How The New VTannual Rating Affects Daylighting

If you’re involved with daylighting commercial buildings, you need to know about optically complex fenestration systems and the new VTannual rating.
   Optically complex fenestration systems are technologically advanced products that use specially engineered light-bending or light-reflecting elements to harvest the wavelengths of light that we want to use to illuminate building interiors. One key example of these new types of optically complex fenestration systems is the tubular daylighting device (TDD), which collects and admits natural light into interiors more effectively than conventional daylighting options.
   Featuring progressive technologies, these optically complex systems use stringent refractive, reflective, and filtering elements to selectively harvest natural light over the course of a year. Compared with traditional skylights, windows, and less-complex TDDs, state-of-the-art TDDs use advanced optics and materials to deliver higher quality visible light with more consistent illuminance, regardless of sky condition or climate. They also significantly reduce the potential for shifting light patterns, glare, and heat transfer issues.

Current rating issues
So how do building designers know which optically complex system offers the best performance for their particular projects? Currently, visible light transmittance (VT) is a factor commonly used by architects, engineers, and contractors to predict a daylighting system’s light output. It’s also a performance rating that is measured using testing and rating protocols established by the National Fenestration Rating Council (NFRC), Greenbelt, MD.
   The issue with the VT rating is that it doesn’t sufficiently account for the light-collection control that can be designed into optically complex fenestration products. These systems are engineered to filter out undesirable wavelengths—such as fabric-fading ultraviolet, heat-carrying infrared, and overpowering midday sunlight—so the collection and transmission of light varies, by design, throughout the day and year. This variance makes product comparisons difficult and the simple VT measurement a poor performance indicator.

Devising a new rating
Measuring simple VT involves direct-normal testing where a single beam of light is aimed into the optically complex system from directly overhead. There are two problems with the test. First, natural light transmits through a surface at a variety of angles throughout the day (depending on the sun’s position in the sky), not just in a perpendicular fashion. Second, this method doesn’t allow the benefits of technology to come into play, such as dome optics or optical tubing reflectance. Every daylighting system performs relatively the same when using this testing protocol, so it does not offer an accurate depiction of a product’s real-life performance. As a result, it doesn’t provide a valuable resource to the consumer when trying to select the best product for a particular application.
   To select the best daylighting system for a given project, commercial building designers must be able to compare product performance with respect to daylighting configuration and geographic location as well as climatic and seasonal variations. Until now, the lack of standard performance metrics that adequately address this new breed of daylighting systems has made the simple comparison and selection of optically complex systems virtually impossible.
   Enter the NFRC Tubular Daylighting Device Task Group. Consisting of members from the NFRC, including technical representatives from the Lawrence Berkeley National Laboratory (Berkeley, CA), testing laboratories, and several major TDD manufacturers, this collaboration has worked for more than four years to develop a new performance testing protocol for collecting and rating visible transmittance data for optically complex systems.
   The outcome of the group’s efforts was a new annualized visual transmittance rating protocol (VTannual), which was implemented by the NFRC in late 2013. The new VTannual protocol offers a more meaningful performance rating that provides an extremely accurate view of how an optically complex system will perform in real-life situations. It will allow building designers to make a true “apples to apples” comparison between daylighting products so they can choose the best system to meet their project goals.

This illustration is a graphical representation of solar angles defined and utilized within the NFRC VTannual rating protocol. Illustration courtesy of NFRC.

This illustration is a graphical representation of solar angles defined and utilized within the NFRC VTannual rating protocol. Illustration courtesy of NFRC.

Calculating VTannual
To calculate the VTannual rating, a specially designed apparatus measures a daylighting product’s:

  • Annual visible transmittance: the annualized amount of daylight transferred through a surface into an interior space.
  • Zonal time (ZT) weighting factors, which are a function that determines the percentage of time the sun spends within a specific patch of sky.

   The apparatus does this by collecting clear-sky, visible-light-transmittance data for a series of vertical planes of data in 10-deg. increments. The measurements span vertical angles for solar altitudes (angles of the sun above the horizon) ranging from 20 to 70 deg. at three specific solar azimuth angles (the compass direction from which the sunlight is coming, i.e., east or west relative to due south) of 0, 30, and 60 deg.

Figure 3 (Figure 2 is not shown) is a depiction of solar altitude angles as measured with respect to the opening of the moveable test apparatus. Illustration courtesy of NFRC.

Figure 3 (Figure 2 is not shown) is a depiction of solar altitude angles as measured with respect to the opening of the moveable test apparatus. Illustration courtesy of NFRC.

