If you live in an older property with single-glazed windows, you already know the problem. Cold draughts creep in from October onwards, condensation builds on the glass and your heating bill climbs every quarter. Full replacement double glazing solves it, but the cost can run into thousands, and if you’re in a listed building or rented property, it might not even be an option.

That’s where acrylic secondary glazing comes in. You’re fitting a clear acrylic panel over the inside of your existing window to create an insulating air gap. It’s the same principle as double glazing, just without ripping anything out. The panel sits against your window frame using magnetic tape or screws, and you can remove it in spring if you want your windows fully open again.

Why Acrylic Works Better Than Glass for This

You could use glass for secondary glazing, but acrylic makes far more practical sense for a DIY job. It weighs roughly half as much, so you can handle larger panels without help. It won’t shatter if you drop it during fitting. And clear acrylic actually transmits slightly more light than glass, around 92% compared to roughly 90%, so your rooms won’t feel any darker.

Acrylic also insulates better than glass of the same thickness. Its thermal conductivity is lower, which means less heat passes through the panel itself. Combined with the air gap between the acrylic and your existing window, you get a noticeable improvement in warmth and a reduction in condensation.

Which Thickness Do You Need?

For most secondary glazing projects, 4mm clear acrylic is the sweet spot. It’s rigid enough to sit flat against the frame without bowing, provides good insulation and handles well during fitting. For smaller windows under about 600mm wide, 3mm works fine and costs a bit less. For anything particularly large, 5mm gives you extra rigidity so the panel doesn’t flex.

We’d steer away from 2mm for anything other than very small panes. It’s cheaper, but larger sheets tend to bow in the middle, which looks poor and reduces the effectiveness of the air gap.

How to Measure Your Windows

Measure the window opening at the frame, not the glass. You need the panel to overlap the frame by at least 15-20mm on each side so the magnetic tape or fixings have a solid surface to attach to. Take measurements in millimetres for accuracy and check both diagonals. Older windows are rarely perfectly square, so measure top and bottom widths plus left and right heights separately. Use the smaller measurement in each case and deduct 3-4mm to allow for thermal expansion.

Fitting Methods: Magnetic vs Screw Fix

Magnetic tape is the most popular option and the one we’d recommend for most homes. It’s a two-part system: a self-adhesive steel strip goes on your window frame, a magnetic strip goes on the edge of the acrylic panel. You press the panel into place and the magnets hold it firm. The beauty of this method is that you can pull the panel off in seconds for cleaning or summer storage, and the steel tape is barely visible on a white frame.

Screw fixing suits larger or heavier panels where magnets alone might struggle. You’ll need pre-drilled holes in the acrylic, which we can do as part of your order, plus small screws into the window frame. It’s more permanent than magnetic fixing but still fully removable. Use a foam compression tape around the perimeter to seal the gap and prevent draughts.

A Few Things to Keep in Mind

Magnetic fixing works on vertical windows only. Don’t use it for rooflights or angled windows as gravity will eventually pull the panel away. Make sure your frame has at least 50mm of flat surface all the way round for the tape to adhere properly. And always fit the panel to the main frame rather than the opening casement, so you can still open your window if needed.

Clean both the frame and the acrylic edges with isopropyl alcohol before applying the tape. Any dust or grease will weaken the adhesive bond. Once fitted, clean the acrylic with a soft cloth and mild soapy water. Avoid household glass cleaners as many contain ammonia or alcohol that can cloud the surface over time.

If you’re ready to get started, our clear acrylic sheets are available cut to your exact measurements in thicknesses from 2mm to 30mm. Order online today.