain Coefficient (RSHGC)    

Relative solar heat gain (RSHG) is essentially the same as SHGC, except for the external shading correction. It is calculated by multiplying the SHGC of the fenestration product by an overhang factor. If an overhang does not exist, then the overhang factor is 1.0.

Overhang factors may either be calculated or taken from

Table 3-7 below and depend upon the ratio of the overhang horizontal length (H) and the overhang vertical height (V). These dimensions are measured from the vertical and horizontal planes passing through the bottom edge of the window glazing, as shown in Figure 3-16. An overhang factor may be used if the overhang extends beyond both sides of the window jamb a distance equal to the overhang projection [§140.3(a)5Cii]. The overhang projection is equal to the overhang length (H) as shown in Figure 3-16. If the overhang is continuous along the side of a building, this restriction will usually be met. If there are overhangs for individual windows, each must be shown to extend far enough from each side of the window.

Figure 3-16 – Overhang Dimensions

Equation 3-1 – Relative Solar Heat Gain

RSHG = SHGC_win x OHF

Where

RSHG = Relative solar heat gain.

SHGC_win = Solar heat gain coefficient of the window.

Where:

H = Horizontal projection of the overhang from the surface of the window in ft,   but no greater than V.

V = Vertical distance from the windowsill to the bottom of the overhang, in ft.

a = -0.41 for north-facing windows, -1.22 for south-facing windows, and -0.92 for east- and west-facing windows.

b =0.20 for north-facing windows, 0.66 for south-facing windows, and 0.35 for east- and west-facing windows.

Table 3-7 – Overhang Factors

To use Table 3-7, measure the horizontal projection of the overhang (H) and the vertical height from the bottom of the glazing to the shading cut-off point of the overhang (V). Then calculate H/V. Enter the table at that point. If the calculated H/V falls between two values in the table choose the next higher value to the calculated H/V value. Move across to the column that corresponds to the orientation of the window and find the overhang factor. Note, that any value of H/V greater than one has the same overhang factor (for a given orientation) shown in the last row of the table.

Figure 3-17graphs the overhang factors of the various orientations as a function of H/V. It shows that overhangs have only a minor effect on the north (maximum reduction in SHGC is only about 20 percent). East, west and south overhangs can achieve reductions of 55–60 percent. The benefits of the overhang level off as the overhang becomes large. (Note: this graph is presented only to illustrate the benefits of overhangs.)

Figure 3-17 – Graph of Overhang Factors

3.2.19  Determining Solar Heat Gain Coefficients    

The solar heat gain coefficient (SHGC) is the ratio of the solar heat gain entering the space through the fenestration area to the incident solar radiation; the lower the SHGC, the less solar heat is gained. For SHGC measurements, the solar radiant energy includes infrared, visible light and ultraviolet. A low SHGC reduces solar heat gains, thereby reducing the amount of air conditioning energy needed to maintain comfort in the building. A low SHGC may also increase the amount of heat needed to maintain comfort in the winter. The technical definition of SHGC is the ratio of solar energy entering the window (or fenestration product) to the amount that is incident on the outside of the window. As with U-factors, the window frame, sash and other opaque components, and type of glazing affect SHGC.

There are four acceptable methods for determining SHGC for use with the Standards (see Table 3-7). The preferred methods are two NFRC procedures:

NFRC 200 for manufactured fenestration, which includes manufactured skylights; and NFRC 100 for site-built fenestration, which includes site-built skylights. The NFRC standard for rating the SHGC of tubular daylighting devices (TDDs or tubular skylights) is appropriate only for attic configurations where the insulation layer is directly on top of the ceiling. For spaces with insulated roofs, use the NFRC or default rating of the top dome only.

A third method is to use the SHGC Defaults from Standards Table 110.6-B or Table 3-7 are on the high side and do not account for special coatings and other technologies that may be part of a proposed fenestration product.

The fourth method, applicable only to skylights and site-built fenestration in buildings with less than 1,000 ft² of site-built fenestration, is to use Equation NA6-2 in the Reference Nonresidential Appendix NA6. This equation calculates an overall SHGC for the fenestration (SHGC_t) assuming a default framing factor and using the center-of-glass SHGC value (SHGC_c) for the glazing from the manufacturer’s literature.

