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, www.nfrc.org 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
||Manufacturer Product Code
||Frame / Sash Type
|A 15 character alpha numeric string
||A product trade or brand name / Double Dome
Let’s take an inventory of what the above information provides a prospective skylight specifier or buyer.
- Manufacturer’s company name
- Trade or brand name for a series of products that the tested skylight is part of.
- Frame material
- U Factor
- Solar Heat Gain Coefficient (SHGC)
- Condensation Resistance (CR)
- Glazing layers (although in most cases this was different than the information provided under “Manufacturer Product Code”)
- 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.
- The specific model or part number of the skylight tested and certified.
- Glazing details such as: material, thickness, color and shape.
- 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.
- Sealant details such as: material and form.
- 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.
- 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
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.