What are impact windows?

Why keeping the opening sealed is the whole idea

The instinctive mental model of hurricane damage is wind tearing at a house from the outside. The mechanism that actually destroys houses is different, and understanding it explains why impact windows are specified the way they are.

When an opening fails during a storm — a window breaks, a door blows in, a garage door buckles — wind enters the building. The air pressure inside rises sharply while the wind passing over the roof is simultaneously lowering the pressure above it. The roof is then being pushed up from inside and pulled up from outside at the same time. That pressure differential is what lifts roofs off houses, and it starts with one failed opening.

An impact window is engineered to keep that from starting. The laminated glass may crack and look ruined, but it holds together and stays in its frame, so the envelope is never breached. A window that has done its job in a storm often looks like a window that has failed.

What laminated glass actually is

Laminated glass is two panes bonded under heat and pressure to a plastic interlayer, most commonly polyvinyl butyral, known as PVB. Some assemblies use a stiffer ionoplast interlayer where design pressures or spans demand it. It is the same construction principle used in car windscreens, which is why a stone chip in a windscreen produces a crack rather than a hole.

This is not tempered glass. Tempered glass is heat-treated to break into small blunt fragments, which is a safety feature about injury, not about keeping an opening intact — a tempered pane that breaks leaves an empty hole. Impact assemblies frequently combine the two, using a tempered outer pane over a laminated inner one, but lamination is what does the work.

How impact windows differ from double glazing

This confuses a lot of buyers, and some sales processes do not go out of their way to clear it up. Double glazing means two panes with a sealed air or gas gap between them, and it exists to slow heat transfer. Lamination means two panes bonded to a plastic interlayer with no gap, and it exists to hold the glass together under impact.

They are independent properties. A window can be laminated and single-glazed, double-glazed and not laminated, or both at once. Only lamination and the tested frame assembly make a window impact-rated. In South Florida the cooling load is dominated by solar heat coming through the glass rather than by conduction, which is why a low-E coating typically does more for your comfort and your electricity bill here than a second sealed pane does.

What they genuinely deliver

Storm protection is the reason they exist, and it is real. But homeowners consistently report two other effects more often, because they are experienced every day rather than once every few years.

The first is noise. The plastic interlayer damps sound transmission in a way a single pane or a standard insulated unit does not, particularly across the mid frequencies that carry traffic, aircraft and neighbourhood noise. On a busy road it is a dramatic change. It is not soundproofing and should not be sold as such, but it is the benefit people mention unprompted.

The second is the insurance wind mitigation credit, which recurs at every renewal for as long as the openings stay protected. There are conditions attached and it does not happen automatically, which is why it has its own page.

Two further effects worth knowing: laminated glass blocks the large majority of UV, which slows the fading of floors, rugs and furniture; and forced entry through a laminated pane is slow, loud and conspicuous, because the glass sags rather than shattering out.

What they do not do, and what they cost

They are not bulletproof, they are not unbreakable, and a severe enough impact will crack them — the design intent is that the assembly stays in the opening afterwards, not that the glass survives unmarked.

They cost meaningfully more than a non-impact replacement window, and considerably more than shutters. Anyone telling you otherwise is selling something. We do not publish a price range because the honest answer depends on opening count and size, operating style, glass specification, frame material, working height and how much stucco and trim repair the tie-ins require — and a range printed on a website that ignores all of that is close to meaningless.

The case for them in South Florida is that the protection is permanent and needs nobody to be home to deploy it, and that the recurring insurance credit and the day-to-day benefits accrue for as long as you own the house.