Guides · The product

How impact windows actually work.

Most explanations stop at “strong glass.” The real answer is a system — a glass sandwich, a plastic interlayer, a frame and an anchoring method — that is tested together and passes or fails together. Here is the whole system, with the real test numbers and the primary sources they come from.

The glass is a sandwich — and it is supposed to crack

Impact glass is laminated: two sheets of glass (the industry calls each one a lite) fused under heat and pressure to a tough plastic interlayer between them. When debris hits, the outer glass may crack — that is expected and allowed. What the test standards demand is that the interlayer keeps the broken glass held in place, so the opening stays sealed and wind and rain stay outside.

That distinction matters more than any slogan: an impact window is impact-resistant, not impact-proof. The pass/fail line in the standards is “did the missile penetrate, and did the assembly survive the pressure cycling afterward” — not “did the glass stay pretty.” A cracked-but-intact window after a storm is the product working exactly as designed.

The interlayer: PVB vs. ionoplast (SGP)

Two interlayer families dominate impact windows. PVB (polyvinyl butyral) is the common, economical choice used in a large share of impact products.Ionoplast — best known by the Kuraray trade name SentryGlas, often abbreviated SGP — is a stiffer polymer. Kuraray’s own published data describes it as about five times the tear strength and one hundred times the stiffness of conventional PVB; treat those as the manufacturer’s numbers, because that is what they are.

The honest way to choose between them: both appear in products that pass the same Florida tests. Ionoplast tends to show up where the engineering is hardest — very large openings, tall buildings, demanding design pressures. PVB remains a rational choice for typical residential sizes. What decides it for your opening is not the polymer’s reputation but the approval document for the exact product, size and configuration you are buying.

The lites: heat-strengthened vs. tempered

The glass sheets themselves come in two heat treatments, and the difference shows up precisely when the glass breaks:

Heat-strengthened glass breaks into large pieces that stay engaged with the interlayer — which is why it is common in impact laminates. A big cracked shard glued to plastic still covers the hole.
Tempered glass is stronger against direct impact but shatters into small cubes. In a laminate, small fragments hold less glass area against the interlayer; tempered lites are used where codes require safety glazing (near doors, floors, wet areas) or the design calls for them.

Neither treatment makes glass “impact-rated” by itself. Non-laminated tempered glass — the kind in shower doors — offers no debris protection once it shatters. The lamination is what keeps the opening closed.

The frame is half the product

A missile test is run on a complete unit: glass, interlayer, frame, hardware, and the anchors installed exactly as the manufacturer’s installation instructions specify. The approval belongs to that whole assembly. Two consequences follow:

Frame material matters. Aluminum frames dominate the strongest coastal applications; vinyl (PVC) impact products exist and carry their own approvals with their own size and pressure limits. Same glass in a weaker frame is a different product with a different rating.
Installation is part of the rating. The anchor type, spacing and embedment in the instructions are part of what was tested. An impact window attached some other way is, legally and physically, an untested assembly. This is one reason the permit and inspection process exists — see our permits and inspections guide.

The tests behind the label

Florida’s High-Velocity Hurricane Zone (Miami-Dade and Broward counties) sets the strictest requirements in the Florida Building Code, and the tests are specific enough to quote. These are the real numbers, from the code text and the standards themselves:

Test	What actually happens	Where it is defined
Large missile	A 9-pound 2×4 lumber missile is fired at the unit at 50 feet per second — once at the center, once at a corner. Applies to products installed up to 30 feet above grade.	FBC §1626.2 · Miami-Dade TAS 201 · ASTM E1996 (missile level D)
Small missile	Thirty 2-gram steel balls (5/16-inch) strike the unit at 130 feet per second, in groups aimed at the center, edges and corners. Applies above 30 feet, where wind carries gravel rather than lumber.	FBC §1626.3 · ASTM E1996 (missile level A)
Cyclic pressure	After the impacts, the same cracked unit endures thousands of inward and outward pressure cycles — 9,000 in total under the HVHZ schedule — simulating hours of gusting on a compromised window.	FBC Table 1626 · TAS 203 · ASTM E1886
Static pressure, air & water	Uniform structural load plus air-infiltration and wind-driven-rain resistance — the everyday performance tests that also anchor the design-pressure rating.	TAS 202

A useful pairing to remember: ASTM E1886 is the test method (how the missile is fired and the cycling run); ASTM E1996 is the specification (what counts as passing). Miami-Dade’s TAS protocols are the HVHZ’s own, stricter versions.

Myths worth retiring

“Every 10 DP points equals one hurricane category.” Invented. Design pressure is an engineering load in pounds per square foot, determined by your building’s height, exposure and location — not a hurricane-category ladder. Our ratings guide shows how DP is actually set.
“The glass carries the approval.” The assembly does — glass, frame, anchors and installation method together.
“Impact windows don’t break.” They crack by design and hold. “Unbreakable” is a claim no test report makes.
“Any laminated window is a hurricane window.” Lamination is necessary, not sufficient. Without the missile-and-cycling test evidence for the exact product and size, it is just laminated glass.

Energy and noise are bonuses — with their own ratings

Impact units are often built as insulated glass (two sealed panes with a gas-filled gap) with Low-E coatings. That improves comfort and energy performance, and laminated glass damps outdoor noise. But those properties carry separate ratings (U-factor and solar heat gain, published per series in the NFRC directory) and have nothing to do with missile resistance. Evaluate them separately, the way the labels do.

Verify instead of trusting: every legally sold window assembly in Florida has a public approval document with its tested sizes and pressures. The window ratings guide walks you through looking one up in minutes — no salesperson required.

Questions we actually get

Do impact windows break when something hits them?

Yes — and that is the design. The glass is allowed to crack; the interlayer is required to hold the broken pieces together and keep the opening sealed. The test standards fail a window that lets the missile through or opens a hole, not one that cracks.

Is the glass what gets the impact rating?

No. The tested unit is the whole assembly: glass, interlayer, frame, hardware and the anchoring method in the installation instructions. Change any of those and the test result no longer describes what is on your wall.

What is the difference between PVB and ionoplast (SGP) interlayers?

Both are used in impact windows that pass the same tests. PVB is the common, economical choice. Ionoplast is stiffer and stronger by the manufacturer’s published data, and is often specified for very large openings and demanding coastal designs. Neither is automatically “better” — the right question is what the approval document for your exact product and size shows.

Are impact windows the same thing as hurricane-proof windows?

No product is hurricane-proof, and the code never uses that word. Impact-resistant windows are tested to resist specific missile impacts and pressure cycles. Marketing that promises “proof” is promising something no test report says.

Do impact windows also help with noise and energy bills?

Laminated glass damps sound noticeably, and insulated Low-E versions improve energy performance. But those are separate properties with separate ratings — a window can be quiet and efficient without being impact-rated, and vice versa.

Sources

Last checked: August 20, 2026. If a source above changes, this page gets corrected — not quietly left to age.

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