· by Michael Blair, Nashville's Windows & Doors
Every new window carries a label with two numbers that decide most of how it performs: the U-factor and the solar heat gain coefficient, or SHGC. The ENERGY STAR program sets limits for both, but not one set for the whole country. It splits the map into four climate zones, and the line between two of them runs through Middle Tennessee. That matters if you are comparing replacement windows for a house in Brentwood against a quote a neighbor got in Hendersonville, because the same window can qualify in one and not the other.
What U-factor and SHGC does a replacement window need here?
In Davidson, Williamson and Rutherford counties, an ENERGY STAR window needs a U-factor of 0.28 or lower and an SHGC of 0.23 or lower; in Sumner, Wilson and Montgomery counties it needs a U-factor of 0.25 or lower and an SHGC of 0.40 or lower. Those are the South-Central and North-Central criteria in the program’s certification criteria table for residential windows, dated October 2023. Both zones, like the other two, also cap air leakage at 0.3 cubic feet per minute per square foot.
The zones pull in different directions. South-Central allows a slightly higher U-factor but holds solar heat gain down hard. North-Central asks for better insulating glass and allows much more of the sun’s heat through. So a window built to one zone’s numbers can miss the other’s on a single line of the label: a U-factor of 0.27 passes in Brentwood and fails in Hendersonville, and an SHGC of 0.30 does the reverse.
Which towns fall in which zone?
ENERGY STAR assigns zones by county, and its climate zone lookup gives the answer by ZIP code. Around Nashville it sorts out like this:
- South-Central: all of Davidson County, including Antioch, Hermitage, Madison, Old Hickory, Goodlettsville and Joelton; Williamson County, including Brentwood, Franklin, Nolensville, Spring Hill, Thompson’s Station, Fairview and College Grove; and Rutherford County, including Murfreesboro, Smyrna, La Vergne and Eagleville.
- North-Central: Sumner County, including Hendersonville, Gallatin, Portland and Westmoreland; Wilson County, including Lebanon and Mt. Juliet; and Montgomery County, including Clarksville.
- Anywhere else, including Columbia and the rest of Maury County, check the lookup by ZIP before you compare labels.
What do the numbers on the window label mean?
The U-factor measures how fast heat moves through the whole window, and the SHGC measures how much of the sun’s heat gets through it. The National Fenestration Rating Council’s guide to its energy performance label puts window U-factors on a scale from about 0.20 to 1.20, where lower is better at keeping heat in, and SHGC on a scale from 0 to 1, where a low number keeps out more solar heat and a high number lets more in.
The same label usually shows visible transmittance, also from 0 to 1, where a higher number lets in more daylight. It is worth reading that third number too before choosing glass for a room you want bright, since the two energy numbers say nothing about how much light comes through.
One window that meets both zones
A window rated at a U-factor of 0.25 or lower and an SHGC of 0.23 or lower meets both sets of criteria, so it qualifies on either side of the county line. That is the combination we look for when we quote windows for a house in Brentwood or for one in Hendersonville. It also means a quote is easy to check: read the two numbers off the label, compare them with your zone, and ask about any window that falls short on either one.
The label is only part of whether new windows make sense at all. Our post on when replacing windows actually pays back goes through the rest. If you want the numbers checked against your county, send us your ZIP code and the windows you are looking at, and we will tell you which zone you are in and whether they qualify.
Sources
- ENERGY STAR: Certification Criteria for Residential Windows (October 2023) — U-factor and SHGC limits for the North-Central and South-Central zones; 0.3 cfm per square foot air leakage in every zone.
- ENERGY STAR: Windows, Doors and Skylights Climate Zone Lookup — Zone by ZIP code: Davidson, Williamson and Rutherford counties South-Central; Sumner, Wilson and Montgomery counties North-Central.
- National Fenestration Rating Council: Energy Performance Label — U-factor scale of 0.20 to 1.20, SHGC and visible transmittance scales of 0 to 1, and which direction is better for each.
FAQ
ENERGY STAR Window Ratings Split Middle Tennessee — questions
What is a good U-factor for windows?
Lower is better, and around Nashville the ENERGY STAR limit depends on the county. In Davidson, Williamson and Rutherford counties it is 0.28 or lower. In Sumner, Wilson and Montgomery counties it is 0.25 or lower. A window at 0.25 or below meets the limit in all six counties.
Is a lower SHGC better for windows?
In the South-Central zone, which includes Nashville, Brentwood, Franklin and Murfreesboro, ENERGY STAR caps it at 0.23, so lower is what qualifies. North of it, in Hendersonville, Gallatin, Lebanon, Mt. Juliet and Clarksville, the cap is 0.40, so a window with a higher SHGC can still qualify there. Check which zone your ZIP code falls in before comparing labels.
What makes windows ENERGY STAR?
A window earns ENERGY STAR by meeting the program’s limits for its climate zone: a maximum U-factor, a solar heat gain coefficient within the zone’s range, and air leakage of 0.3 cubic feet per minute per square foot or less. The ratings come from the window’s NFRC label.
What is visible transmittance on a window label?
Visible transmittance is how much daylight passes through the window, rated from 0 to 1 on the NFRC label. A higher number means more light. The U-factor and SHGC do not tell you how bright a room will be, so compare this number too when two windows have similar energy ratings.
Is Hendersonville in the same ENERGY STAR window zone as Nashville?
No. ENERGY STAR puts Davidson County, which includes Nashville, in its South-Central zone, and Sumner County, which includes Hendersonville and Gallatin, in its North-Central zone. The two zones have different U-factor and solar heat gain limits, so check the label against the right zone for your ZIP code.