Knoxville needs windows that work both ways
Knoxville sits in IECC climate zone 4A, called mixed-humid. That label is accurate. July averages about 78°F around the clock, with roughly 36 days a year reaching 90°F, while January averages about 38°F and the city has recorded −24°F (January 1985). A window here has to hold heat in through the winter and keep sun out through the summer, which is why both label numbers matter.
Reading the NFRC label
- U-factor: how fast heat leaks through the whole window. Lower is better. Double-pane low-e units typically land well under 0.35.
- Solar heat gain coefficient (SHGC): the share of sunlight that turns into heat inside. Lower blocks more summer sun but also less free winter warmth.
- Visible transmittance: how much daylight gets through. Some heavy solar coatings darken the glass slightly.
- Air leakage: how much air passes through the closed window. Casements and fixed units usually test tighter than sliders.
Code minimum versus ENERGY STAR
| Standard | U-factor | SHGC |
|---|---|---|
| 2018 IRC, climate zone 4 (code floor referenced by the City of Knoxville) | 0.32 or lower | 0.40 or lower |
| 2021 IECC, climate zone 4 | 0.30 or lower | 0.40 or lower |
| ENERGY STAR v7, North-Central zone | 0.25 or lower | 0.40 or lower |
| ENERGY STAR v7, South-Central zone | 0.28 or lower | 0.23 or lower |
ENERGY STAR assigns zones by county. Sources disagree on whether Knox County falls in North-Central or South-Central, so look up your county on the ENERGY STAR climate zone map and compare quotes against that row. Under the residential code, replacement windows in an alteration are expected to meet the same table values as new construction.
Glass packages that hit those numbers
Almost every quote you see will be dual-pane insulated glass with a low-emissivity (low-e) coating and argon gas between the panes. The differences are in the coating and spacer:
- Single-silver low-e lets more winter sun through. A good fit for north walls and shaded lots.
- Double- or triple-silver low-e cuts more summer heat. Better for unshaded west and south glass.
- Warm-edge spacers reduce condensation at the bottom edge of the glass on cold mornings.
- Triple-pane lowers U-factor further and quiets road noise, at a higher price and heavier sash.
If fogged or failed glass is the only problem, see double-pane windows before replacing the whole unit.
Rebates and credits in 2026
- TVA EnergyRight: its residential rebate schedule has listed $15 per replaced window, installed by a member of the Quality Contractor Network. Confirm it is still offered through KUB or your local power company before you sign.
- Home Uplift: TVA EnergyRight and KUB offer income-qualified KUB customers free weatherization upgrades, averaging about $10,000, which can include windows and doors.
- Federal tax credit: the 25C credit stopped covering windows once 2025 ended; anything installed in 2026 gets nothing.
Where the savings come from
New windows cut drafts, stop condensation on the glass in January and make rooms on the sunny side of the house comfortable on August afternoons. They are rarely the cheapest way to lower a power bill on their own, so be wary of payback claims in a sales pitch. Ask the installer to show the label numbers and let comfort and maintenance drive the decision. Typical installed costs are on the cost page.
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