Best ICF Blocks for Building a Hot Tub: What to Look For

Last updated: September 2026

ICF stands for insulated concrete form. ICF blocks are hollow blocks made of expanded polystyrene (EPS) foam. You stack them, put rebar inside and fill them with concrete, so you end up with a reinforced concrete wall that’s insulated on both sides. For a hot tub, that’s a big deal: the foam keeps the heat in, and the concrete does the structural work.

Over the last few years I’ve had more and more customers asking to build with ICF, and there are several ICF builds in the case studies on this blog and on my YouTube channel. If I built my own hot tub again, I’d use ICF rather than block. So in this guide I’ll explain what I look for in an ICF block for a hot tub, which features matter and which don’t, and how some of the main systems compare.

What are ICF blocks?

ICF blocks are formwork that stays in place. Each block is two panels of EPS foam held apart by plastic or steel ties (sometimes called webs). They slot together dry, course on course, a bit like Lego for adults. Once the walls are up, you drop rebar into the cores and fill them with concrete. The foam never comes off, so the finished wall has insulation on the inside and the outside.

Why use ICF for a hot tub?

• Speed and ease. For me, this is the biggest advantage. If, like me, you don’t lay blocks very often, you’re probably slow at it. ICF forms are much bigger than individual blocks and just drop into place, so the walls go up in a fraction of the time.
• They’re light. DIYers often work alone. Lugging ICF blocks around is no problem at all. The same can’t be said for concrete blocks.
• Insulation built in. The foam on both faces keeps heat in the water rather than in the ground, which cuts running costs. It’s also why ICF suits cold plunges and swim spas.
• Easy plumbing. You cut a hole in the foam, push the pipe or jet body through, and the concrete locks it in. No core drilling through solid masonry.
• Strength. Once filled, it’s a solid reinforced concrete wall, well able to hold a tub full of water.

There are some honest downsides too. ICF usually costs more than plain block. The walls must be braced and the concrete poured carefully, or a form can burst. And foam can be tunnelled by insects, so protect any exposed foam, especially in areas with termites. I’ve gone into these in my ICF wall guide.

What about the cost?

On the surface ICF looks considerably more expensive, but when you break it down it’s not as much as you might think. Yes, the blocks cost more than concrete blocks. But you save a lot of time on construction, and you don’t need to buy separate PIR insulation boards. Compared with timber formwork for a poured wall, ICF saves you buying a lot of expensive timber. In my mind, ICF is worth it. Do get real quotes for your own build, though, because prices vary a lot by region and supplier.

What to look for in an ICF block for a hot tub

1. Keep it simple

I’m a big fan of keeping things simple. All I want is a set of blocks that can be quickly and easily glued or slotted together to form the shape of my hot tub. Anything more is overkill, because we’re only building around three and a half courses, four at most. We’re not building a two-storey house.

2. Don’t pay for structure you don’t need, but don’t skip the rebar

Many ICF systems on the market have quite complex internal structures: lots of ties, grid-shaped cores or special interlocks. These are mainly there to hold the form together under the weight of wet concrete, to give fixing points for cladding, or to suit tall house walls. They make the block cost more to produce, and a hot tub doesn’t need them.

What those ties don’t do is replace the steel. Whichever block you choose, your hot tub wall still needs the rebar your plans or engineer specify, tied into starter bars from the slab. A regular ICF form plus regular rebar is all we need. There’s more in my rebar guide.

3. Core size and wall thickness

The core is the space the concrete fills. Pool systems commonly use a 6in (150mm) core. For example, BuildBlock’s radius pool forms fit its 6in product line. Your plans set the size you need. Remember the overall wall is the core plus the foam on both sides, so allow for it when you size the dig.

4. Insulation value

Check the U-value (UK) or R-value (USA) of the finished wall. More foam means better insulation, but a thicker wall. As an example, the TubBlox spec (below) is 0.32 W/m²K, which is roughly R-18 in US units.

5. Block height and course count

Taller blocks mean fewer courses and fewer joints. For a hot tub around 1m (39in) deep, blocks around 250–300mm (10–12in) high give you that three and a half to four courses.

6. Corners, ends and curves

Check that the system has corner blocks and end caps, and radius pieces if you want curved walls. Otherwise you’ll be cutting and bracing a lot of custom joints. Some manufacturers make pre-formed curves (BuildBlock offers radii from 2ft to 20ft). Others let you cut straight forms to make a curve.

7. Plastic or steel ties

Most blocks use plastic ties, which are easy to cut through with a saw when you need to trim a block. Some, such as Polysteel’s Polyblok, use steel ties, which the maker says keeps the blocks stable while they’re filled with concrete.

8. Local availability and a proper manual

ICF blocks are bulky, so shipping them a long way gets expensive fast. Buy what’s available near you, and choose a supplier with a clear installation manual. Ideally it should have pool-specific guidance on bracing, pour heights and finishes. BuildBlock publishes a pool construction manual, and Nudura offers an ICF pool installation booklet.