Note: Buildings that have 1,000 ft² or more of site-built fenestration cannot use the Alternative Default Fenestration Procedure, Equation NA6-1 or NA6-2.

Windows are not allowed SHGC credit for any interior shading such as draperies or blinds. Only exterior shading devices such as shade screens permanently attached to the building or structural components of the building can be modeled through performance standards compliance. Manually operable shading devices cannot be modeled. Only overhangs can be credited using the relative solar heat gain procedure for prescriptive compliance.

Table3-8 – Methods for Determining SHGC

3.2.20  Determining Visible Transmittance (VT)    

Visible Transmittance (VT) is a property of glazing materials that has a varying relationship to SHGC. VT is the ratio of light that passes through the glazing material to the light that is incident on the outside of the glazing. Light is the portion of solar energy that is visible to the human eye. VT is an important characteristic of glazing materials, because it affects the amount of daylight that enters the space and how well views through windows are rendered. Glazing materials with a very low VT have little daylighting benefit and views appear dark, even on bright days. The ideal glazing material for most of California’s summer climates would have a high VT and a low SHGC. Such a glazing material allows solar radiation in the visible spectrum to pass while blocking radiation in the infrared and ultraviolet spectrums. Materials that have this quality are labeled “spectrally selective” and have a VT that is up to 2.2 times the SHGC. The center-of-glass VT for a given insulated glass (IG) is found in manufacturer literature, through the NFRC product directory or by use of the Component Modeling Approach (CMA).

Figure 3-18 – Solar Heat Gain Coefficient and Visible Transmittance

The prescriptive requirements of Tables 140.3-B and 140.3-C of the standards prescribe specific VT values for all climate zones and glass types.  The visible light transmittance is used in the performance method in the calculation of the interior illumination levels and lighting energy savings due to daylight controls. This is discussed in more detail in Chapter 5 of this 'manual.

3.2.21  Site-Built Fenestration Roles and Responsibilities    

Manufactured fenestration products are factory-assembled as a unit, and the manufacturer is able to assume the burden of testing and labeling. However, with site-built fenestration, multiple parties are responsible. Architects and/or engineers design the basic glazing system by specifying the components, the geometry of the components, and sometimes, the method of assembly. An extrusion manufacturer provides the mullions and frames that support the glazing and is responsible for thermal breaks. A glazing manufacturer provides the glazing units, cut to size and fabricated as insulated glass (IG) units. The glazing manufacturer is responsible for tempering or heat strengthening, the tint of the glass, any special coatings, the spacers, and the sealants. A glazing contractor (usually a subcontractor to the general contractor) puts the system together at the construction site or their shop and is responsible for many quality aspects. Predetermining the energy performance of site-built fenestration as a system is more challenging than for manufactured units.

One of the parties (architect, glazing contractor, extrusion manufacturer, IG fabricator, or glass manufacturer) must take responsibility for testing and labeling of the site-built fenestration system under the most recent NFRC 100 procedure. The responsible party must obtain a label certificate as described in §10-111 of the Standards.

It is typical for the glazing contractor to assume responsibility for the team and to coordinate the certification and labeling process. A common procedure is for the design team to include language in the contract with the general contractor that requires that the general contractor be responsible; the general contractor typically assigns this responsibility to the glazing contractor, once the responsible party has established a relationship with an NFRC.

It is not necessary to complete the NFRC testing and labeling prior to completing the compliance documentation and filing the building permit application. However, plans examiners should verify that the fenestration performance shown in the plans and specifications and used in the compliance calculations is “reasonable” and achievable. This requires some judgment and knowledge on the part of the plans examiner. Generally, designers will know the type of glass that they plan to use and whether or not the frame has a thermal break or is thermally improved. This information is adequate to consult the default values for U-factor and SHGC in Reference Nonresidential Appendix NA6.

After the construction contract is awarded, the glazing contractor or other appropriate party assumes responsibility for acquiring the NFRC Label Certificate. Each label certificate has the same information as the NFRC temporary label for manufactured products, but includes other information specific to the project such as the name of the glazing manufacturer, the extrusion contractor, the places in the building where the product line is used, and other details. The label certificate remains on file in the construction office for the building inspector to view. After construction is complete, the label certificate should be filed in the building office with the as-built drawings and other operations and maintenance data. This will give building managers the information needed for repairs or replacements.