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!)

3. Select the brush and click the "To Entity" button in the lower right. Choose func_breakable in the drop-down list. (If you want the glass to completely shatter, choose func_breakable_surf instead.)

4. Go to the Material Type key value and select Glass.

5. Add an env_cubemap to the surface of the glass texture. (Put it right on there.)