So this is just a thought I have in mind and I want to clarify.
The 他動詞 used in the テ形 with いる expresses a continuos state of action/being
e.g 窓が開けている。 Window is open,
here we have to use the transitive verb.
Another thing we can do is say 窓が開けてある, which has a slightly different meaning.

My question is, how would the meaning be if I used intransitive verbs anyway (they always take the が particle after all, atleast usually).
so using 開く instead of 開ける, we get:
窓が開いている
窓が開いてある

But does this have any usage or real meaning in Japanese language at all? I just have this habit of digging into things since I am an Engineer. Sorry for inconvinience caused
ありがとうございます。

窓が開けている is not grammatically correct; since it's a transitive verb you have to mark the object with を. This sentence is somewhat uncommon, it would typically mean that someone is opening a number of windows.

窓が開いている means "The window is open".

窓が開いてある is not grammatically correct; the てある form can only be used with transitive verbs.

Thanks for pointing out my mistake yudantaiteki, I have spent some time reading on it again, the books that I have are not very clear about this but I realized that there is infact an ambiguity over the interpretation of: intransitive verb~te form + iru. And this is what I have been wondering about as well.

The books I have read say that we can used ~te iru to expressed the gerund or progressive form of a verb. And give a few examples, thats all. They don't go into intransitive verb~te form + iru or transitive verb~te form + iru explicitly.

Anyway, thanks again for pointing out the mistake dude.