Adam’s ICF Hot Tub: Following a DIY Build with ICF Blocks

Last updated: September 2026

Have you ever dreamed of having your own hot tub in the garden? You’re not alone. But what if you don’t have the budget for a pool or spa builder to come and build it for you? That’s where DIY comes in.

In this post, we’re following Adam as he builds his own hot tub using ICF blocks. ICF stands for insulated concrete forms: hollow foam blocks that you stack, reinforce and fill with concrete. They’re becoming increasingly popular for DIY builds because they’re light, easy to work with and insulate the tub as you build.

Preparation and design

Adam had just finished building his new home, and he wanted to add a hot tub to the back garden. Given the space available, this was going to be a custom design project, because the designs in my online store just weren’t the right dimensions.

Step 1: Digging the hole

The first step in any in-ground hot tub project is digging the hole. It’s a back-breaking job, and the hole needs to be big enough for the tub plus everything around and under it. Adam’s hot tub is custom-designed, so he dug the hole to the exact dimensions on his drawings. If you’re at this stage, my in-ground hot tub build guide covers the dig, base and walls in detail.

Step 2: Laying the concrete slab

Once the hole is dug, it’s time for the concrete slab. This is the foundation of the hot tub, so it needs to be level and solid. See how to build a concrete slab for a hot tub.

Step 3: Installing the ICF blocks

Now comes the fun part: the ICF blocks. They’re like giant Lego blocks that snap together, made from expanded polystyrene foam, which is both insulating and lightweight. Once they’re stacked and the steel is in, they’re filled with concrete. For more on choosing them, see the best ICF blocks for a hot tub and TubBlox and ICF hot tub walls.

Step 4: Adding the plumbing

Before the rebar goes in, the plumbing has to be added. This is a crucial step. The pipes carry water to and from the tub, and once the walls are filled you can’t easily change them, so they need to be right first time. My DIY hot tub plumbing guide and how to install gunite jet bodies go through it.

Step 5: Rendering the surface

Once the plumbing is in, the inside of the ICF walls is rendered. This gives a smooth, true surface ready for the waterproofing and tiles.

Step 6: Waterproofing

After the render has cured, the tub needs a proper waterproofing membrane. Choose one that’s designed for pools and permanent immersion, not a general-purpose sealer. I cover the options in hot tub waterproofing, tile adhesive and grout.

Step 7: Building the control room

The control room houses the pumps, filter, heater connections and electrical equipment. It needs to be big enough for all of it, with room to work on it, and it needs to be properly ventilated. See hot tub control room design considerations.

Step 8: Water testing

Before you tile or finish your hot tub, fill it and test it. This shows up any leaks or problems while they’re still easy to fix. Once you’re sure there are no leaks, you can go ahead and finish the tub. That was Adam’s next job.

Update, early May 2024: gas heater, waterproofing and testing

It had been a little while since we last caught up with Adam, and things had moved on a great deal, so here’s a summary.

First, there was quite a lot of plumbing to do so Adam could hook up his external gas heater. He chose gas rather than an air source heat pump because of the location. There wasn’t enough space to put a large heat pump anywhere other than under a window. Just like a split air-con system, they make some noise, and for Adam that simply wasn’t viable. On the plus side, with the gas heater, Adam will get the tub up to temperature in no time at all. If you’re weighing up the same choice, see how noisy is an air source heat pump?, hot tub gas heaters and my guide to air source heat pumps for hot tubs.

With the plumbing complete, it was time for the walls to be rendered and waterproofed. Adam used my European recommendation, Ardex S 7 Plus. It’s not the cheapest material, but it’s very good. For those of you in the USA, Hydro Ban from Laticrete is a great product and easy to get hold of.

With the walls waterproofed and the plumbing done, it was time for Adam to test the systems and run the pumps. As DIYers, we don’t usually have the equipment to pressure test properly. The next best thing is to fill up and run everything before the finishes go on. That way, it’s not too messy if anything needs repairing or replacing, which you really don’t want to be doing once the tiles are in.

I’m pleased to report everything checked out fine for Adam. Next up: tiles.

Update: Adam’s hot tub is almost finished

It’s been a busy few months for Adam, but he’s finally starting to see the light at the end of the tunnel. I’ve been following the whole process on my YouTube channel and here on the blog.

The tiles are finally in, and I’m absolutely loving how they look. Adam went with a blue mosaic tile that really pops against the Cotswold stone. It’s exactly the look he was going for. He was just waiting for the grout and adhesive to fully cure before he could really enjoy it. For choosing your own, see hot tub tiles.

Adam was also in the final stages of building the cover for the underground control room. It had been a bit of a challenge, but once it’s done he can fully seal up the control room.

The best part? The custom hot tub cover I had made for Adam was due to arrive the following week. There’s more on covers in the ultimate guide to hot tub covers.

I get a lot of questions about how to prepare for the first fill, so here are a few tips:

• Remove debris: before filling the tub completely, run the pumps for a short time to flush out any dirt or debris.
• Protect your system: take the filter out and use a submersible pump or wet vac to extract any remaining particles.
• Prevent air locks: turn the jets on when the water is about halfway up the tub as you fill, to avoid air locks and get the water circulating properly.

There’s a full walkthrough in how to fill a hot tub the correct way.

Latest update

Thanks, Adam

Thanks for following along on this journey, and thank you, Adam, for sharing your build. If you’re thinking about ICF for your own tub, I compare it with block and poured concrete in DIY hot tub construction: block, ICF or poured concrete?, and there’s another ICF build in ICF block hot tub: a case study.

Planning your own ICF hot tub?

If a standard size fits your space, my DIY hot tub plans include CAD drawings, the plumbing layout, a parts list and the video course. If, like Adam, you need something designed around your garden, my custom hot tub design consultancy gets you a design and my support through the build, or I can turn your own layout into architectural CAD and a 3D plumbing design.

FAQs

Can you build a hot tub with ICF blocks?

Yes. ICF blocks are stacked, reinforced with rebar and filled with concrete, then rendered, waterproofed and tiled like any concrete tub. The foam gives you insulation built in.

Do ICF blocks need waterproofing?

Yes. The foam and concrete aren’t a waterproof finish. The inside needs rendering and a pool-rated membrane before tiling.

Should I test my hot tub before tiling?

Yes. Fill it and run the pumps before the tiles go on, so any leak can be fixed without ripping out finished work.

Got a question about an ICF build like Adam’s? Drop it 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