Need advice big gaps wood floor and dirt floor basement

[Tammy Richards]

Have a 4 unit multi built @1900.  Wood refinished floors have huge gaps/cracks, and they are layed over what appears to be what was called a subfloor in the Victorian era, which is wood which also has gaps.  From the basement, I can see straight through these gaps to the apartment.  The basement has a dirt floor.  I want to lay floors (hopefully myself) on top of the wood floors, to improve air quality and cleanliness - vinyl plank will be fine for my section 8 tenants.  Would like to lay vinyl plank or floating engineered myself  I'm fine with filling gaps from below the basement in with Great Stuff or something, but hoping to avoid laying in a plywood subfloor (I don't have the tools, equipment, or time for that). Wondering if sealing gaps with great stuff and putting an underlayment which is also a moisture barrier would do?   Flooring is pretty level, and in good condition.   I'd appreciate recommendations/advice from folks. 

[Andy Hudgins]

@Tammy Richards

The old, "charming" floors are not the problem --- the dirt basement floor is.  Heat rises in a home so it "draws" or "sucks" directly at the floor level.  It draws air out of the basement through the leaky floor and carries moisture, mold particles, and other airborne STUFF with it.  Please search "stack effect".  There's lots of very small air passages in the floor you cannot see, same as with all floor systems over a crawl.  The floor isn't broken, the building envelope is. There is no "boundary" on the lowest portion of your investment.  There's no barrier between the earth, elements, and your investment. 

Look into

http://www.crawlspacedepot.com/

I've got older posts on this topic.  If I can help regarding specifics of this type of installation let me know.  FYI the large, nationwide installer of this type of product are a money printing machine.

Andy