This question is not about the meaning of and the difference between the words insulation and isolation, it has been already answered here: What's the difference between "insulated" and "isolated"? and here Difference between "isolated" and "insulated".

In other languages these two concepts are usually expressed by one single word.

They have practically the same etymology:

  • http://www.etymonline.com/index.php?search=isolate

    "standing detached from others of its kind," 1740, a rendering into English of French isolé "isolated" (17c.), from Italian isolato, from Latin insulatus "made into an island", from insula "island" (see island (n.)). English at first used the French word (isole, also isole'd, c. 1750), then after isolate (v.) became an English word, isolated became its past participle.

  • http://www.etymonline.com/index.php?search=insulate

    1530s, "make into an island," from Late Latin insulatus "made like an island", from insula "island" (see isle). Sense of "place in an isolated situation, cause (someone or something) to be detached from surroundings" is from 1785. Electrical/chemical sense of "block from electricity or heat" (by interposition of a non-conductor) is from 1742. Related: Insulated; insulating.

Do I understand properly that the word was imported into English twice, once directly from Latin (1530), and then again via French (1740)? Then it is funny that the notion which sounds more modern is carried by the older word. Or was it one word which somehow "split" into two?

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    In English we need two words, since they mean different things. – Hot Licks Mar 7 '16 at 14:05
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    @HotLicks your answer is kind of "cheap". I always find fascinating thinking about why one language needs two words while another doesn't. E.g. some languages do not distinguish between can and know, borrow and lend, boring and bored, fetch and bring - and English has only one word to be where Spanish has three words. I think that you "solved" it too fast... – Honza Zidek Mar 7 '16 at 14:25
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    I appreciate the effort that went into framing the question, but your very title starts from a false premise ("nearly the same concept") and I am quite curious which other languages fail to distinguish between "protection" and "separation". (Though it is certainly true that the "protection" aspect comes from being separated.) – Hellion Mar 7 '16 at 14:26
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    The Q is based on a misconception. – Kris Mar 7 '16 at 14:28
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    FWIW, it's not rare to find cognate doublets in several languages with intimately related meanings contrary to the economy principle. There are several of them in English in the linked page, and a few I can think of in Spanish are "oreja" and "aurícula", which are related only metaphorically, and "indudable" and "indubitable", which mean virtually the same thing. – Yay Mar 7 '16 at 15:11

The -atus ending insasmuch as it is formed from the past participle of the verb (and thereby conveys the idea of action-done-to) is suited to the meaning "protected by means of a barrier"; the earlier borrowing from Latin insulanus (Middle English insulan, island-dwelling) shows an adjectival ending appended to nouns to convey the idea of origin or possession.

The insul-- form was already here since the mid 15th century.

The Middle English concept of "island-dweller" already went beyond the geographic definition to include the more abstract idea of not being subject to certain forces or influences:

Eke, for he is a insulane, therefor he doth no subjeccion onto no man.

The idea of "being shielded from" is lurking there.

The Latin-derived -ate ending was already productive in Middle English technical/scientific writing in the first quarter of the 15th century.

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