   Ultimately, 18 distinct points of paired data are collected, then factored in with the historical position of the sun for a preselected site location which, for the NFRC rating, will be a standard Middle America location at 40 deg. north latitude, i.e., Boulder, CO. These can then be used to generate functional, annualized, visible-light-transmittance ratings for any site location in the world, accounting for how an optically complex product is designed to selectively increase or reduce light collection for specific times of the day and year.
   It’s important to note that the VTannual rating is based on clear-sky conditions only. Thus, the new rating will be less useful for people who live in predominantly overcast or cloudy climates.

Obtaining a rating
To obtain a VTannual rating, a manufacturer works with a third party testing organization to conduct the test. The results are then sent to an independent inspection agency to review and verify the test data and rating results. If the data are deemed to be accurate and conform with the testing standard, an NFRC label with the rating is issued to the manufacturer for use on its packaging. The data are also uploaded to the NFRC Certified Product Database.
   The VTannual rating is designated as a single number that represents the annual average clear-sky visible transmittance of a daylighting product for a standard Middle America location. This accounts for the actual time-weighted path the sun travels during the course of the year, and is expressed as a number between 0 and 1. This differs from the static direct-normal VT rating, also expressed as a number between 0 and 1, which, for a skylight, represents the ideal maximum light transmittance of a product when the sun is directly overhead, a condition that never happens for all but a few hours each year for sites within the tropics near the equator.

Taking a new approach
Optically complex systems are forcing a paradigm shift in commercial-building design. With their ability to collect, filter, and redirect daylight, they have made it easier for natural light to become the primary daytime illumination source, with electric lighting taking a supplementary role. These systems are not your average TDDs, but fully vetted lighting equipment that has been proven to perform.
   The adoption of the VTannual rating protocol is a crucial part of this new approach to commercial lighting. It is a significant advancement in how fenestration products are evaluated because it allows those involved with building design to make educated decisions based on a product’s real-life performance, and eventually the data collected in the NFRC VTannual rating process may even allow annual performance values to be calculated relative to the building’s actual geographic location.
   Architects can now make direct comparisons, which allows them to specify and select the best product for the application. They can even calculate how much useful light is available, making it possible to estimate how much electric light is needed to make up for any deficiencies during any hour of the year. Look for the new performance rating on NFRC labels starting in the Fall of 2014.

Author
Neall Digert, Ph.D., MIES, is vice president of product enterprise, Solatube International Inc., Vista, CA.

Five Myths of Tubular Daylighting Devices

Are these myths preventing you from specifying/purchasing tubular daylighting devices for your commercial facility?

Michael Sather, commercial marketing manager at Solatube International Inc., Vista, CA

Michael Sather, commercial marketing manager at Solatube International Inc., Vista, CA

Many people are familiar with the concept of tubular daylighting devices (TDDs), often generically referred to by more informal names such as solar tubes, sun tunnels, light pipes, or tube lights. The general concept is simple: A dome, attached to a roof with a self-mounted flashing or mounted on a curb, captures sunlight, transfers it into the building through a highly reflective tube, and delivers it into the interior space through a diffuser lens mounted at the ceiling level or at the end of the tube in an open ceiling.
   In the past 13 years, TDDs have revolutionized the way buildings are illuminated. When applied correctly, a building can be fully daylit using only the natural light supplied by the TDDs for 90% or more of the occupied hours of the year, relying on electric lights only as a backup during extremely overcast days or at night.
   That said, how do you know if TDDs are the right choice for daylighting your project? What key aspects should you consider when selecting the best TDD for a specific application? To help answer these questions and give you a better understanding of this product category, let’s explore five myths of TDDs.

When applied correctly, a building can be fully daylit using only the natural light supplied by the TDDs for 90% or more of the occupied hours of the year, relying on the electric lights only as a backup during extremely overcast days or at night.

When applied correctly, a building can be fully daylit using only the natural light supplied by the TDDs for 90% or more of the occupied hours of the year, relying on the electric lights only as a backup during extremely overcast days or at night.

Myth 1: Tubular daylighting devices are only for residential applications or small spaces.
The original TDDs that appeared in the U.S. market in the early 1990s were strictly designed for residential spaces. In the past two decades, the TDD category grew to rival and eventually surpass traditional skylights for residential applications.
   Building on that residential-market success, the world’s first commercial-grade TDD appeared on the scene in the year 2000. This new technology boasted a 21-in.-dia. tube and a transition box for a grid ceiling system, which allowed a round tube to accommodate a square diffuser, simply by replacing a 2 x 2-ft. ceiling tile. Open-ceiling models also debuted at this time and featured a diffuser lens attached directly to the tube bottom. As a result, the approach to daylighting commercial buildings was greatly simplified and the daylight fixture concept was born.

Specular reflectance, which refers to a concentrated bundle of light transferred down the tube through the diffuser, is the key factor in determining how effective a TDD is at delivering light to an interior.

