L4D2 Tutorial covers:
Creating breakable glass
Attaching breakable glass to a rotating door
*compatible with L4D1
1. Creating Glass
Use simple brushes and position where your glass is going to be.
Assign nodraw texture on all of the glass surface faces. The entire brush needs to be in nodraw texture.
Depth of the glass is best at 2 units.
2. Assign Glass Material
Texture browser choose the glass you want for your window. I chose glass/urban_glass_03
Assign the glass texture only to one face.
Now that you have the glass positioned where you like it and have the texture applied to the single face with the rest of the faces as nodraw texture.
Select the glass brush and press Cntr+T.
Tie to Entity = func_breakable_surf
Do this for each of the brushes that are glass.
4. Attaching the glass to a moving object (door)
Open up the door's properties and under name, name your door.
Name can be anything you want.
Now open up the func_breakable_surf properties and where it says Parent, type in the door's name.
Final step inside the editor is to insert an entity, env_cubemap in front of the door/window.
Compile your map and when the game starts go into console and type buildcubemaps
Now you are done. The door rotates and the glass stays attached with it.