@matthiasott I would say this is language specific. In Czech (whose typography rules are heavily influenced by German) we use both. Using space means a hunderd percent as a noun. Without a space it's an adjective. Difficult to explain in English where the same word without any change may act as a noun, adjective or even verb.

In English i've learnt to use it without a space in any case. Bit odd to me, but English has more oddities.