I think this would make a lot of people uncomfortable. I'm relatively pragmatic and untrusting, but I would be offended if someone I knew did this to me. And if it were a neighbour I didn't really know, I would be a bit worried: now I have the responsibility of keeping their key secure, and if I lose it they will suspect me of stealing it, and all for the privilege of being asked to do a favour for a borderline stranger who doesn't even consider me trustworthy.
Its for both your peace of mind. You come home from vacation, something you misplaced makes you concerned its gone, but the key from next door is still in the envelope. So no suspicion there.
Maybe works better for business. I did this with the insurance agent next door when I was running an Engineering office.
I agree with GP (retsibsi). The envelope thing says, "I trust you enough to give you a key... but I don't really trust you, so I'm also giving it to you in this weird signed envelope thing." You either trust them or you don't, and you're asking them to do you a favor.
Plus, if they have a sealed envelope with the key, they can get enough information about the key (the key bitting) to make a copy. They don't need to open the envelope, they just need to press the envelope down enough to see the shape of the key.
I should add to my other replies: I don't mean to criticise you personally, or your actions in your own particular case. In the general case I think the envelope proposal would often cause offence, but as a compromise with someone initially reluctant, it sounds like it was a clever solution.
It does make sense, but I still don't think I could avoid that emotional reaction. It's the combination of asking for a favour while simultaneously signalling that you don't trust me. If we're in zero-trust stranger-to-stranger mode, why should I do this favour at all? It only has downsides for me, including the possibility that something goes wrong (envelope lost or damaged) and I come under suspicion.
