Breakable Glass

1. Create a brush where you want the glass to be, using the nodraw texture. Like real glass, it should be very thin.
2. Apply a nice-looking glass texture to one face. (Not on both sides!) Make sure you use glass/glasswindowbreak070a Don’t use glass/glasswindowbreak070b. All other faces should be textured with tools/toolsnodraw.
3. Select the brush and click the "toEntity" button in the object toolbar. Choose func_breakable_surf in the drop-down list. (If you want the glass to completely shatter all at once, choose func_breakable instead.)
4. Go to the Material Type keyvalue and select Glass.
5. Add an env_cubemap to the surface of the glass texture. (Put it right on there.)