FARBER ENERGY DESIGN

 

home page

contact information

firm profile

Title 24 Standards

efficiency incentive programs

energy analysis checklists

energy-efficient design

resources/links

 

FENESTRATION

 

Click on topic listed below to jump to that topic:  

 

Fenestration Definitions, Categories, Values

 

 

 

Rating Fenestration Efficiency

 

 

 

Determining Fenestration Energy Values for Energy Code Compliance

 

 

 

Energy-Efficient Fenestration

 

 

 

Nonresidential Fenestration Considerations

 

 

 

Residential Fenestration Considerations

 

 

 

Apparent Motion of the Sun

 

 

 

General Definitions:

 

·   Fenestration:  “Fenestration Productis any transparent or translucent material, plus any sash, frame, mullions, and dividers, in the envelope of a building.  Fenestration includes (but is not limited to): windows, glass doors, skylights, curtain wall glazing areas, and garden windows.  You will find a definition of fenestration in the Standards and in both the Residential and Nonresidential Title 24 Compliance Manuals.

 

·   Low E Glass:  Low E stands for “low emissivity”.  Low E glass is selective in reducing transmission of radiant heat while allowing visible light to pass through.  All Low E glass reduces thermal heat flow.  Some Low E glass allows a fairly high level of solar heat transmission (typically “hard-coat” Low E technology), while “soft-coat” Low E glass acts to reduce solar heat gain as well as thermal transmission.  Most modern Low E glass products are “soft-coat” type, with low solar transmission.  Low E coatings are available on both clear and tint glass. 

 

 

Fenestration product categories (for energy rating purposes):

 

·   Manufactured Fenestration: Fenestration products that are fully assembled at a factory (i.e. glass and frame assembled into a unit).  Windows and skylights sold at “home improvement” stores are an example of products in this category.

 

·   Site-Built Fenestration: Fenestration constructed on-site (or at a glazing subcontractor’s shop) from manufactured components designed to be field-glazed.  Storefront systems and curtainwall systems are considered to be site-built. 

 

·   Field-Fabricated Fenestration: Fenestration constructed on-site from standard dimensional lumber or other materials that were not formed with the specific intention of being used to create fenestration.

 

 

Fenestration energy efficiency is measured by three values:

 

·   U-factor (or U-value): A measure of conductive and convective heat flow through the product (i.e. thermal transmission).  Lower U-factors mean less thermal transmission.

 

·   SHGC (solar heat gain coefficient): A measure of the ratio of total available solar radiant heat that the product will transmit.  The higher the SHGC, the more solar heat that will pass through the product; conversely, the lower the SHGC, the less solar heat that will be transmitted.

 

·   Visible Transmittance (VT): A measure of available daylight that will be transmitted through a fenestration product (i.e. accounts for both the glass and the frame).  The higher the VT, the more daylight that will pass through the product.  VT is the daylight value used by NFRC (see next section, below). 

 

Note: the measurement of daylight transmittance through glass only is called Visible Light Transmittance (VLT).

 

 

return to top of page

 

RATING FENESTRATION EFFICIENCY: General Concepts

For Title 24 compliance, fenestration (glass + frame and related components) energy values are modeled.  Glass-only values are never modeled.  There are two basic sources for fenestration energy factors (U, SHGC & VT):  NFRC certified product values, and Title 24 default fenestration values [exception: “site-built” fenestration in small projects can use an alternate formula based on glass type and frame type – see next box below].  NFRC values are derived from actual product testing, and are considered to be fairly accurate. 

 

The Standards include Title 24 “default” SHGC and U-factor tables, which contain values for a selection of glass types and frame materials.  However, glass choices only include 1-pane and 2-pane, clear and tint.  The available default values are conservative – they almost always provide less energy credit than a window of identical construction that has NFRC values.  This is especially true for windows with high-performance (i.e. Low E) glass. 

Note 1: Some manufacturers report “NFRC values” that are not certified NFRC values.  These manufacturers will have their products tested in NFRC-certified labs, but do not participate in the NFRC fenestration certification process.  Unless a product is actually certified with NFRC, the tested values are not acceptable for Title 24 compliance.  Factory-assembled NFRC certified products will include an NFRC label on them.  If you are uncertain as to whether a product is actually NFRC certified, you can check NFRC’s website to see whether the product is listed.

