Every company that makes or installs PVC windows or doors in your town will have 1/8 inch thick PVC sheet, typically in a 20 inch width. They use it to make PVC "returns" on the window they install. Often, the walls of the buildings they install their windows into are thicker than the windows themselves, and so they need something to make up that difference between the window frame thickness and the wall thickness.

If your intention is to reduce the noise from your air conditioner, then insulation isn't going to help much. When a sound wave hits a wall (for example) the sound wave DOES NOT travel through the wall. Instead, the wall moves slightly as a result of the sound wave and that motion recreates a second sound wave on the other side of the wall. It's that recreated wave you hear, not the original.

It's this simple as dirt method of sound propogation through walls that's the basis of the MASS LAW of Acoustics. The mass law says that for every doubling of the weight of the wall (per square foot) OR for every doubling of the frequency of the sound wave hitting the wall, the amplitude of the recreated sound wave on the other side of the wall is reduced by 6 decibels, or to 1/4 of it's former amplitude.

That's because the more massive the wall, the less it moves in response to the sound wave hitting it, and that means the amplitude of the recreated sound wave is also reduced. Also, the more massive the wall, the less able it is to can change it's direction of motion quickly. So, the higher the frequency of the sound wave hitting the wall, the less capable the wall is of reproducing that sound wave. In the ultimate instance, the inertia of the wall would prevent the wall from responding to a high frequency sound wave at all, and the result would be silence on the opposite side of the wall.

So, to stop the noise from your air conditioner, a smarter gameplan would be to install heavy shutters on that window that could be closed. You want to avoid having air paths through the shutters that the sound could travel through, so your shutters would either be solid and block the light, or you could have the shutters made out of 1/2 inch thick glass, perhaps.

Also, have curtains made for that window from the thickest and heaviest drapery you can find. Almost certainly MOST of the sound you hear is coming through the closed glass window, and so that's where you should concentrate your efforts at reducing the noise. Put as much mass between you and the air conditioner as you can, and that will stop more noise than your insulation idea.

ARCHIVED - Sound Transmission Through Building Components - IRC - NRC-CNRC

http://archive.nrc-cnrc.gc.ca/obj/ir...s/bpn/56_e.pdf