Not sure if this is the right forum. But, I thought you guys would know the answer. I ran across this today in a crawlspace. It’s a foil with a paper backing. It’s stapled to the floor joists.
Is it considered insulation? If not, what’s it for?
Thanks in advance.
Vapor barrier.
…and it appears to be improperly installed.
That depends…What part of the country do you live in? I believe it is ok to install a vapor barrier like that in areas with hot summers and mild winters. If you live in a cold winter climate, then it is incorrect.
Yes, my world revolves around winter, hence the word “appears”. 
In addition to being a vapor barrier it is also a radiant heat barrier which can be thought of as insulation in that it will reduce heat loss/gain.
Hot summers and mild winters here. It’s very rare to see a vapor barrier installed like this here. This is a one in two hundred install!