Note 2: NFRC ratings are not available for field-fabricated fenestration. 

 

 

 

DETERMINING FENESTRATION ENERGY VALUES FOR ENERGY CODE COMPLIANCE

Manufactured” fenestration energy rating requirements in the 2016 Standards:  Manufactured windows must either be NFRC certified, or their energy values must come from the Title 24 default tables that do not give credit for high-performance glazing.  Most manufactured windows targeted to the single-family residential market are NFRC certified/labeled; significantly, NFRC certification is much less common among “commercial grade” windows.  All NFRC certified factory-assembled products are listed in the NFRC Certified Products Directory, at http://www.nfrc.org/certified-product-directory/  This NFRC rating system is called the Product Certification Program.

Site-built” fenestration energy rating requirements in the 2016 Standards:  Projects with over 1,000 sq. ft. of site-built fenestration must either have the site-built fenestration NFRC tested and rated, or their energy values (U-factor and SHGC) must come from Title 24 default tables that do not give credit for high-performance, low SHGC glazing.  For projects that have less than 1,000 sq. ft. of site-built glazing, the fenestration SHGC and U-factor can be based on the actual glass and frame properties.  However, even when very efficient glass and thermally-broken frames are used, this alternate method will not result in energy values that meet prescriptive compliance fenestration required values for new buildings. 

Under past energy codes, it has been rare for site-built glazing to need NFRC ratings, both because previous versions of the energy code were not as stringent overall, and because they allowed buildings with up to 10,000 sq. ft. of site-built fenestration to use an alternate energy value method based on actual glass values and frame type.  With the greatly reduced glazing area allowed to use the alternate energy values methodology, and with the more stringent prescriptive compliance energy values in the 2016 code, expect that “site-built” fenestration will often need to receive NFRC certification. 

NFRC certification of “site-built” fenestration:  NFRC allows site-built fenestration to obtain certified NFRC energy values either through the Product Certification Program (originally established for factory-assembled products), or through the Component Modeling Approach (CMA).  Under the CMA method, frame manufacturers and glass manufacturers have their products tested to establish their “component” energy rating.  Glazing system manufacturers and glazing contractors who are licensed under NFRC to produce CMA type certification (using NFRC’s CMAST program) can then produce NFRC certificates – as long at the chosen components (frame, spacer, glass) are CMA rated.  Of course, this does not guarantee that the results will be favorable in terms of SHGC, U, and VT – hence the importance of selecting high-performance components.   

Site-built fenestration that is certified under the Product Certification Program are listed, with their energy values, at http://www.nfrc.org/certified-product-directory/ .  Note: This directory does not include a separate category for “site-built” products.  You will find “site-built” products listed under “windows” and “doors”, and the product will typically be noted as being storefront or curtainwall.

Site-built fenestration certified under the CMA program are listed, with their energy values, at http://cmast.nfrc.org/ .  Unfortunately, this site does not have a pull-down menu that shows all of the manufacturers that have products listed.  But if you click “products” on the cmast home page, and then type ‘*’ on the Manufacturer line, all of the CMA certified products will show up.  Or you may search for a particular name, like Oldcastle.

For site-built fenestration products, I suggest that you ask the manufacturer or the vendor a) whether the product is available with NFRC certified energy values, and b) for products with NFRC certified values, whether they are CMA type or Product Certification Program type.

The CMA program is described in section 3.3.1.3.F of the ’16 Nonresidential Compliance Manual, starting on page 3-41.

Using “site-built” fenestration that is not NFRC rated (on projects with over 1,000 sq. ft. of site-built glazing), and/or using  manufactured windows and skylights that are not NFRC rated, can put a project at a very large disadvantage in meeting the Title 24 energy standards.

 

 

return to top of page

 

ENERGY-EFFICIENT FENESTRATION

In order to reduce solar heat gain, glass used to rely on tint and reflective coatings.  With the advent of modern Low E coatings, these older glass treatments are becoming much less popular.  Low E coatings are available that, on clear glass, block up to around 75% of available solar heat, while achieving high daylight transmittance.  For nonresidential buildings, the energy code emphasizes use of fenestration products that possess both high daylight transmittance and low solar heat gain.

