Most buildings that need better protection already exist, so retrofits do most of the real-world work in protective design. The goal is usually modest and practical: keep glass and wall fragments out of occupied rooms and give people a better chance of walking out uninjured. This comparison covers the main retrofit families, security window film, catcher systems, blast curtains, laminated glazing upgrades and polymer wall retrofits, with what each does well, where each falls short, and how to approach a retrofit responsibly.

Start with the right question

Before comparing products, a qualified engineer should answer three questions through a risk assessment: which rooms and occupants need protection, what level of protection is appropriate, and what the existing glass, frames and walls can support. Without that, it is easy to buy a product that looks protective but fails at its weakest link. Our explainer on how buildings fail covers why balanced design matters.

Security window film

Security window film is a clear, multi-layer polyester laminate applied to the room-side surface of existing glass. When the glass breaks, the film holds fragments together. How it is attached determines how it behaves:

• Daylight application. Film covers the visible glass but stops short of the frame. It reduces loose shards, but the filmed pane can come out of the frame as a single sheet.
• Wet-glazed (structural silicone) attachment. A bead of structural silicone bonds the film edge to the frame, helping keep the pane in the opening.
• Mechanically attached. Metal battens or bars screwed to the frame clamp the film edge, providing a stronger connection where the frame can support it.

Film has a finite service life, and adhesives and silicone age with UV exposure. Plan for periodic inspection and eventual replacement on the manufacturer's recommended schedule.

Catcher systems

Catcher systems sit inside the window and catch glass and film fragments that would otherwise enter the room. Common forms include horizontal catch bars, tensioned cable systems and energy-absorbing assemblies, often combined with film. They are useful where frames are too weak to anchor film properly, because the catcher can be anchored to the surrounding wall or structure instead. Their effectiveness depends on that anchorage, so the wall must be evaluated too.

Blast curtains

Blast curtains are fabric panels, often made of high-strength synthetic mesh, hung inside a window from a top anchor. Extra fabric is stored at the bottom, typically in a sill box or trough, so the curtain can billow and catch glass fragments. Modern versions can look like ordinary sheer drapes and still allow daylight. Blast curtains are within the scope of ASTM F1642/F1642M, which covers retrofit systems not directly attached to the glazing, such as curtains, cables, shades and architectural mesh.

Their big limitation is human: a curtain that is tied back, removed for cleaning or blocked by furniture provides no protection.

Laminated glazing upgrades

Replacing existing panes with laminated glass, or adding a laminated inboard pane or secondary window insert, addresses the hazard at its source. When frames and anchorage are designed together, this is often the most robust glazing option, and laminated glass also improves wind-borne debris resistance, forced-entry resistance and acoustic performance. It is also usually the most expensive and disruptive option, especially in occupied buildings or historic facades.

Polymer and polyurea wall retrofits

Unreinforced masonry and lightweight walls can break into debris under sudden loads. Starting in the late 1990s, researchers at the Air Force Research Laboratory (AFRL) at Tyndall Air Force Base studied spray-applied elastomeric polymers on the interior face of such walls. Their publicly released findings indicated that a tough, highly elastic coating could help hold a wall together as it deformed and reduce debris entering the room. Later government and university research examined variables such as coating thickness, material stretch and strength, combining polymers with fabric reinforcement, and how the coating is anchored at the wall edges, which proved important.

Those findings made spray polyurea and similar elastomers a recognized retrofit approach. Manufacturers such as ArmorThane publish physical property data for their polyurea systems, including elongation and tensile strength. Those properties are inputs an engineer considers, but protective performance must be established for the specific wall, coating thickness and anchorage detail through engineering analysis and test data. A generic coating job is not a protective retrofit.

Side-by-side comparison

Measure your windows before you call

Retrofit pricing is driven largely by glazing area and by the linear feet of frame where film or attachments must be anchored. This calculator turns a quick window survey into the numbers installers ask for.

How to approach a retrofit

1. Get a risk assessment. Define who and what you are protecting and the appropriate protection level, using the frameworks in our standards guide.
2. Survey existing conditions. Record glass type and size, frame material, frame condition, anchorage and wall construction around openings.
3. Match options to conditions. Weak frames point toward catchers or curtains anchored to the wall; sound frames may support attached film; replacement projects favor laminated glass.
4. Specify tested systems. Require test reports or engineering analysis for the exact configuration you will install.
5. Inspect installation. Verify attachment details, silicone bead dimensions, batten fastening and coating thickness against the specification.
6. Maintain. Schedule inspections of film, silicone, curtains and coatings, and train staff to keep curtains deployed.

Choosing a qualified designer matters as much as choosing a product. See our guide on how to choose a protective design engineer.