The sticker on a new window is the only part of that window nobody in the showroom wrote. The brochure is marketing. The figure a salesperson quotes about the glass is usually true and almost always the flattering half of the truth. The label is a test result — the manufacturer submitted that unit, in that glass package, to an independent certification program, and the numbers came back in a fixed format so the window beside it can be read the same way.
Then most people peel it off and put it in the trash before they read it.
We are not going to tell you which brand to buy. What is worth knowing is what each number on that sticker describes, which of them carries the most weight in Central Indiana, and which one your installer can ruin in an afternoon.
What the label actually is
The certification label — NFRC across the top, for the National Fenestration Rating Council — reports whole-window performance. That phrase carries the whole section. The numbers cover the frame, the sash, the spacer at the edge of the glass and the glass itself, averaged over the entire unit and tested as a complete assembly.
Sales literature usually quotes center-of-glass performance instead, because the middle of a sealed unit is the best-performing part of a window and it makes the figure look better. A frame conducts heat. A metal spacer conducts heat. The edge of the glass runs colder than the middle. The certified number carries all of that; the brochure number does not. If you are handed a performance figure and cannot tell which kind it is, ask.
The sticker also names the manufacturer, the model and the glazing. Keep it. A window series is not one product — the same frame takes several glass packages, and each package gets its own label, so comparing brand to brand without matching the glass compares nothing. The paper comes off eventually, but there is normally a permanent mark etched into a corner of the glass, and that is what you fall back on years later.
U-factor: the rate the house gives up heat
U-factor is the rate heat moves through the assembly. Lower is better, and that is genuinely the whole instruction. R-value is its inverse, which is why glass gets advertised in R-value — a bigger number reads as more. The label uses U-factor, whole-window, and in this climate it is the number to weight hardest. Heat loss is the load that runs the most hours in a Hamilton County house.
What you feel is not the utility bill. It is the cold plane. Sit near a poor window on a January night and the air is not moving, but the glass is pulling radiant heat off you and the air touching it is cooling and falling into the room. It reads as a draft. It is not one, and chasing it with weatherstrip does nothing.
Winter condensation on the inside of the glass is the same story from the other side. Heating season air is dry, so water forming on the pane says the surface is cold rather than the house is wet — the same season that has wood shrinking and moving. Left alone, it rots the sill and stains the casing under it.
SHGC: how much of the sun you let in
Solar heat gain coefficient is the share of the sun's energy that ends up inside — what passes straight through the glass plus what the glass absorbs and radiates inward. Lower admits less. Higher admits more. Neither is better until you say which wall it is on.
Orientation changes the answer
South glass in December is free heat. West glass in July is the afternoon cooling load, arriving low and straight into the room at the hour the house is already warmest. One specification for the whole house is a compromise between those two facts. Ordering different glass by elevation is possible and mostly gets skipped, because it complicates a purchase order. If you have a west wall of glass over the room that is always the hottest, raise it before the order goes in.
Shade does the same work from outside, so count what you have or plan to build — a deep overhang, a mature tree, the rafters of a pergola. Exterior shading stops the heat before it reaches the glass rather than after.
Visible transmittance is a different number
The label carries VT alongside SHGC: how much daylight comes through. Some low-SHGC glass gets there with a tint that drops the light along with the heat, while spectrally selective coatings hold more of the light and still cut the heat. In a room that is already dark — a north bedroom, anything below grade — read VT as carefully as you read SHGC.
Air leakage, and the number your installer decides
Air leakage is measured in a laboratory, on a new unit, clamped into a test wall by people who do it every day. It describes what gets past the weatherstrip, the sash and the joints of the frame. It is a real number and a useful floor.
It says nothing about the gap between the window and your framing. That gap is where most air in a real house moves, and it belongs entirely to whoever installs the unit: the sill pan, the direction the flashing laps, backer rod and low-expansion foam in the shim space instead of a hard-setting can foam that bows the jamb and binds the sash, the way the housewrap gets folded over the nailing flange. None of that appears on any sticker.
Which is why the install method deserves more attention than the badge on the frame. Full-frame or insert are two different jobs with two different scopes, and an insert leaves the original frame — and whatever air sealing it had — around the outside of a brand new window. Some labels also carry an optional condensation resistance rating, where higher is better; it is not required, so its absence means nothing.
Inside the glass: gas, spacers and panes
Gas fill
Argon sits between the panes because it is denser than air and slows convection in the cavity. Krypton does the same in a narrower gap and costs considerably more. Both leak, slowly, through the seal across the life of the unit, and nobody can tell you the day it went. That is not a scam. It is the honest limit of a sealed glass unit, and it means the seal is the real product — when a seal fails you get fog between the panes that no cleaning touches. The gas is already priced into the U-factor on the label, so you do not need to shop for it separately.