Specular reflectance, which refers to a concentrated bundle of light transferred down the tube through the diffuser, is the key factor in determining how effective a TDD is at delivering light to an interior.

Myth 2: Tubular daylighting devices are only for the top floor.
Specular reflectance, which refers to a concentrated bundle of light transferred down the tube through the diffuser, is the key factor in determining how effective a TDD is at delivering light to an interior. It is often confused with total reflectance, which refers to scattered light that is reflected in every direction. Total reflection is not an indicator of throughput since this would include light reflecting back up the tube.
   When daylight moves through a TDD, it reflects (or bounces) off the tubing surface. With each bounce, a small amount of that light is lost. For each 90-deg. turn, only about 5% of the light is lost. This makes possible tube runs of great distances, spanning multiple floors, running down chases in the walls, and using multiple 90-deg. turns to be able to deliver daylight deep into the interior of multistory buildings.

When daylight moves through a TDD, it reflects (or bounces) off the tubing surface. With each bounce, a small amount of that light is lost. For each 90-deg. turn, approximately only 5% of the light is lost.

When daylight moves through a TDD, it reflects (or bounces) off the tubing surface. With each bounce, a small amount of that light is lost. For each 90-deg. turn, approximately only 5% of the light is lost.

Myth 3: Tubular daylighting devices are only effective at certain times of the day or year.
Factors affecting seasonal consistency are a combination of specular reflectance, dome optics, spectral selectivity, color temperature maintenance (CTM), and solar heat gain. Lower end TDDs will have a greater difference in daily and seasonal variation due to a lack of the above mentioned properties.
   Advanced TDDs offer daily and seasonal consistency by incorporating dome technologies with passive internal reflectors or Fresnel-lens optics to help efficiently collect low-angle sunlight. This can greatly increase performance in the early morning or late day. During the winter months, when the sun is low in the sky, this is an especially important consideration in Northern latitudes.

Myth 4: Tubular daylighting devices are unpredictable.
While dome optics and tubing material will play a major role in the predictability and consistency of a TDD, you must also take into account the overall design. Even the most advanced TDDs can be designed incorrectly into a space. If you use too many units, the results can be overwhelming; if you use too few, the results can be disappointing. Most TDD manufacturers offer daylight dimming devices that provide total control over the amount of daylight entering the space.

Myth 5: All tubular daylighting devices are the same.
This statement is equivalent to saying all cars are the same. To ensure you select the right TDD for your particular project needs, there are three main considerations: the manufacturer, the product, and the partner:

  • The manufacturer. Significant differences exist in the product offerings and core focus of companies manufacturing TDDs. Some manufacturers specialize in TDDs as their sole business, whereas other companies may only offer TDDs as a small part of their overall product line.
  • The product. Be sure to specify a product that meets the needs of the space. Most TDD manufacturers will offer a wide range of models and component options to create the right configuration for the specific application and climate.
  • The partner. Once a manufacturer is selected, it is probably best to make sure there is a factory-trained distributor or representative to assist with the project. Most TDD manufacturers will have a partner who works with you at a local level from project conception through completion to help you meet your daylighting goals and stay within your budget. These companies typically offer installation services as well as installation training for subcontractors to ensure your project is a success.

Michael Sather is the commercial marketing manager at Solatube International Inc., Vista, CA.

A rare, and enlightening, trip to Walmart

 

Walmart's new logo to go with it's new, fresh look.

Walmart's new logo to go with its new, fresh store look.

It’s a rare day that I set foot in a Walmart store. Truth be known, when it comes to shopping, I have a disappearing act that makes David Copperfield look clueless, so it’s really not a Walmart thing. Nonetheless, I was in a Walmart the other day that was in the process of a complete overhaul to incorporate their new fresh look—colors, logo, no hyphen in the name.
   It works.
   My wife was with me and we were in the store to get one thing, which was at the back of the facility. While there, we decided to pick up some paper goods, cleaning supplies, etc., which necessitated my walking all of the way to the front to get a cart, and say hi to the greeter person for the second time (If you’re a Jeff Dunham fan, a joke comes to mind at this point). While strolling through the store, I started to realize that the lighting was very pleasing, more so than any store I’d been in recently. It created a crisp, clean look and felt good, i.e., I didn’t mind being in the store.
   I looked up. Much to my surprise, I saw rows and rows of skylights. Yes, other than around the edges and a few places where artificial lighting made sense, the entire store was lit by daylight. Fluorescent lights were up there, too, but those irritating tubes were turned off, for the most part. I stopped and just took in the lighting and the very pleasing effect it was having on the store environment. I saw first hand why studies keep telling us that daylighting in retail, healthcare, and education facilities has a positive overall effect. It really does work. A tip of the cap to Walmart’s store designers.
   However, as nice as the experience was, I’ll still avoid shopping like the plague.—Gary L. Parr