Reading a window label: U-factor, SHGC and air leakage
A window label is a short list of numbers. Knowing three of them lets you compare quotes fairly.
- U-factorHow fast heat flows through the window. Lower is better at holding heat in during winter.
- Solar heat gain coefficient (SHGC)How much sun heat passes through. In a cold-winter, humid-summer climate, many owners balance it against summer cooling.
- Air leakageHow much air passes around the sash. Lower is better and affects drafts directly.
- Visible transmittanceHow much daylight comes through. Lower numbers mean a darker look.
- The NFRC label is the independent rating label found on many windows.
- ENERGY STAR certification is based on climate zones, so check that the product is certified for yours.
- Federal tax credits for windows have changed; check current IRS guidance before you count on one.
- Ask your electric or gas utility, PECO or Philadelphia Gas Works, whether any efficiency program is open, since offers change.
- Climate zone
- Philadelphia sits in IECC climate zone 4A, with cold winters and humid summers, so both U-factor and sun control matter.
- Energy code
- The energy code can set maximum U-factors for replacement windows, so ask the installer to show that the product meets the requirement that applies.
- Income-based help
- The Pennsylvania Weatherization Assistance Program, run through the state Department of Community and Economic Development, serves income-eligible households and may cover air sealing and insulation; ask the local provider what is included.
- Do not skip air sealing
- A tight window in a leaky wall still leaves a cold room. Insulation and air sealing often matter as much.
Insert vs full-frame
Which replacement method fits your frames and walls.
Frame materials compared
Vinyl, fiberglass, wood and aluminum-clad, plainly compared.
Window replacement in West Philadelphia
Victorian twins, bays and tall sash in University City and beyond.
Window replacement in Northeast Philadelphia
Mid-century rows, twins and single homes.
What is a good U-factor?
Lower is better, and the number that counts is the one required by your energy code and the one on the NFRC label of the product quoted. Ask your installer for the figure rather than a general claim.
Is triple pane worth it?
It can help comfort and sound, but costs more and adds weight. It depends on your goals, budget and the condition of everything else in the wall.
Is the federal credit still available?
The rules for the federal home energy credits changed recently. Check current IRS guidance or ask a tax professional before relying on it.