In my first libgdx 3D game i now switched from
normal.x = 0;
normal.y = 1;
normal.z = 0;
Perhaps not directly related, but still important to note: don't use createRect or createBox for anything other than debugging/testing. Instead combine multiple shapes into a single model/node/part. Or even better, use a modeling application where possible.
You didn't specify how you disabled backface culling. But keep in mind that you should not change the opengl state outside the shader/rendercontext (doing so will result in unpredicted behavior). To disable backface culling you can either specify it using the material attribute
IntAttribute.CullFace (see: https://github.com/libgdx/libgdx/wiki/Material-and-environment#wiki-intattribute), the DefaultShader (or default ModelBatch) Config
defaultCullFace member (see http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/graphics/g3d/shaders/DefaultShader.Config.html#defaultCullFace) or the (deprecated) static
DefaultShader#defaultCullFace member (see http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/graphics/g3d/shaders/DefaultShader.html#defaultCullFace).
Whether a face is front or back is based on the vertex winding. Or in other words: the order in which you provide the corners of the rectangle is used to decide which side is front and which side is back. If you use one of the rect methods, you'll notice the arguments have either the
11 suffix. Here, when looking at the face, 00 is lower-left, 01 upper-left, 11 is upper-right and 10 is lower-right.
For a rectangle, it's normal is the perpendicular facing outwards the rectangle. For example if you have a rectangle on XZ plane with it front face on the top, then its normal is X=0,Y=1,Z=0. If its front face it facing the bottom, then its normal is X=0,Y=-1,Z=0. Likewise if you have a rectangle on XY plane, its normal is either X=0,Y=0,Z=1 or X=0,Y=0,Z=-1. Note that the normal is not used for face culling, it's most commonly used for lighting etc. Specifying an incorrect/opposite normal will not cause the face to be culled (it might cause incorrect/black lighting though).