insubstantial vs flimsy

insubstantial is not a noun while flimsy is a noun.

Word	Noun	Adjective	Verb	Adverb
insubstantial	No	Yes	No	No
flimsy	Yes	Yes	No	No

As adjectives, insubstantial and flimsy are synonyms defined as:

insubstantial (noun)	flimsy (noun)
	a thin strong lightweight translucent paper used especially for making carbon copies

insubstantial (adjective)	flimsy (adjective)
lacking material form or substance; unreal	not convincing
lacking in nutritive value	lacking solidity or strength
lacking solidity or strength	lacking substance or significance