[] Polygons inside Huge Fireplace chimney extend too far down

Inside the Huge Fireplace, the polygons for the inside of the chimney extend further down than they should, and they are single-sided.

Steps to Reproduce

Place a Huge Fireplace and noclip inside, then look up at the chimney.

What I expected to happen

The chimney should begin at the hole.

What happened

The chimney begins slightly below the hole, with exposed single-sided polygons.

Notes / Media

Also, while I’m at it: The glass material on this model is one-sided as well, though I’m unsure if this is intentional.