First off windows are in fact movers. Here’s how you make them quickly and easily.

Build a brush with the size of your windowsill and a thickness of 1. Place the brush in midair. add space. texture it with a texture found preferably in "coretekglass". Right click on the 2 viewable sides one at a time and set the surface properties so that it is translucent. This is a checkable box in the main screen. Now build a brush larger than the floating window and completely surround it without the brush touching any other solid (object floor wall etc). Intersect the brush.

Move the brush into where you want the window to be. browse to classes, go to brushes, go to movers, go to deus ex movers and highlight breakable glass by clicking on it. Now click the Add mover button. Its the cube with the squiggles. Drag the red brush out of the way. Right click the purple mover frame. Go to keyframe1. Now DRAG THE MOVER FRAME COMPLETELY OFF OF THE MAP for keyframe1. this is the key. now when damaged enough the window will trigger effect broken glass and move to new location which as far as the player concerned is nowhere. Voila, broken window.

For a breakable wall you do the exact some thing, note u can be more creative with your shapes but still intersect them and move them into position and drag them completely off screen for the "destroyed" position. The only difference between the set principles of the wall and the glass is the damage type. The destructible wall cannot be destroyed by any damage other than explosive. You can use this knowledge to make high-strength glass or weak walls. Also for walls to get good shapes for the destructible part i strongly recommend making a "star" shaped brush using the 2D shape editor. I will explain its function in a later tutorial so for now just use squares. Always texture the destructible wall the same as or similar to the regular wall. otherwise it looks way too obvious. Also instead of doing translucent for windows feel free to make one side "mirrored" its a checkable box this will in effect make a 1 way mirror. Not sure if anyone is interested in an interrogation room. this technique also works with non movable(destructible) brushes.