Plastics for Architectural Glazing

By B. Decker | Laird Plastics | Architectural Glazing

Plastic glazing outperforms glass in weight, impact resistance, and installed cost across a wide range of architectural applications, but only when the right material is specified. Cast acrylic, solid polycarbonate, and multiwall polycarbonate each have a distinct performance profile, and the differences between them aren't always obvious until a project is already designed.

This guide covers what architects, glazing contractors, and specifiers actually need to know: UV degradation timelines, IBC code classifications, impact ratings, thermal expansion math, and installation requirements that most product data sheets bury in footnotes.

Plastic glazing refers to any transparent or translucent thermoplastic sheeting used in place of glass for skylights, building envelopes, canopies, machine guards, or architectural features. Three materials handle the vast majority of commercial and architectural applications: cast acrylic (PMMA), solid polycarbonate (PC), and multiwall polycarbonate. 

The Three Plastic Glazing Materials Worth Knowing

Quickest way to make a bad glazing call is to treat these three materials as equal options ranked by price. They're not. Each one was developed to solve a specific problem, and specifying the wrong one could produce visible, expensive results including yellowed skylights, cracked panels, or a code compliance issue.

Cast Acrylic (PMMA)

If optical clarity is the primary requirement, cast acrylic is where the conversation starts. It transmits 92% of visible light, more than standard glass, and more than any other common plastic glazing material. Cast acrylic's UV stability comes from its molecular chemistry, not a surface coating. The PMMA backbone absorbs and dissipates UV energy without the yellowing and embrittlement that afflict most other plastics outdoors.

Properly specified cast acrylic will keep it's clarity for 10 to 20 years in outdoor service, with some grades tested at less than 3% degradation over a decade.

There's a distinction worth pointing out here: cast acrylic and extruded acrylic are not the same material for outdoor purposes. Cast acrylic has higher molecular weight, better UV stability, and better long term optical performance. So for any outdoor glazing application, specify cast, not extruded.

Impact resistance is 4 to 8 times stronger than glass, which is pretty solid performance for glazing applications. Where it can fall short is under sharp, concentrated point impact, it's a brittle failure mode, not a ductile one. Acrylic is not the right specification for security glazing, hurricane-rated applications, or any project where the IBC requires a tested impact rating.

Thermal expansion is something you have to design for. Acrylic's coefficient of linear thermal expansion (CLTE) is 0.000041 in/in/°F, which is significantly higher than glass or aluminum framing. If you ignore it your panels could buckle in summer or crack at fasteners in winter. However if you account for it correctly then your material will perform without issue. See the math below.

IBC classification: CC2 burning rate per ASTM D635.
Best for: Skylights, canopies, retail storefronts, display windows, and architectural glazing where optical clarity and long term appearance are the main requirements.

Solid Polycarbonate

Polycarbonate's main characteristic is impact resistance, which is 250 times stronger than glass. This is the main reason why the material is specified for machine guards, bullet-resistant glazing, hurricane-rated applications. And any installation where the IBC requires safety glazing with a tested impact rating. A polycarbonate panel absorbs and deflects impact that would shatter acrylic or glass. Its temperature service range runs from –40°F to 240°F, which covers most of what buildings encounter.

Light transmission is 88% which is slightly lower than acrylic, but still high enough for most glazing applications where you're not optimizing for maximum clarity.

The most important thing to understand about polycarbonate is UV degradation. Without a UV-protective coating, polycarbonate yellows and becomes brittle within 2 to 5 years of direct sun exposure. This is a known characteristic that the industry solved decades ago with co-extruded UV-stabilized surface layers. Quality UV-coated polycarbonate maintains clarity and impact performance for 10 to 15 years in temperate climates. You can run into problems when you don't verify UV coating is included, or when uncoated sheet gets used outdoors because it was cheaper. Always confirm UV protection on any polycarbonate going outside.

IBC classification: CC1 burning rate per ASTM D635, better fire performance than acrylic, permitted in a wider range of applications under Section 2606.
Best for: Safety glazing, machine guards, hurricane and impact-rated applications, greenhouses, and applications requiring wide temperature service range.

Multiwall Polycarbonate

Multiwall polycarbonate is a different product category than the solid sheet versions above, and it solves a different problem. The hollow channel panel structure, twin-wall, triple-wall, or multi-wall configurations, delivers something neither solid acrylic or solid polycarbonate can: meaningful thermal insulation.

Twin wall panels achieve R-values around 1.5. Triple-wall and multi wall configurations reach R-2.5 to R-3.5 and above. Single pane glass sits right around R-1. For any cold climate application where energy performance matters, that gap is significant.

The tradeoff is optical clarity. Light transmission ranges from 40% to 80% depending on panel configuration and finish, opal and diffused finishes drop transmission further in exchange for uniform light distribution, which is often exactly what a greenhouse or skylight system needs. 

Weight is 60 to 80% lighter than glass at equivalent functional thickness, which reduces structural support requirements and simplifies installation. 

