Acrylic, polycarbonate or glass for secondary glazing?
Quick answer: For magnetic DIY secondary glazing, clear cast acrylic is the best all-rounder: about half the weight of glass, slightly clearer, far harder to break and it doesn't yellow. Polycarbonate is tougher still but scratches and yellows more easily and costs more. Glass is too heavy for magnets and, in some spots, must be Grade A safety glass under AS 1288.
| Cast acrylic (PMMA) | Polycarbonate | Float glass | |
|---|---|---|---|
| Weight, 3mm | about 3.6 kg/m² | about 3.6 kg/m² | about 7.5 kg/m² |
| Light through, clear | about 92% | about 88% | about 90% |
| Breaking | Much tougher than glass; cracks rather than shatters | Near unbreakable | Breaks into sharp pieces unless toughened or laminated |
| Scratching | Fair; wipe with a soft cloth | Scratches easily | Best |
| Yellowing in sun | Stays clear | Yellows over time without a UV coat | None |
| Heat flow (k) | about 0.19 W/m·K | about 0.20 W/m·K | about 1.0 W/m·K |
| Works on magnets? | Yes | Yes | No, too heavy; needs a frame |
Typical published values for each material. Exact figures vary by brand and grade.
Why the material barely changes warmth
All three are thin. The still air between the panel and your window does most of the insulating, so a sealed edge and a 15mm-plus gap matter more than the panel material. Acrylic and polycarbonate conduct heat about five times less than glass, which helps a little at the edges. See the thickness comparison.
Glass inner panes and AS 1288
AS 1288 is the Australian Standard for glass in buildings. It requires Grade A safety glass (toughened or laminated) in places people can walk into or fall against, such as low windows near the floor, doors, side panels and bathrooms. If you add a glass inner pane, it has to meet those rules and it needs a proper frame. Acrylic isn't glass, weighs half as much and doesn't shatter into shards, which is why it's used for removable panels. If you do want glass, our sister business Affordable Glass Online cuts toughened and laminated glass to size.
Questions
Is acrylic or polycarbonate better for secondary glazing?
Acrylic for most homes. It is clearer, resists yellowing, scratches less and costs less. Choose polycarbonate only where impact is the main worry, such as a window kids or balls might hit.
Can I use glass for magnetic secondary glazing?
Not really. Glass weighs about twice as much as acrylic, so magnetic tape won't hold it safely, and in many spots AS 1288 requires Grade A safety glass in a proper frame.
Does acrylic go yellow?
Cast acrylic is UV stable and stays clear for many years. Polycarbonate without a UV coating can yellow in direct sun.
How do I clean acrylic panels?
Use warm soapy water and a soft microfibre cloth. Avoid paper towel, window spray with ammonia and anything abrasive.
Is acrylic a fire risk?
Acrylic is a plastic and will burn, like curtains or blinds. Keep panels away from heaters and flames.