This page resolves a reporting mismatch: which U-factor goes in which document. It is not a Florida climate-zone limit table. For limit overviews, use ACG’s energy-code compliance page and then verify current code text for the project.
Two definitions, carefully
Public NFRC procedure language (ANSI/NFRC 100) distinguishes:
- Total fenestration product U-factor (Ut): the U-factor representative of the total system. In practice this is what people mean by whole-window or whole-product U-factor. It accounts for center of glazing, edge of glazing, frame, dividers where applicable, and related components under the NFRC rating method.
- Center-of-glazing U-factor (Uc): the U-factor representative of the center-of-glazing area only. NFRC defines the center-of-glazing area as glazing away from the frame/sash/divider edge zone (commonly described as outside a 2.5 in / 63.5 mm band from those edges in NFRC 100 definitions).
NFRC also explains in public education material that labeled U-factor is whole-product performance, not glass alone. Cite the controlling NFRC document edition on the project submittal rather than paraphrasing from memory.
Why the numbers differ
Frames and edge-of-glass regions usually transfer more heat than the center of an insulating glass unit. For the same glass makeup, whole-product U-factor is typically higher (worse) than center-of-glass U-factor. That is normal physics under area-weighted assembly rating, not a vendor error.
This draft intentionally includes no sample U-factor numbers. Pull configuration-specific values from manufacturer NFRC documentation, CMA reports, or certified label certificates for the sizes and frame types on your schedule.
Which value belongs in which document
Use this worksheet at buyout and submittal. It is a template, not a project record.
TEMPLATE ONLY. Assign one reporting basis per document. Blank cells mean pull project docs. No invented U-values.
| Document | Typical value to show | Label the basis as | Do not |
|---|---|---|---|
| Division 08 performance spec | Whole-product U-factor (and SHGC if required) for the specified product type/size basis | NFRC whole-product / Ut | Answer with unlabeled COG brochure values |
| Energy model / COMcheck-type input | Assembly U-factor the model form requests (often fenestration product) | Match the software field name exactly | Paste COG into a whole-product field |
| NFRC label / label certificate | Certified whole-product ratings for the configuration | NFRC certified product | Edit or round certified values |
| Glass schedule / makeup table | Often COG U and SHGC for comparing coatings and IGUs | Center-of-glass / Uc | Present COG as if it were the NFRC label |
| Shop drawings / product data | Both if useful: whole-product for compliance, COG for glass note | Two clearly separated lines | One ambiguous “U-factor = …” line |
| Bid form / VE proposal | Same basis as the owner’s specified limit | State basis next to every number | Compare COG alternate to whole-product basis of design |
| Owner finish sample review | Appearance and VT notes; U-factor secondary | N/A or COG for glass only | Use thermal sheets as color samples |
Bid-day mistakes to avoid
- Comparing a glass-only COG from a coating brochure to a whole-window U-factor limit in the spec.
- Omitting frame type (thermally broken vs non-thermally broken) when stating whole-product U-factor.
- Using a residential NFRC size basis for a commercial curtain wall or storefront without checking the commercial rating path (CMA / site-built as applicable).
- Changing Low-E and recycling old thermal sheets without re-pulling the configuration. See Low-E change vs approved system.
Submittal language that stays clean
Prefer two lines over one vague line:
- “Whole-product U-factor (NFRC / total fenestration product): [value from certified docs for config X]”
- “Center-of-glass U-factor: [value from glass maker sheet for makeup Y]”
Leave the brackets empty until documents are in hand. Do not invent fillers for bid day.
How to ask ACG for the right package
Send elevations, the opening schedule, and the specification section that states U-factor and SHGC. Note whether the energy modeler needs whole-product inputs. State material-only or furnish-and-install.
ACG works from West Palm Beach, Naples, and Tampa. Call (772) 486-7711. FL CGC #1531993.
Frequently Asked Questions
What is whole-window U-factor?
In NFRC terms, the total fenestration product U-factor (often called whole-window or whole-product U-factor) represents heat transfer for the complete assembly, including center of glazing, edge of glazing, frame, and related components as defined in ANSI/NFRC 100. It is the value used for product comparison on NFRC labels and certificates.
What is center-of-glass U-factor?
Center-of-glass (center-of-glazing) U-factor represents the central glazing area only, excluding frame and edge effects as defined in NFRC procedures. It is useful for glass makeup comparisons. It is not a substitute for whole-product U-factor when the specification or code path asks for fenestration product performance.
Which U-factor belongs in a commercial submittal?
Use the value the project documents require, stated with its basis. If Division 08 or the energy model calls for NFRC whole-product U-factor, do not answer with center-of-glass alone. If a glass schedule asks for center-of-glass for makeup selection, label it COG explicitly.
Why do bids fail when U-factors look better on paper?
A common failure is comparing a center-of-glass brochure number to a whole-window specification limit. COG is usually lower (appears better) than whole-product U-factor for the same makeup. Always state the reporting basis next to the number.