Learn Farsi: این اتاق جادار است، اما عایق ندارد. - This room is spacious, but it has no insulation.

It is made by combining جا (place or space) with دار (the present stem of داشتن, to have). Attaching دار to a noun is a highly productive way in Farsi to form adjectives meaning "possessing" or "having" that noun. So, a room that is جادار literally "has space" (is spacious).

The sentence says عایق ندارد (literally "it doesn't have insulation"). Could I use the verb بودن (to be) instead?

Yes, you could say عایق نیست (it is not insulated). The word عایق can act as both a noun (insulation) and an adjective (insulated or impermeable). Both phrasings are natural and convey the exact same meaning.

Is there any difference between using اما and ولی for "but" in this sentence?

Functionally, no. اما and ولی both mean "but" and can be used interchangeably here. اما can sound a tiny bit more formal or emphatic in writing, but both are standard and universally understood for linking contrasting clauses.

How could I rewrite this sentence to use the sequential markers اولاً and ثانیاً from this lesson?

You can use them to list the room's attributes step-by-step, contrasting the pro and the con. You would write: اولاً این اتاق جادار است، ثانیاً عایق ندارد (Firstly, this room is spacious; secondly, it has no insulation). Notice that when you use ثانیاً to introduce the second point, you can drop the .