ENERGY STAR estimates that swapping old single-pane windows for certified ones saves a typical house $493 to $547 a year in the two climate zones that cover Tennessee. If you already have double-pane windows, its estimate drops to $253 to $336. Those numbers rest on a model house, and there are good reasons to replace windows that never show up on the bill. Here's how to read the numbers, answered one question at a time.
Where do those numbers come from?
They're on ENERGY STAR's savings map, from modeling done for the program in 2023. The model is a 2,376-square-foot, two-story, single-family house, run in 132 U.S. cities with each state's average gas and electric prices from the Energy Information Administration. The savings assume you replace every window in the house with windows that meet ENERGY STAR Version 7.0.
| ENERGY STAR zone | Replacing single-pane clear glass | Replacing double-pane clear glass |
|---|---|---|
| North-Central | $493 a year, 11 percent | $253 a year, 6 percent |
| South-Central | $547 a year, 12 percent | $336 a year, 7 percent |
The percent is a share of the whole home's yearly utility bill, not only heating and cooling. ENERGY STAR says your real savings will vary with your local weather, your utility rates and your house.
Which zone is Knoxville in?
On ENERGY STAR's climate zone map, the line between North-Central and South-Central runs right across Tennessee. Rather than squint at the map, use the program's climate zone finder, which asks for your state and county.
If you live in a county near that line, the finder settles it quickly. The zone matters more for which windows qualify than for the savings, since the two zones' estimates sit close together.
What do U-factor and SHGC mean?
A certified window's label carries two numbers worth reading.
U-factor is how easily heat moves through the window. Lower is better. A low U-factor keeps the heat inside in January and outside in July.
SHGC stands for solar heat gain coefficient. It's the share of the sun's heat that comes through the glass, and lower means less of it gets in. In a place with long, hot summers, a low SHGC is what keeps a west-facing bedroom bearable late in the afternoon.
Here are ENERGY STAR's window criteria for the two zones:
| Zone | U-factor | SHGC |
|---|---|---|
| North-Central | 0.25 or lower | 0.40 or lower |
| South-Central | 0.28 or lower | 0.23 or lower |
Both zones also limit air leakage to 0.3 cubic feet per minute per square foot of window. The South-Central zone asks for a much lower SHGC, which tells you the program sees keeping summer sun out as the bigger job there.
What about comfort?
This is where we think the real value sits. ENERGY STAR's windows page says the inside glass of a certified window stays warmer in winter, so a seat by the window stops feeling like a seat by the door. It says the coatings cut fading by up to 75 percent, which matters for wood floors and furniture in a sunny room. It also says certified windows resist condensation better, though they can still fog up in a hard cold snap. That matters in any room where you can feel cold air sliding off the glass on a January night.
What if my windows are already double-pane?
Then the math changes. ENERGY STAR's estimate for replacing double-pane clear glass is $253 a year in the North-Central zone and $336 in the South-Central zone, a good deal less than its single-pane numbers. If your double-pane windows still open, close, lock and seal, the bill alone is a weak reason to replace them. Fog trapped between the panes means the seal between them has failed, and that's a fair reason to replace the sash or the whole window.
Does the install matter?
A lot. ENERGY STAR's own page says even the best windows can be drafty if they're installed poorly. A certified window set out of square, or left with gaps around the frame, leaks air like the one it replaced. Ask how the gap between the window and the wall framing will be sealed and insulated, and who finishes the trim inside and out. You can read about the window work we offer on our windows and doors page.
Is it worth it for the savings alone?
Our opinion: buy replacement windows for comfort, and count the lower bill as a bonus. A whole house of windows is a big purchase, and a few hundred dollars a year takes a long time to add up to it. Drafts, fog trapped between the panes, soft wood at the sills or sashes painted shut are reasons on their own.
If you're weighing it, choosing replacement windows covers the types and frame materials. The window lines we work with are on our products page, and you can read about paying over time.
Questions
Do I have to replace every window at once?
No. ENERGY STAR's estimates assume the whole house, so a few windows save less, but starting with the worst rooms is a sensible way to spread out the cost.
Will replacement windows fix a hot upstairs room?
They can help, mostly on the west and south sides, where a lower SHGC blocks more of the afternoon sun. Check the attic too. Heat coming down from a hot attic is a separate problem with its own fix.
