Increase your window’s STC and minimize window noise

Working from home? You might be familiar with the blast of a leaf blower from your neighbor’s driveway, or a steady flow of delivery trucks rattling up your street. On alternate days, you might hear tree trimmers, power washers, moving vans, or road construction.

Your windows are closed, but they sound like they’re open. And you wonder how to reduce that window noise without wasting money on something that doesn’t work. Or, if you’re a renter, doing something that’s forbidden by your landlord.

The answer might be D.I.Y. plexiglass window inserts. No, not the ones that you custom order from a well-known company on the internet. Those might cost almost as much as a new window. Instead, you can buy some plexiglass and cut it to fit snugly inside the window frame or around the exterior of the window frame. This layer of plexiglass — with an air space in-between — creates an extra barrier against noise.

 

The recipe:

    • 1/4-inch thick plexiglass (6.35 mm), cut to the dimensions you need
    • Foam weatherstripping or felt strip lengthwise along the windowsill. The bottom of the plexiglass will sit on it to reduce vibration from the windowsill. The strip should be about 1/8″ deep. (This is revised and updated from what I showed in the video.)
    • Brackets for wall mounting (if you choose an outside mount). You might also need shims.
    • Adhesive-backed foam strips (about 1/2 to 1-inch wide, and 1/8-inch deep) if you create a mount against the drywall. These strips will go on the back of your plexiglass (around the sides and the top) and the foam will press against the wall. I used joist tape, but it’s bright green.
    • Joiners if you have multiple panels that need to fit together.

For mounting inside your window frame:

Measure the inside of your window in all four corners, and reduce the measurements by about 1/4 inch on each side. No window is perfectly squared, and it’s better to have the plexiglass slightly shorter than to be too large for the opening on all sides. You’ll pad the plexiglass edges with foam or silicone to make it fit in place securely.

For mounting outside your window frame:

Measure the dimensions of your window frame and add an additional 1 inch to 1.5 inches on the top and on each side of the window frame. The bottom edge of your Plexiglass sheet will sit on the windowsill, and the sides and the top will extend past the window opening. They’ll be held firmly in place by the brackets.

As an example, if your window opening is 36 inches wide and 48 inches tall, and you want to mount the plexiglass on the outside of the window frame, then cut the plexiglass so that it’s at least 37.5″ tall and 51 inches wide. This would add an extra 1.5″ at the top, and an extra 1.5″ on either side of the window opening. You could even make it larger, but not smaller.

Joining together two pieces of plexiglass (for large windows assembled in multiple pieces)

If you have a large window and you need to assemble your plexiglass insert in multiple panels, you should hold the pieces together with a “joiner,” like this one sold at TAP Plastics. The joiner should have a track on both sides, which will hold two pieces together like a seam. Any gap in your finished window will let some noise in.

roll of green acoustical tape that was used on back of plexiglass
I used this green-colored joist tape around the edges of the plexiglass where it touches the drywall. Obviously you might want a more neutral color.

The cost

To get a cost estimate, visit the TAP Plastics site and enter the dimensions into their Extruded Acrylic Plastics estimator. For example, a 38-inch by 48-inch piece cost $134.00.

 

The results

Though not completely soundproof, your new plexiglass window inserts should help to reduce incoming window noise up to 10 decibels. They’re most effective at airborne noise, light traffic noise, and directional noise. Louder, more-vibrational, low-frequency noise such as truck engines and road construction might get slightly muted by window inserts, but not enough to truly quiet your home. Decibel reduction will be less significant in those cases. That’s because this type of noise also comes through your walls, building frame, doors, and other rooms of your home.

Ensure that you can remove the plexiglass easily if needed

This improvised gate latch helps to keep the plexiglass window in place, but also makes it easy to remove, if needed.
This gate latch secures the plexiglass over the window opening, and also makes it easy to remove the plexiglass to open the window behind it. That greenish color on the left side is joist tape on the back side of the plexiglass.

I changed out some of the brackets on the large window after I installed it. On the far-right-side panel, I removed the stationary brackets and replaced them with gate latches as handles. I reversed the direction of the latch so that it would sit above the plexiglass, and then used a foam shim between the handle and the plexiglass to ensure that the plexiglass stays pressed against the wall with no air gap.

 

See it in action

This Soundproofist video details the process of ordering and installing the window inserts, with before-and-after examples.

 

In part two, you’ll see how to join multiple panels of plexiglass together to make a large window insert:

A noise & decibel test

When a loud truck was parked outside this window, the noise lasted long enough to perform a test.  Soundproofist inserted a mobile phone in the empty space between the original window and the plexiglass insert, and the other smartphone was placed outside the insert. Both mobile phones ran the same Decibel Meter app at the same time to measure the decibel level inside the air space and the decibels that escaped into the home, which passed through the extra layer of plexiglass.

Plexiglass/acrylic resources

Learn more about window noise and inserts:

How to reduce window noise with plexiglass inserts
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