Lesson 2 of 3

Read the label.
Then read the warranty.

Every window that meets code comes with a sticker that tells you how it performs, and a warranty that tells you how long the maker expects it to. Both are written to be skimmed. Here is how to read them slowly.

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The NFRC sticker

Five numbers. Two of them matter most here.

The National Fenestration Rating Council label is on every new window. Same test method for every maker, so the numbers compare straight across.

U-factor is how much heat gets through the whole window, frame and glass. Lower is better. Middle Tennessee is climate zone 4, and the 2021 energy code calls for 0.30 or lower. SHGC is how much of the sun's heat comes through the glass. Lower is cooler in August. On a house with big west windows, this is the number that decides the power bill.

Visible transmittance is how much light comes through; higher is brighter. Air leakage is how much air moves through the closed unit; lower is tighter, and it is the number that separates a well-built sash from a loose one. Condensation resistance is optional on the label; higher means less sweating at the glass edge, which comes back to the spacer from Lesson 1.

Label line	What it measures	Better is	Middle Tennessee
U-factor	Heat loss through the whole unit	Lower	0.30 or lower to meet code; low 0.20s is premium
SHGC	Solar heat coming through the glass	Lower (for cooling)	Matters most on west and south glass
Visible transmittance	Daylight through the glass	Higher	Trade-off against SHGC; ask for the glass package options
Air leakage	Air through the closed window	Lower	0.3 cfm/ft² is the code ceiling; good units are well under
Condensation resistance	Sweating at the glass edge	Higher	Warm-edge spacer moves this number

Ask

Which glass package is the quote priced on? The same window can carry three or four different labels depending on the Low-E coating and argon. Make sure the label you are reading is the unit you are buying.

The structural rating

DP is the number nobody prints on the price sheet.

Design Pressure is the wind load, in pounds per square foot, that the window held in a test without failing. It comes from the NAFS standard (AAMA/WDMA/CSA 101/I.S.2/A440), which also rates air leakage, water penetration, and structural test pressure. It is the closest thing to a strength number a window has.

It matters most on large units, mulled assemblies, second-story openings, and anything on a ridge or open field. A frame that has been tested to 75 psf is a different object from one where the maker does not publish a number, and the difference is the cladding and the corner construction from Lesson 1.

Ask

What is the DP rating on this unit at this size? Ratings drop as the window gets bigger. A rating quoted on a small test unit does not carry to the 6-foot picture window in the great room.

The warranty

Two words to look for: years, and transferable.

"Lifetime" is the most quoted word in window sales and the least defined. Read the document, not the headline. Look for four things.

Glass seal, in years. This is the failure that actually happens. Twenty years stated plainly beats "lifetime" that turns out to mean ten and prorated. Finish, in years, against fading. A maker who meets AAMA 2604 says twenty years; 2605 says thirty. Prorated or not. Prorated pays a smaller share every year, and by year eight it pays for very little. Transferable or not. If you are building to sell, this is the one. A transferable warranty goes to your buyer with the deed and belongs in the listing.

Then read what is excluded and whether labor is covered. A parts-only warranty on a fogged unit means somebody is paying a glazier.

Read for	Weak warranty	Strong warranty
Glass seal	"Lifetime", prorated, undefined	Stated years, not prorated (20 years is the premium mark)
Finish	"Durable finish", no term	Named AAMA standard with years against fading
Wood or frame	Not mentioned	Stated years against rot or structural failure
Transfer	Original owner only	Transfers to the next owner
Labor	Parts only	Says what is covered, in writing

Before You Price a Window

Know what you are buying.

Three short reads. Ten minutes total. Written for the builder who has to answer to the money, and for the money.

Lesson 1 What you are paying for Five things that separate a window that lasts from one that looks the same on the truck. Each one you can check on a sample in our showroom. Read it → Lesson 2 Read the label The NFRC sticker, the DP rating, and the warranty. What the numbers mean in Middle Tennessee, and the two words to look for in every warranty. You are here Lesson 3 The second window is the expensive one A window is priced by the opening and paid for over the life of the house. Run your own numbers, then hand the page to whoever is funding the build. Read it →

Frequently Asked

Labels and warranties, answered.

What U-factor do I need for windows in Middle Tennessee?

Middle Tennessee is IECC climate zone 4, mixed-humid. The 2021 energy code prescriptive value for windows in zone 4 is a U-factor of 0.30 or lower. Lower is better. Most Low-E argon double-pane windows land between 0.27 and 0.30; triple-pane and better spacers get into the low 0.20s. Check which code year your county has adopted, and ask your Midway estimator to confirm the number on the actual unit you are quoting.

What is SHGC on a window label?

Solar Heat Gain Coefficient, from 0 to 1. It is how much of the sun's heat comes through the glass. Lower means less summer heat gain. In a Tennessee house with a lot of west-facing glass, a lower SHGC matters more than a slightly better U-factor. Energy Star sets an SHGC target by climate zone; ask which zone your county falls in.

What does a window DP rating mean?

Design Pressure, in pounds per square foot, is the wind load the window was tested to hold without failing, under the AAMA/WDMA/CSA 101/I.S.2/A440 standard (NAFS). A higher DP means a stronger window. It matters for large units, second-story openings, and exposed sites. A window that is structurally tested to 75 psf is in a different class from one that does not publish a number.

What does a "lifetime" window warranty actually cover?

Usually less than it sounds like. Read for four things: how many years the insulated glass seal is covered, whether coverage is prorated (it pays a shrinking share each year), whether it transfers to the next owner, and whether labor is included. "Lifetime" is often defined as the life of the product as the maker decides it, non-transferable, parts only. A warranty that states plain years, in writing, with transfer, is the stronger one even when the number is smaller than "lifetime".

Is a transferable window warranty worth anything?

Yes, and especially if you are building to sell. A transferable warranty goes to the buyer with the house. A non-transferable one ends at closing, which means the buyer is paying for windows with no coverage. Your listing can say the windows carry a twenty-year transferable glass warranty. It cannot say that about most builder-grade lines.

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Read the label.Then read the warranty.