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@Sphere
Hmm, do you even now why it doesnt works? ^^
In the example, you see 6 Worldbrushes outside. These Worldbrushes "Closes" the map from the void.
Otherwise the map will have a leak. And every map must be closed. (Otherwise vrad (a file from the compile process) will say "Leaked entity in ... ").
In my example the room is VERY tiny. You could just expand the 6 outside worldbrushes to make the map larger, and then your models should fit in. OR you just make 6 Worldbrushes in your map, who will "Cover" the map from the void.
Here an site from the vdc: https://developer.valvesoftware.com/wiki/Leak
Maybe you now know more about that 
I'm looking at the VDC, but I can't select only one side of a brush. D:
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
VDC is an Internet Site, or what the heck are you trying to do? 
Sorry for being confusing.
What I meant is that:
1. I read the tutorials at VDC
2. I seem to be unable to select only a single side of a brush in hammer.
(This message took too many edits to fix)
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
Nevermind. *I realized Noobishness got to me and that you can't select individual sides*
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
So now I'm making something that is truly good enough for science. May not be aperture-quality science yet, but hey, it's a test chamber!
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
Please use the EDIT button.
Did you now fixed your problem?
He started mapping in hammer, "hm well, first i will make an exterior map"
Hey @Sphere, if i where you i would first learn the basics of hammer, the basics of the tools, how maps and brushes are working, and how to change textures.
That all you will learn in some tutorials, and some sites on the vdc, for beginners.
Or watch some videos from tophattwaffle, because where i started mapping, i looked how i make a room with a button and a door. And not starting bts maps
Hey @Sphere, if i where you i would first learn the basics of hammer, the basics of the tools, how maps and brushes are working, and how to change textures.
That all you will learn in some tutorials, and some sites on the vdc, for beginners.
Or watch some videos from tophattwaffle, because where i started mapping, i looked how i make a room with a button and a door. And not starting bts maps
In addition to tophattwaffle, I also recommend solarchronus's channel
Aperture Science: We do our science asbestos we can!