The installation requirement that usually catches contractors off guard is edge sealing. The open cell ends of multiwall panels must be sealed before installation with aluminum tape on the top edge, vented closure strips on the bottom edge. This isn't optional, unsealed ends allow moisture into the channels, which produces condensation staining, algae growth, and panel degradation that can't be reversed. It's the most common reason for failure in multiwall PC systems, and it's entirely preventable.

Best for: Greenhouse glazing, skylight systems, pool and spa enclosures, walkway covers, and cold climate applications where thermal insulation value is a design requirement.

Full Property Comparison

The table below is the reference architects and glazing contractors reach for when moving from material selection into framing design and code compliance work.

Every value here is verifiable against manufacturer datasheets or the referenced standards.

CLTE values: acrylic per Plexiglas/Acrylite manufacturer data; polycarbonate per HighLine Polycarbonate technical data. Always verify against the specific product datasheet, values vary by grade and manufacturer.

Code and Compliance: What IBC Section 2606 Actually Requires

This is where a lot of glazing specifications get into trouble. Architects and contractors who are familiar with glass often don't realize that plastic glazing operates under a separate code framework and that the CC1/CC2 fire classification isn't just a label, it has direct consequences for area limits and permitted applications.

CC1 vs. CC2 — What the Classification Actually Means

Under IBC Section 2606, light-transmitting plastics must meet one of two burning classifications per ASTM D635:

CC1 — the material burns 1 inch or less in the test. Polycarbonate (both solid and multiwall) typically meets CC1. This is the better fire rating, and CC1 materials are permitted in a broader range of building types, occupancy classifications, and locations under Section 2606.

CC2 — the material burns at 2.5 inches per minute or less. Cast acrylic typically meets CC2. It's still permitted for the majority of common glazing applications, skylights, canopies, patio covers, greenhouses, but carries tighter area limitations and more restrictions near property lines and in certain high-occupancy buildings.

The practical consequence that matters most for skylight work: CC1 materials are allowed to cover up to 33.3% of the floor area of the sheltered room; CC2 materials are limited to 25%. If you're specifying a large skylight array in a commercial building, that 8-point difference in permitted area can determine which material you can use or whether a sprinkler system addition changes the math.

OSHA Machine Guarding — 29 CFR 1910.217

For industrial machine guard glazing, OSHA 29 CFR 1910.217 governs guarding requirements. Solid polycarbonate is the standard material for machine guard glazing because its 250× impact resistance over glass means it absorbs impacts that would send glass fragments into the work area. Minimum thickness is application-dependent contact Laird Plastics' specification team for guidance on specific machine guarding scenarios.

Wind and Impact Ratings — ASTM E1886 / E1996

Hurricane rated and wind driven rain applications require glazing that has been tested to ASTM E1886 (performance of exterior windows, curtain walls, and doors under impact and pressure) and ASTM E1996 (specification for impact protective systems).

Polycarbonate can meet these requirements while Acrylic can't. If a project is in a hurricane exposure zone, or if the Florida Building Code or a similar jurisdiction's requirements apply, polycarbonate is the only option. This also applies to Miami-Dade NOA requirements so always verify the specific product has the required approval, not just the material type.

Installation Details That Actually Cause Problems

Thermal Expansion Clearance — Do the Math

Both acrylic and polycarbonate expand and contract more than the framing systems holding them. Get the clearance wrong and you'll be replacing panels. The formula is simple:

Movement (inches) = panel length (inches) × CLTE × temperature range (°F)

For a 48-inch cast acrylic panel across a 60°F temperature swing (a reasonable summer installation-to-peak assumption in much of the U.S.):

48 × 0.000041 × 60 = 0.118 inches of movement

For a 48-inch polycarbonate panel across the same swing:

48 × 0.0000375 × 60 = 0.108 inches of movement

Standard practice: provide a minimum of 1/8 inch clearance on each side within the frame, use fastener holes 1/8 inch larger than the fastener diameter, and never overtighten fasteners against the panel face. Dark-colored panels in direct sun absorb more heat and move more and ACRYLITE's installation guidance recommends applying a 1.4 safety factor to the calculated movement for dark sheets.

Frame Compatibility

Aluminum framing with EPDM or silicone gaskets is the standard system for exterior polycarbonate glazing. Try to avoid using PVC glazing channels with polycarbonate in any environment where solvent based cleaners or industrial chemicals could be present. Certain chemicals attack the interface between PVC and polycarbonate, which could cause crazing or joint failure that isn't visible until the damage is already done.

Multiwall PC Edge Sealing — Don't Skip This

Every multiwall polycarbonate installation needs sealed edges, and this step gets skipped more often than it should. The top edge (the highest installed point of the panel) gets sealed with aluminum tape. The bottom edge needs a vented closure strip, one that blocks insects and debris but still allowing condensation to drain out rather than accumulate. Sealing both ends with solid tape traps moisture inside the channels, which produces exactly the kind of visible algae staining and degradation that makes the specification look bad down the road.

Frequently Asked Questions

What is the best plastic for skylight glazing?

