Vapor barrier.

@Ron Flatt it’s moisture vapor, it’s constantly passing through an uncovered slab.

We then got a second opinion which lead to a recommendation of putting a vapor barrier under the home due to the potential of moisture getting under the home and the heat causing the water to raise into the home, creating the moisture issue.

A lot of tile guys use Dragonskin on the hardiboard in addition to a vapor barrier....

From what I understand I would still have to add a vapor barrier and insulation to the sunroom, but I haven't yet talked to any contractors about all the work that would be required.

. $2,000) - Fence post fix ($350) - New Furnace ($5,500) - Bird infestation removal resealing attic ($850)- New Vapor Barrier in crawl space ($975)Property #2: - New Water Heater ($2,000) - Repaired main water line ($950)- Hot tub removal ($600) - Tree removal ($1,235) - Bathroom fan replacement ($500) - Towel Bar replacement ($150) - New microwave ($450) - Pergola removal ($400) - (Due to damage from roofing materials crushing part of it during a strong wind) - New Roof ($13,695)I have probably missed a few items as well..... 

Whether permitted or not, the slab likely has little to no footing and it's not likely it includes a vapor barrier, plus the span on the 24" on center ceiling joists is likely too far, and when you sheath and shingle it will get worse...assuming there is even a world where it would be viable to insulate sufficiently and pass Title 24 etc. 

Vapor Barrier, sump pump, drainage all done after the rehab...cost a bunch of time and money.

Quote from @Kenneth Garrett: @Kristen SchaeferIf you are only replacing tile no permit is required.Some inspectors will want to see the new vapor barrier.....I agree if you can save the tub, do it.

Radiant heat works through carpet in a bedroom.Your selection with a slab floor should take into consideration flooring techniques for your area, applications are different for Idaho than Florida, with vapor barries and find out if the slab was insultated.