So you bought a sparkly new above‑ground pool. Awesome! But before you fill it up and cannonball in, let’s talk about what goes *under* it. A bare grassy patch? That’s a one‑way ticket to root‑poked liners and rocky bottom feet. You want something soft, smooth, and protective. Enter: foam board insulation. But not all foam is created equal. Let’s find your pool’s perfect squishy mattress.
The Short Answer (For the Impatient)
Use XPS foam board – extruded polystyrene. It’s the golden child of pool pads. Closed‑cell, water‑resistant, strong enough to hold thousands of gallons without turning into a crumbly mess. Look for 1‑inch thick sheets with high compressive strength (25+ psi).
The Long, Fun Answer (Because You Like Learning)
There are three main foam types you’ll see at the hardware store. Let’s meet them.
1. XPS (Extruded Polystyrene) – The Pool Champion
XPS is your best friend. It comes in pink (Owens Corning Foamular), blue (Dow Styrofoam), or green depending on the brand. Here’s why it rocks:
- Closed‑cell structure – Water can’t sneak in. Your pool will leak? Maybe. But the foam won’t turn into a soggy sponge.
- High compressive strength – It laughs at heavy water pressure. No squishing, no settling.
- Smooth surface – No sharp bits to poke your pool liner. Happy pool = happy you.
- Resists rot and mold – Because dirt + moisture + time = yuck. XPS says “not today.”
Get 1‑inch thick XPS for most pools. If your ground is extra rocky or you’re super picky about comfort, go 1.5 or 2 inches. Your feet will feel like they’re walking on clouds.
2. EPS (Expanded Polystyrene) – The “Maybe” Guy
EPS is that white, beady foam you’ve seen in packaging (or cheap coolers). It’s lighter and cheaper than XPS. So why not use it?
- Open‑cell-ish – Those little beads can absorb water over time. Wet EPS becomes heavy and sad.
- Less compression strength – Under a pool? It might crush unevenly, leaving dips and lumps.
- Still usable… if you’re careful – Some people double‑bag it with a tarp or use it only for temporary pools. But for a permanent setup? I’d pass.
Verdict: EPS is like that friend who’s fun for a day but flakes on moving day. Cute, but not reliable.
3. Polyiso (Polyisocyanurate) – The Fancy No‑Show
Polyiso has a foil face and high R‑value. Sounds fancy! But here’s the catch: it loses insulation power when cold (thermal drift) and hates moisture. Under a pool, where condensation happens, polyiso can get grumpy and degrade. Plus, it’s usually more expensive. Save it for your attic.
4. Plain Old Beach Sand or Carpet Pad? – Just No ❌
Sand shifts. Carpet pad compresses and rots. Foam board is the modern, clean, happy answer. Trust the foam.
How to Install Foam Board Under Your Pool (The Mini Version)
1. Prep the ground – Remove rocks, sticks, and ninja star glass. Level the dirt. Rake it smooth.
2. Lay down a weed barrier or geotextile fabric – Keeps grass from trying to hug your foam from below.
3. Roll out the foam – Put XPS boards edge‑to‑edge. No gaps! Tape the seams with waterproof tape (house wrap tape works great).
4. Add a top layer – A tarp or pool liner pad is optional but nice for extra smoothness.
5. Set your pool on top – Fill slowly, watch the magic happen.
Pro tip: If your pool has a metal frame, cut the foam to fit inside the frame – not under the legs. The legs need firm ground. Foam is for the floor area only.
A Few Lighthearted Warnings
- Don’t use foil‑faced foam – The foil can react with pool chemicals or trap moisture. Plain XPS is better.
- Don’t skip the tape – Gaps let sand or little critters sneak up. No one wants an ant farm under their pool.
- Don’t buy thin craft foam – That ½ inch craft store stuff is for kid projects, not holding 10,000 gallons of water. Go thick or go home.
Final Cheerful Verdict
Buy XPS foam board – 1 inch thick, pink or blue, high density (25‑30 psi compressive strength). It’s a little more expensive than EPS, but think of it as insurance against lumpy bottoms, popped liners, and chilly toes. You’re building a backyard paradise – splurge on the foundation.
Now go forth, level that ground, and make your above‑ground pool feel like a luxury spa. Your feet (and your floating self) will send you a thank‑you note. ♂️✨