For skylights where clarity and long-term appearance are the priority, cast acrylic is the standard choice. It delivers 92% light transmission and inherent UV stability that holds for 10 to 20 years outdoors without coating maintenance. For skylights in hurricane exposure zones or where the IBC requires a tested impact rating, solid polycarbonate is the correct material.

For cold climate applications where insulation value matters like greenhouses, northern commercial buildings, multiwall polycarbonate is the right call. There isn't a universal answer; it depends on what the application is actually asking of the glazing.

Does acrylic yellow outdoors?

Cast acrylic doesn't yellow under normal outdoor conditions, UV stability is inherent to the PMMA chemistry. This is one of the genuine performance advantages cast acrylic has over polycarbonate for long term outdoor applications. The caveat is the word "cast": extruded acrylic has lower molecular weight and and also lower UV stability than cast acrylic. For any exterior glazing application with a multi year service life, always specify cast acrylic, not extruded.

Is polycarbonate better than acrylic for roofing panels?

Better is the wrong frame for this question. Polycarbonate is the right choice when impact resistance, safety glazing requirements, hurricane ratings, or a CC1 fire classification are required. Acrylic is the right choice when maximum optical clarity, inherent UV stability without coating maintenance, and a CC2 fire classification are acceptable.

For greenhouse and insulated roofing where thermal performance matters, multiwall polycarbonate is specified over both solid materials. The answer depends entirely on what the roof panel needs to do.

What is multiwall polycarbonate used for?

Primarily applications where thermal insulation value is a design requirement: greenhouses, skylight systems, pool and spa enclosures, walkway covers, and cold climate agricultural buildings. Its R-values around 1.5 for twin-wall up to 3.5+ for multi-wall configurations are simply not achievable with solid acrylic or solid polycarbonate sheet, both of which sit near R-1. It's also 60 to 80% lighter than glass at equivalent functional thickness, and available in clear and diffused finishes for applications where uniform light distribution matters more than direct view.

What is the fire rating of polycarbonate sheet?

Solid polycarbonate and multiwall polycarbonate both typically classify as CC1 under IBC Section 2606, tested per ASTM D635. CC1 means the material burns 1 inch or less in the horizontal burn test it's the better of the two plastic glazing fire classifications. Cast acrylic classifies as CC2. Under Section 2606, CC1 materials are permitted in a larger percentage of roof area and face fewer location restrictions than CC2. Always verify the specific product's classification with the manufacturer not all polycarbonate products achieve CC1, and locally adopted code editions may have amendments that affect permitted use.

How do you account for thermal expansion when installing plastic glazing?

With math. Movement (inches) = panel length (inches) × CLTE × temperature range (°F). For cast acrylic the CLTE is 0.000041 in/in/°F; for polycarbonate it's approximately 0.0000375 in/in/°F. A 48-inch acrylic panel across a 60°F temperature swing moves about 0.118 inches is enough to buckle a panel or crack it at a fastener if the frame doesn't allow it. Standard practice is minimum 1/8-inch clearance per side in the frame, fastener holes 1/8 inch oversized relative to fastener diameter, and no overtightening. This is where most DIY and contractor installation errors occur.

The expansion calculation takes two minutes; the panel replacement takes considerably longer.

What plastic can replace glass for building windows?

Cast acrylic and solid polycarbonate are both used as glass replacements in building windows, depending on what the application requires.

Acrylic offers better optical clarity and inherent UV stability; polycarbonate offers 250× the impact resistance and CC1 fire rating. Neither material is a code compliant drop in replacement for glass in all applications, IBC Section 2606 governs plastic glazing separately from glass glazing, with specific area limits, thickness requirements, and occupancy restrictions. Run the code review before finalizing the spec, not after.

Laird Plastics Architectural Glazing Inventory

Laird Plastics stocks cast acrylic, solid polycarbonate, and multiwall polycarbonate across multiple thicknesses and sheet sizes, with cut-to-size capability at locations nationwide. For specification support, sampling, or material certification documents, the Architectural Glazing industry page is the right starting point.

Browse by material:

• Acrylic sheet — cast and extruded
• Polycarbonate sheet — solid
• Multiwall polycarbonate panels

The Bottom Line

Cast acrylic is the clarity and longevity choice. Solid polycarbonate is the safety and impact choice. Multiwall polycarbonate is the insulation choice. Those three sentences are a starting point, not a complete specification but they're the right lens for the initial decision.

What actually gets glazing projects into trouble is treating these materials as equivalent options and letting price drive the selection, or assuming outdoor plastic glazing performs like indoor glazing, or skipping the IBC code review because glass glazing doesn't require it. Plastic glazing operates under a separate code framework, expands significantly more than its framing, and has UV performance requirements that vary by material and product grade.

Specify correctly at the design stage, verify with the AHJ, and the materials perform. Laird Plastics' team works with architects and glazing contractors through the specification process before the order is placed, not after the panels are in.

This article is for informational purposes only. Material selection, thickness specifications, code compliance, and installation requirements must follow project specifications, manufacturer datasheets, and applicable building codes and standards. Verify current standards, certifications, and material grades with the manufacturer and your design professional before specifying.