Bristolite Advantage
Our mission at Bristolite is to provide our customers with the highest quality products and supreme service at an exceptional value. We also aim to provide the industry with an abundance of accurate and useful information relative to daylighting and energy conservation. We take our corporate responsibility to our employees, associates, industry colleagues and customers very seriously and we see ourselves as stewards for the efficient use of sustainable carbon free energy.

NFRC Certification

How useful is NFRC Certification of plastic unit skylights?

In our review of all NFRC certified plastic unit skylights (which the NFRC identifies as domed skylights) on the NFRC web site, on September 7, 2011, we once again noted that none of the skylights showed a value for light transmission (VT) or had a detailed description of the units tested and certified.

Following is the total information presented when checking a manufacturer’s certified skylight under the Certified Products Database. No other information is available on the NFRC web site or by request.

Manufacturer: Company Name
Series Name: A Trade or Brand Name
Operator Type: XXXX

CPD # Manufacturer Product Code Frame / Sash Type U-factor SHGC VT Condensation Resistance Glazing Layers Low-E Gap Widths Spacer GapFill Grid Divider
A 15 character alpha numeric string A product trade or brand name / Double Dome AI/NA 0.74 0.42 0 41 1     N   N NA

Let’s take an inventory of what the above information provides a prospective skylight specifier or buyer.

  1. Manufacturer’s company name
  2. Trade or brand name for a series of products that the tested skylight is part of.
  3. Frame material
  4. U Factor
  5. Solar Heat Gain Coefficient (SHGC)
  6. Condensation Resistance (CR)
  7. Glazing layers (although in most cases this was different than the information provided under “Manufacturer Product Code”)
  8. And lastly, the last 6 items in the table apply to glass only and do not apply to plastic. 

Now, let’s take an inventory of what the above information DOES NOT provide.

  1. The specific model or part number of the skylight tested and certified.
  2. Glazing details such as: material, thickness, color and shape.
  3. Frame and frame cap details other than material such as: frame and frame cap dimensions, thicknesses, thermal break or not and if so what design, and frame coatings.
  4. Sealant details such as: material and form.
  5. Identity of the actual NFRC test reports (by the testing laboratory’s name and test report number) that were used for certification of the performance numbers.

The fact is a specifier or buyer really doesn’t know anything about the skylight that was tested other than three performance numbers presented as certified. Consequently, there is no way to verify that the skylight specified or purchased is the same model, same design and same materials of construction as the skylight that purportedly produced the NFRC certified numbers.

  1. And, MOST SIGNIFICANTLY of all.
    A certified Solar Heat Gain Coefficient (SHGC) without a corresponding LIGHT TRANSMISSION (VT) performance number isn’t very useful.

Light transmission and solar heat (SHGC) have a direct and opposing relationship. It is not possible to reduce solar heat without reducing light transmission. A specifier or buyer selecting an NFRC certified skylight based on a low SHGC needs a corresponding light transmission (VT)

The reason the NFRC does not have VT in their plastic unit skylight certification program is they do not have a standard or testing procedure for VT testing of plastic unit skylights. We would hope that at sometime in the future they would adopt an ASTM VT standard and include this in their certification program. We do not know why specific information on the tested and certified skylight is not provided.

Of the three most important performance measures of a skylight: light transmission, U Factor and SHGC, light transmission has by far the greatest impact on a skylight’s ability to reduce energy costs by turning off electric lights. On a lumens or foot candles versus wattage comparison sun light is much more efficient than electric lights. Approximately 80% of an electric lighting system’s energy is emitted as heat rather than light. This is in contrast to approximately 50% for sun light.

Reducing light transmission to reduce solar heat in skylights and daylighting systems is very often a poor economic trade off in terms of building energy savings. To prove this point, let’s run a few SkyCalcs on two skylights, one with a low SHGC and a correspondingly low VT, and the other with a terribly high SHGC and a good VT. To isolate our comparison on VT and SHGC, both skylights have the same U Factor and are employed on the same building. We will also run the SkyCalcs in the two very different ASHRAES of Los Angeles and Minneapolis.

Building: A 100,000 square foot retail operation with a 30 foot ceiling and an electric rate of $0.12kWh.

Daylighting System: Five percent of the roof area skylighted with a quantity of 166, 5 foot by 6 foot skylights.

Skylight A - 47% VT, .42 SHGC, U Factor .74

Skylight B – 70% VT, .65 SHGC, U Factor .74

Results: In review of the following SkyCalc Tabular Results it’s clear where the “money” is. For daylighting it’s all about turning of the electric lights and turning off electric lights is all about VT. Daylighting’s affect on heating and cooling energy cost combined is generally only about 10% of the effect it has on lighting energy costs.

In the Los Angeles ASHRAE skylight B (terrible SHGC and good VT) produced 24% more annual energy savings than skylight A (very low SHGC and correspondingly lower VT).

In the Minneapolis ASHRAE skylight B (terrible SHGC and good VT) produced 112% more annual energy savings than skylight A. (very low SHGC and correspondingly lower VT).

We started this document with the question how useful is NFRC Certification of plastic unit skylights? In our opinion, until light transmission and detailed descriptive information of the skylight tested and certified is added to the program the answer is, it’s just not useful.