Transparency Problems In Doom Legacy

I'm currently trying to add windows to a map using Doom Builder, and every time I test the map the windows are solid. I'm using the create floor (transparent) linedef tag and it's just a solid wall.

Linedef transparency works in the software renderer by default, but if you're using OpenGL, you have to enable it manually because it's a bit experimental.

Open the console and type:

gr_transwall 1