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Both are used to mean "sealed airtight".

気密 is a Noun, while 密閉 can also be used as a suru verb "to seal airtight"

Apart from these, is there any meaningful difference?
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word-choice words nuances
What is the difference between 密閉 and 気密?
Just from the Kanji, my guess would be that 気密 is about air 気 not being able
to go out or in, whereas 密閉 is more about being sealed/closed off 閉
completely.

The overall meaning is the same but the nuance might be a bit different, maybe
a bit like "airtight" and "sealed off".
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[dictionary.goo.ne.jp entry for
密閉](https://dictionary.goo.ne.jp/srch/all/%E5%AF%86%E9%96%89/m0u/):

> みっ‐ぺい【密閉】
>
> ［名］(スル)すきまのないように、ぴったりと閉じること。「容器を―する」

Translation (mine): **To be closed, such that there are no gaps. "(seal) a
container"**

[dictionary.goo.ne.jp entry for
気密](https://dictionary.goo.ne.jp/srch/all/%E6%B0%97%E5%AF%86/m0u/):

> き‐みつ【気密】
>
> 密閉して気体の流通を妨げ、気圧の変化の影響を受けないようにすること。「―構造」「―性が高い」

Translation (mine): **To be sealed in order to prevent the circulation of
gases or be unaffected by changes in atmospheric pressure.**

Note that the definition of 気密 actually uses the term 密閉. So 密閉 really just
means "seal" (close something so that there are no gaps or cracks, etc).
Whereas 気密 is a sub-category of 密閉 where something is explicitly sealed (密閉した)
in such a way that it is also totally airtight.

密閉 could be used in situations where in English we might use "airtight" in a
figurative sense (i.e. "The security was airtight", etc), but it does not
_literally_ mean "airtight", it only really means "sealed tightly". On the
other hand, 気密 actually literally means that no air (or other gasses) can get
in or out.
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As a technical term, the tightness is different (from
[here](https://www.yueki.com/faq/faq68/index.html)). From most tight to less
tight: 密封容器＞気密容器＞密閉容器.

In ordinary usage, as Foogod says, 気密 is generally the word for 'air-tight'
and 密閉 means just sealing (contrary to the above technical definition, 密封
feels sealing less tightly).

Practically 気密 is only used when speaking of air-tightness as in 気密性. It
cannot be used as a verb: ×気密する.

密閉 is used much more broadly, generally meaning sealing tightly enough. Note
that there are words like 密閉空間, which can be just a room where all the doors
are closed - so not necessarily air-tight.
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