Yes it is, if you make it big enough. You could build it at ground level and as it heats up naturally due to the warmth generated in any city, it will float up into the atmosphere. It doesn’t need any lifting gas. Make it kilometer scale.

That was the idea for Buckminster Fuller's "cloud nine" was a design for an entire city, one kilometer in diameter, floating in the atmosphere. A sphere as large as that would be so low in mass, if constructed as a tensegrity sphere, relative to the mass of the atmosphere that just a one degree increase in temperature would be enough for it to float.

As a tensegrity sphere it would also be very robust in any weather conditions. There's never been any need to build such a structure, and maybe there never will be, nor did he expect there to be, but it seems generally agreed that the engineering for it is sound. He thought of them only as an "exercise to stimulate imaginative thinking".

So, going on from that, there was a serious proposal in 1980 by doctors Ernst Okress and Robert Brown of the Franklin Institute to build a large platform called STARS, half a mile to a mile in diameter, as an upper atmosphere research station. See Solar Thermal Aerostat Research Station (STARS) and the news story about it in the Washington Post:

"Solar Powered Balloon Station Proposed For the Edge of Space".

For an artist's impression of STARS, see Peter Elson's "Orion Shall Rise" painting, an illustration from Poul Anderson's novel of the same name, Orion Shall Rise, which features the STARS aerostat.

One of the covers of Poul Anderson's "Orion Shall Rise" featuring the STARS aerostat floating at 100,000 feet, which in his novel is an old pre-war relic from our time. Painting by Peter Elson.

This construction would have flown at a height of about 100,000 feet. And it would have been kept aloft only by the heat of the air inside it, like Buckminster Fuller's "cloud nine", not needing hydrogen or helium or any kind of lifting gas

In this paper he discusses temperatures between 27 C and 100 C for the interior, kept that warm basically by the greenhouse effect on a transparent balloon if I understand them right.

This is an extract from my article Can Giant Airships Accelerate To Orbit (JP Aerospace's Idea)?

The same idea has also been proposed in the Venus upper atmosphere. There ordinary air would be a lifting gas a bit like Helium in the denser carbon dioxide gas of the Venus upper atmosphere. Turns out it’s at just the right temperature and pressure for humans. The Russians had ideas to build “cloud colonies” based around these orbital airships.

Russian idea for a cloud colony in the upper atmosphere of Venus, proposed in 1970s
 original article (in Russian) - and forum discussion of the article - includes rough translation (I think anyway), probably by non native English speaker.

This illustration is from Aerostatical Manned Platforms in the Venus atmosphere - Technica Molodezhi TM - 9 1971

For more about this, see my Will We Build Colonies That Float Over Venus Like Buckminster Fuller's "Cloud Nine"?