What about using Styrofoam insulation on the ground under POOL liner?

It's not recommendable to use expanded polystyrene (EPS), such as Styrofoam, as insulation under a swimming pool liner. Despite its insulating properties, EPS is not the ideal material for this application due to several reasons:

1. Buoyancy: EPS is buoyant and can cause floating if it becomes waterlogged or if the water table is high. This can result in wrinkles, bumps, or tears in the pool liner, compromising the integrity of the pool.

2. Compression: Over time, EPS can be compressed under the weight of the pool and water, losing its insulating effectiveness and possibly leading to damage or deformation of the pool structure.

3. Durability: EPS is not as durable as other insulation materials, and it can deteriorate due to exposure to moisture, sunlight, and chemicals found in swimming pools.

4. Environmental Concerns: EPS is non-biodegradable and can leach harmful chemicals into the water, affecting water quality and potentially posing health risks to swimmers.

Instead of EPS, it's better to use specially designed insulation materials for swimming pool floors and walls. These pool-specific insulation options are more suitable and provide better performance in terms of thermal insulation, durability, and safety.