9. How you’ll finish the inside face

US pool systems often finish the foam with pool plaster or a liner. Fox Blocks describes plaster applied directly to the foam, or a liner. For a tiled hot tub, you need a surface your waterproofing membrane and tile adhesive will bond to, so check what both the block maker and the membrane maker recommend before you start. John’s build, where a membrane delaminated, shows why this matters. See John from Kentucky’s 8×8 DIY hot tub and my waterproofing, grout and tile adhesive guide.

Some ICF systems compared

These are examples of ICF systems with pool guidance. I haven’t built with all of them, so treat this as a starting point for your own research, not a recommendation.

System	Where	Worth knowing
TubBlox	buildahottub.com	My own simple hot tub ICF block (see below). Not currently listed in my shop.
BuildBlock	USA	Pool manual, pre-formed radius forms (2–20ft), plaster applied to the foam
Fox Blocks	USA	Pool guidance covering liners and plaster finishes, with engineer-specified rebar and bracing
Nudura	North America	ICF pool installation booklet
Polysteel (Polyblok)	UK	Steel-tied blocks aimed at pool walls, usually with a vinyl liner

TubBlox: my own take on it

OK, I’m biased here, as TubBlox are my own brand. I designed them around everything above: a simple EPS form, sized for hot tubs and small pools, that you glue together, reinforce with regular rebar and fill with concrete. No complicated internal structure. Accessory pieces let you make most hot tub and pool shapes. If we were building a house or a garage, there are probably better systems out there, but for a hot tub, simple is best.

TubBlox specification	Metric	Imperial
Block dimensions	1000 x 250 x 300mm	about 39 x 10 x 12in
Blocks per square metre of wall	3.33	about 0.31 per sq ft
Concrete per square metre of wall	0.14m³	about 0.46 cu ft per sq ft
Thermal rating (U-value)	0.32 W/m²K	roughly R-18 (US units)

TubBlox aren’t currently listed in my shop. If you’re interested in them for your project, fill in the form at the bottom of this page and I’ll let you know what I can do. If not, don’t worry. Everything in this guide applies to any simple ICF block, and the full build method is in my TubBlox and ICF wall guide.

Building with ICF: the key steps

1. A level, reinforced slab with starter bars where the walls go. See my in-ground build guide.
2. Get the first course dead level and square. Everything above follows it.
3. Stack the courses, staggering the joints, and fit the rebar as you go.
4. Fit the plumbing through the foam before the pour, and seal around the pipes.
5. Brace the walls and check they’re plumb.
6. Pour in lifts, with a mix made for ICF. That’s 4,000psi with fibres in the USA, or C25/C30 in the UK. Consolidate it well and watch for bulges.
7. Waterproof and flood test before you tile.

You can see it done in these ICF builds: Adam’s ICF hot tub, a Tennessee ICF hot tub, an ICF block hot tub case study and out with the cedar, in with an ICF in-ground hot tub.

Common mistakes with ICF

• Pouring too fast or too high in one go, and bursting a form.
• Not enough bracing, so the walls end up out of plumb.
• Leaving out rebar because the block “looks strong enough”.
• Forgetting the thickness of the foam when sizing the hole and the internal dimensions.
• Leaving foam exposed to sunlight, garden pests or strimmers. Cover it or clad it.
• Tiling onto a surface the membrane can’t bond to. Follow the membrane maker’s instructions.

Not sure ICF is right for you? Compare it with block and poured concrete in Hot tub wall construction: what are the options? and Cinder block hot tub: a DIYer’s guide.

Planning your own build?

Whichever block you use, the wall is only as good as the drawings behind it. My DIY hot tub plans give you CAD drawings with every dimension, the plumbing layout, a full parts list and the video course. If your tub is a custom shape, design it in the free Tub & Pool Designer Pro software and I can turn it into architectural CAD and a 3D plumbing design.

FAQs

Are ICF blocks good for building a hot tub?

Yes. They’re quick, light, easy to plumb through, and they give you a reinforced concrete wall with insulation built in. That’s why I’d use ICF if I built my own tub again.

Do ICF hot tub walls need rebar?

Yes. The ties in the block hold the form together during the pour. They don’t replace reinforcement. Follow the rebar spacing in your plans or from your engineer.

Is ICF more expensive than concrete block?

The blocks usually cost more, but you save on labour, timber formwork and separate insulation, so the overall difference is smaller than it first looks.

What core size do I need?

Pool systems commonly use a 6in (150mm) core, but your plans or engineer set the size for your tub.

Can I tile directly onto ICF?

Not directly onto bare foam. The inside face needs preparing and waterproofing to suit your membrane and tile adhesive, so follow both makers’ instructions.

If you’d like to ask about TubBlox, or talk through ICF for your own project, fill in the form below and I’ll get back to you.

Or drop a question in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi