Vapor barrier placement for slab over EPS geofoam when filling an indoor concrete pool – GreenBuildingAdvisor

I’m converting an unused indoor concrete pool into usable floor space and would appreciate guidance on vapor barrier placement before installing geofoam.

A picture of the pool area is attached. The hot tub in the far left corner will be demolished and filled to floor grade.

The pool is approximately 35′ × 15′, with an average depth around 4–5 ft (7 ft deep end / 3 ft shallow end). The entire pool room is 48′ × 24′.

The plan is to fill the pool with EPS geofoam blocks (likely EPS19) and then pour a 4″ reinforced concrete slab over the entire room, including both the existing flagstone floor and the former pool area, so the finished slab is continuous.

The pool has been empty for about 10 years, and the concrete shell appears to be in good condition.

One installer recommended the following assembly:

1. Line the pool shell (walls and bottom) with vapor barrier

2. Install the geofoam blocks

3. Install another vapor barrier over the geofoam

4. Pour the reinforced slab

My understanding is that the critical vapor barrier is the one directly under the slab, primarily to control vapor transmission into the slab and any future flooring.

Typical geofoam details I’ve seen look more like:

Concrete pool shell

→ EPS geofoam blocks

→ 10–15 mil vapor barrier

→ reinforced concrete slab

So my questions are:

1. Is there a building-science advantage to lining the pool shell with a vapor barrier before placing the geofoam, or is this generally unnecessary?

2. Could a liner under the foam potentially trap moisture against the concrete shell, or create a slip layer under the blocks?

3. Since the slab will extend across the entire room, would it be preferable to install one continuous vapor barrier across the full floor area prior to the pour, rather than only over the geofoam section?

4. I’ve also seen some discussions about drilling drainage holes in the bottom of the pool shell before filling. In a case like this (indoor pool, empty for ~10 years, shell appears sound), is there any benefit to doing that, or is it typically unnecessary and/or might create other issues?

The finished space will be used as a home theater and gym, so loads are residential but somewhat heavier in the gym area.

Any insight into best practices for this type of project would be greatly appreciated.

Thank you.