The spacer
The spacer holds the panes apart at the edge and carries that seal. An aluminum spacer conducts heat straight around the perimeter, which is why condensation and frost start at the edge of a pane and work inward. Warm-edge spacers use lower-conductivity materials to break the path. You do not have to adjudicate the marketing here either — whole-window U-factor already includes the spacer's effect. A window with a better spacer tests better.
A third pane
Triple glazing improves U-factor and adds weight, and weight is the trade: heavier sash, harder-working balances and hinges, deeper frames that eat into the glass area. On fixed glass it costs you nothing but money. On a casement or a double-hung somebody opens daily, it is a hardware question over decades.
Frame material, and what each one trades away
- Vinyl. Welded corners, strong thermal numbers for the money, nothing to finish. It expands and contracts with temperature more than the glass does, which shows at corners and on long units, and dark exterior colors make that a real consideration rather than a taste one.
- Fiberglass. Expands at close to the rate of glass, so joints stay put. Takes paint. Narrower frames for the same rough opening, so more glass.
- Wood, usually clad. Wood inside, aluminum or vinyl skin outside. The interior is why people buy it. It fails at the exterior sill and the joint where the cladding meets it — the same exterior joints that let water into everything else outside.
- Aluminum. Needs a thermal break to perform at all here, and is mostly a commercial product.
Whatever the frame, the joint between it and the wall takes the same wetting and freezing that takes apart stoops and mortar here. Sealant is a maintenance item, not a permanent one.
The label will not flag the other thing worth checking: glass area. Two windows in the same rough opening can deliver noticeably different amounts of glass, because the certified numbers are per unit area of the whole window. A wide frame with excellent glass still tests moderately, and you lose the view either way. That matters most over a sink or along a run of upper cabinets, so settle it with the cabinet layout if the window sits in a wall you are remodeling a kitchen against.
What the label does not tell you
Whether the opening is legal
This is the largest gap between what a sticker says and what a room needs. A basement sleeping room requires an emergency escape and rescue opening, and Indiana Residential Code Section R310 sets it in net clear terms: a net clear opening of at least 5.7 square feet, a net clear width of at least 20 inches, a net clear height of at least 24 inches, and a sill no more than 44 inches above the finished floor, operable from inside without a key, a tool or special knowledge. There is an exception in the IRC for certain openings, and the office that inspects yours is the office that decides whether it applies.
Net clear opening is the hole when the sash is thrown fully open — not the frame, not the glass, not the catalog size. A casement swings clear and gives you nearly all of itself. A slider gives you one leaf. A double-hung gives you the lower sash. Two windows with identical labels in an identical rough opening can land on opposite sides of that line, which is arithmetic worth doing early. If the wall is below grade, the well outside is part of the same calculation — its size, its ladder, how much light it lets down — and all of it gets settled before anything is ordered for a finished basement.
The wall around it
A certified window in an uninsulated wall is a good component in a poor assembly. Once a wall is open, what is in the cavity and how it is air-sealed is part of the job, and on a permitted job somebody looks at it before the drywall goes back.
Whether you need a permit
Cutting a new opening or enlarging an existing one is structural, and structural is one of the triggers. In Fishers, the city says the trigger for a permit is a structural, electrical, plumbing or HVAC change, applications run through its online portal, and a residential application typically takes no more than 15 working days when the submittal is complete. Permitting and Inspections is 317-595-3120. Carmel, Noblesville and Westfield each run their own authority, so confirm with the one covering your address rather than assuming Fishers' rules travel.
Reading two quotes with the labels beside them
Once you know what the numbers mean, the useful questions are short:
- Which model, and which glass package, written out. A series name is not a specification — the same reason "white subway tile" is not one.
- The certified whole-window U-factor, SHGC and VT for that exact package, not the brand's best line.
- If more than one package is quoted, which elevations get which.
- Full-frame or insert, and the plan if a frame comes out rotted — an allowance in the estimate, or a change order later.
- What the sill pan, flashing and air-sealing detail is at the opening.
- What the manufacturer's own paperwork covers and whether it transfers on sale. That is a separate document from the label.
A quote that answers all six is not necessarily the cheapest one. It is the one you can compare against another.
The short version
Read the label in this order here. U-factor first, because heat loss is the load that runs longest in Central Indiana. SHGC second, decided by wall rather than by house. Air leakage third, as a floor — and then stop, because the gap around the window matters more than the gap through it, and no sticker on earth covers that.
The label compares products. It does not compare installations, and it has no idea what your opening is required to do. If you want someone to measure the openings you already have before anything gets ordered, ask for an estimate and we will walk them with you.