 

To reduce thermal transmission, efficient fenestration uses dual-pane (or triple-pane) glass combined with a low thermal transmission type frame.  Low E coated dual-pane glass is slightly more efficient than uncoated glass in reducing thermal transmission.  Non-metal frame/sash (wood, vinyl or fiberglass) are much better at reducing conduction than metal frame.  Thermally-broken metal frames are more efficient than standard metal frame, but not as efficient as non-metal frame choices. 

 

For more detailed information about fenestration consideration particular to nonresidential buildings and to residential buildings, see below. 

 

 

NONRESIDENTIAL FENESTRATION CONSIDERATIONS

Because nonresidential buildings usually have relatively high internal heat loads from lighting, office equipment and occupants, solar heat is rarely desired (except in smaller, residential-scale buildings during winter heating conditions).  Therefore, low SHGC glazing is typically used.  Under the energy code, prescriptive fenestration energy values are based on efficient thermally-broken metal frames. 

 

External shading of fenestration (overhangs, fins, etc.) can be even more effective than low SHGC glass in reducing solar heat gain, depending on the geometry of the glazing and external shading devices.  Fenestration facing south and north is preferable to glazing facing east and west, as south and north oriented glass will not be subject to as much solar heat gain during the warmer time of year.

 

The energy code also includes a prescriptive fenestration minimum VT requirement (first enacted under the 2013 code).  In order to save lighting power, the energy code both encourages natural daylight transmission into the building, and mandates automatic daylight controls on most interior lighting adjacent to daylit areas (adjacent to windows and to skylights).  To achieve both a high VT and a low SHGC, use of low SHGC type Low E coatings on clear glass works best.  If tint glass is a priority, consider blue and green tints, because gray and bronze tints, as well as reflective coatings, block much more daylight than do blue and green tints. 

For copies of the nonresidential prescriptive fenestration energy requirements tables, open the following:

  16 Stnds-new NR bldg Presc Fenest Reqts.pdf

  16 Stnds-Table 141-A Alt Fenest Reqts.pdf

 

return to top of page

 

RESIDENTIAL FENESTRATION CONSIDERATIONS

In most areas, solar heat gain is beneficial in winter.  However, high SHGC glass, if unshaded, will result in high cooling loads in areas with hot summers.  For this reason, low SHGC fenestration using Low E coatings has become very popular for most residential projects.  In fact, except in a few coastal climate zones, low SHGC fenestration is the Title 24 standard.  The 2013 code lowered the residential prescriptive SHGC requirement from 0.40 to 0.25.  These values continue under the 2016 code.

Contrary to what the energy code is encouraging, higher SHGC glazing is best in any area that has winter heating conditions, but only if the glazing is adequately shaded in summer.  It is possible to meet the energy code with higher SHGC products, under the performance approach, when proper shading is part of the design.  A couple of considerations to note in this regard:

1.   Energy compliance calculations can only account for exterior fixed shading devices.

2.   Most manufacturer’s “default” windows use low SHGC glass.  Where higher SHGC glass is an option, it may require special ordering.

 

South-facing glazing is most beneficial for winter heat gain, and is also easy to shade in summer with a modest overhang.  East and west-facing glazing is much more difficult to shade adequately in summer (see diagram below).  In areas with hot summer weather where air conditioning is common, any unshaded glazing should use low SHGC glass, i.e. 0.25 SHGC or lower.  In any climate, low U-factor products that use dual-pane glass and wood, vinyl, or fiberglass frames are beneficial.

For a copy of the low-rise residential new building prescriptive energy requirements table, which includes fenestration requirements, open  16 Stnds-RES Presc Fenest Reqts.pdf

 

APPARENT MOTION OF THE SUN

 

In winter, the sun rises in the southeast, follows a relatively low arc through the sky, and sets in the southwest.  In summer, the sun rises in the northeast, follows a high arc through the sky, and sets in the northwest.  The sun is always due south at “solar noon”.  In winter, the sun’s relatively low mid-day rays most easily penetrate south-facing glazing.  In summer, the sun’s rays most readily penetrate east-facing, horizontal, and west-facing glazing, as these surfaces are more perpendicular to the summer sun’s morning, mid-day, and afternoon rays.

 

 

return to top of page