Reading NFRC and ENERGY STAR labels on a skylight

A skylight label packs five ratings into a small space, and the terms are unfamiliar enough that many people skip straight to the price. That is a missed chance, because the numbers are the only way to compare two units on the same footing. This page explains each rating as the National Fenestration Rating Council (NFRC) defines it, how the ratings are produced, and where the ENERGY STAR mark comes in. It describes a label in general terms rather than any one design. For the bigger picture of how these ratings play out in a house, see skylights, energy and comfort.

Who NFRC is and what a label promises

NFRC describes itself as an independent, nonprofit certification body for the energy performance of windows, doors and skylights. Its own About page is clear about limits: it does not sell, service or recommend products, does not endorse manufacturers, and does not install or guarantee the performance of improperly installed products. It is also not a code body and does not set or enforce building codes. A label therefore tells you how a product was rated, not whether it is the right choice for your roof.

The ratings come from a defined process. NFRC's residential program says computer simulation is used to determine U-factor, solar heat gain coefficient and visible transmittance for product lines, with physical testing used to validate those simulations, and that physical testing is mandatory for air leakage. An independent inspection agency reviews test reports and conducts in-plant inspections. Certified products are listed in a public directory and must display the label showing their ratings.

The five ratings in plain terms

NFRC's home page lists the ratings a label can report: U-factor, visible transmittance, condensation resistance, solar heat gain coefficient and air leakage. The table below puts each one next to its direction of better, using NFRC's own wording where possible.

U-factor

The U-factor measures the rate of heat flow through a product when the indoors and outdoors differ in temperature. The Efficient Windows Collaborative glossary says values are normally given for winter conditions of 0 degrees Fahrenheit outdoors, 70 degrees indoors, a 15 mph wind and no solar load, and that the number can describe the glass alone or the entire unit including frame and spacer. Whole-unit values are the ones to compare. The glossary also notes that R-value is the inverse of U-factor (R equals 1 divided by U), so a U-factor of 0.27 corresponds to an R-value of about 3.7.

If winter heating is your main worry, NFRC's label sheet says to look for a low U-factor. For many readers in colder regions this is the rating that gets the most attention, though the right priority depends on climate.

Solar heat gain coefficient

The SHGC is the fraction of solar radiation admitted through the product, including heat that is absorbed and then released inward. NFRC's label sheet says a lower number lets less heat in and can reduce cooling costs, while a higher number can reduce heating costs. There is no single best value. A skylight that gets hot afternoon sun in a warm region may call for a low SHGC, whereas one in a cold region may benefit from more passive warmth. ENERGY STAR makes the same point about windows by side of the house: in colder climates a higher SHGC suits south-facing glass, while east- and west-facing windows should have a low SHGC or be shaded. Those statements are about wall windows, so how they translate to a particular roof slope is a design question covered in skylight placement and room orientation.

Visible transmittance

Visible transmittance is the share of visible light, weighted by the sensitivity of the human eye, that passes through the glazing. NFRC's label sheet says a high VT may reduce dependence on artificial lighting. Since the reason to install a skylight is usually light, VT is the rating that measures what you are paying the thermal penalty for. The glossary offers a way to relate it to heat: the light-to-solar-gain ratio is VT divided by SHGC. Using the numbers on NFRC's sample label (VT 0.51, SHGC 0.25), that ratio would be about 2.0. A higher ratio means more light for each unit of solar heat admitted.

Air leakage

Air leakage describes how much air passes through the product itself under a test pressure. The glossary gives the unit as cubic feet per minute per square foot of frame area, and says the lower the rating, the better the airtightness. NFRC's label sheet calls the label entry an optional rating and says products at 0.3 or less produce fewer drafts. This rating applies to the unit as manufactured. It does not tell you how well the opening around the skylight was sealed during installation, which ENERGY STAR notes can make even the best product drafty if done poorly.

Condensation resistance

Condensation resistance reflects how likely moisture is to form on a product's interior surface. The glossary describes the related condensation resistance factor as an indication of a window's ability to resist condensation, based on an AAMA standard: the higher the number, the less likely condensation is to occur. NFRC's residential page refers to this as the condensation index. Note that this is a comparison of products under test conditions. Whether condensation appears in a home depends just as much on indoor humidity, which is explored in skylight condensation and moisture.

Conditions and size matter

NFRC's sample label carries a statement that ratings are determined for a fixed set of environmental conditions and a specific product size, and that NFRC does not recommend any product or warrant its suitability for a specific use. Two practical consequences follow. First, compare labels for the same type of product at similar sizes where you can. Second, treat the numbers as a fair test, not a forecast of your energy bills.

Where ENERGY STAR fits

NFRC is the official certification body for ENERGY STAR windows, doors and skylights, and products must be NFRC certified before applying for ENERGY STAR certification. ENERGY STAR itself, run by the U.S. Environmental Protection Agency, sets requirements by climate zone. It says windows, doors and skylights must meet U-factor and, where applicable, SHGC requirements for the zone, and that the current specification took effect on October 23, 2023. When buying, it advises looking for the ENERGY STAR label for your climate zone next to the NFRC label on the glass, and saving both labels as documentation.

Because the criteria differ by zone, a unit certified for one region may not carry the mark for another, even though its NFRC numbers are perfectly accurate. Check the zone on the label against your own location using the zone finder on the ENERGY STAR site. For choosing among glazing materials, see glass versus acrylic skylight glazing. If a permit or inspection is involved in your area, the general process is described in building permits and codes for roof openings. Local codes, product instructions and a qualified professional have the final word.

The short version

An NFRC label reports U-factor, SHGC, visible transmittance, air leakage and condensation resistance, each rated by a consistent method. Low is better for U-factor and air leakage, higher is better for VT and condensation resistance, and SHGC depends on whether you need to block or capture sun. ENERGY STAR builds climate-zone requirements on top of those NFRC ratings.