ICF Block Hot Tub? Absolutely! Case Study

Can You Build a Hot Tub from ICF Blocks – Case Study, John, UK

An ICF Block Hot Tub? Yes please! They’re one of my favourite kinds of building material due to all the benefits that they offer. 

In this Case Study, we will be following John from the UK and his DIY Hot Tub build using ICF Blocks. Stay tuned because by the end of this article, you’ll almost be an expert in building your own ICF Block Hot Tub

 

Background to John’s ICF Block Hot Tub

John from the UK has actually “graduated” from the Online Course that I have available on the website. He took the course in late December 2021 with the aim of starting his DIY Hot Tub build in the Spring of 2022.

As you will see from the images and narrative below, John is well on his way with the build.

What is particularly interesting with the build is the materials that John has opted to use.

Having built and remodelled his basement using ICF (Insulated Concrete Forms) blocks he decided this would be his material of choice. In this case study, we will follow his progress to completion of an ICF Built Hot Tub.

What is an Insulated Concrete Form or ICF block?

Insulated Concrete Forms (ICFs) are a modern construction system that combines reinforced concrete with rigid thermal insulation. These forms stay in place permanently, serving as both interior and exterior surfaces for walls, floors, and roofs.

ICF blocks interlock like Lego bricks and are dry-stacked without mortar before being filled with concrete. This creates a durable, energy-efficient structure that meets strict building codes for disaster resistance and insulation.

ICF construction is widely used in both residential and commercial buildings, especially as energy efficiency and structural durability become bigger priorities.

Why should you use an ICF Block?

ICF blocks offer energy efficiency, durability, sound insulation, fire resistance, and environmental benefits, making them a superior choice for modern construction.

Energy Efficiency

• Superior Insulation – ICF walls minimize air leaks, reducing heat loss and improving comfort.
• High R-Value – Typically R-17 or higher, saving energy compared to traditional masonry.
• No Thermal Bridges – Continuous insulation prevents heat loss, unlike framed walls.
• Thermal Mass Benefits – Helps regulate indoor temperatures, especially in fluctuating climates.

Strength & Durability

• 10x Stronger – ICF walls provide superior structural integrity compared to wood framing.
• Resistant to Rot – Both the concrete core and insulation materials do not degrade when exposed to moisture.
• Withstands Natural Disasters – ICF structures are more resilient against extreme weather.

Sound Absorption

• Superior Noise Reduction – ICF walls achieve sound ratings (STC 46-72), far better than standard drywall and fiberglass insulation.

Fire Resistance

• 4 to 6-Hour Fire Rating – Offers significantly more protection than conventional wood framing.
• Meets Building Codes – Requires a thermal barrier (e.g., drywall) for added safety.
• Cement-Based Forms – Some ICF blocks use materials like cement-bonded fibers for inherent fire resistance.

Indoor Air Quality

• Regulates Humidity – Helps prevent mold growth without requiring plastic vapor barriers.
• Comfortable Interior – Maintains a healthier indoor environment with stable humidity levels.

Eco-Friendliness

• Uses Recycled Materials – Some ICF blocks incorporate recycled content, reducing environmental impact.
• Criticism of Concrete Use – While concrete production contributes to greenhouse gas emissions, its long lifespan and energy efficiency help offset this.

Pest & Vermin Resistance

• Sealed Structure – No gaps for insects or rodents to travel through, unlike wood-framed walls.
• Difficult to Tunnel – The concrete core and cement-bonded forms create a strong barrier against pests.

Therefore, it’s safe to say that ICF blocks are an excellent investment for builders and homeowners seeking energy savings, durability, and long-term performance in their structures.

Let the Digging Commence!

As with any DIY Hot Tub project, the fist stage is always clearing the area and this is no difference for John. He has got a bit of a helping hand with the digger that you can see in the background. Excellent choice John, saves back pain no end!

 

Foundations for the Hot Tub Base

Once John was down to depth, the next stage is preparing the area for the base to be poured. What you need to do here is have a 10cm/4″ layer of compacted Type1 / crushed rock or ballast that you use a “Whacker” to then pound flat.

What this layer does is that it helps spread the and distribute the weight of the concrete evenly and also stops it subsiding.

What John has done on top of this is put a layer of  black aggregate. This black aggregate at the bottom of the hole is foamed glass which he used on top of type 1. It’s got a great R value and he was worried about foam insulation in the base but also wanted some added insulation given the lengths he is going to on the sides with the ICF block.

Bring on the membrane!

If you have read the blog here or watched any of my YouTube videos, you will know that the membrane is there not to keep the water in, but to keep dirty water ingress out of your hot tub.

What you don’t want is dirty water passing through your blocks and concrete and entering your hot tub. So, to stop that we put down a damp proof membrane as you can see John has done in the

ICF Block Hot Tub Time!

This is where the ICF blocks really come into their own. The speed at which you can put them together is just fantastic. They slot together just like lego blocks and don’t need any fixings or to be cemented in place (until you fill them of course)

The advantage of using these for the DIY Hot Tub is that it is also really easy to cut the holes in the polystyrene for the plumbing. This makes life very easy rather than having to core holes in regular block work.

The other advantage is of course the thermal properties of the blocks. They add R value and you don’t need any additional insulation for your tub.

When it comes to sealing everything, you are pouring concrete into them so you get a perfect seal around all of your plumbing too.

You will also see that additional vertical rebar has been added to the ICF Blocks to give them additional strength and tie them all together when the concrete is poured.

Also, you will also notice John has removed the bottom 8” or so of inner insulation. This is because he wanted to do a mono pour where he poured the slab at the same time as the walls in order to avoid cold joints. Very clever John!

Once you have the blocks in place, you can also add the plumbing as you can see how John has done that. What is really cool with the ICF Blocks is that you are cutting holes in polystyrene (or similar material) rather than coring holes in blocks which is much more work.

When it is time to pour the concrete into the centre of the ICF blocks, you will also get a perfect fit and seal on the plumbing going through wall too. This is certainly one of the advantages of using the ICF Blocks.

Bracing the ICF Block Structure

Once John has the blocks in place, it was time to add the wooden bracing. Concrete weighs an awful lot so it is really important the that structure is braces correctly for the pour. This ensures that the walls remain vertical and do not bow under the weight of the concrete.

Once the structure is dry and cured, the rebar takes over and holds everything together so the bracing is literally just for the pour.

ICF Block Hot Tub Pour Time

Once you have the ICF Blocks in place, fully braced, then it is time to pour some concrete.

According to John, the guys initially had the concrete mix too thick (as the concrete pump guys didn’t think pouring it all at once would work at all) so they had quite a few issues vibrating it down the walls.

However, it was ok once they had got the mix right (they were about 2/3 way through it at that point). Pour time was ~three hours. John made the point that for his basement he had a massive concrete pump crane as it had to come over the house and in hindsight having something like that would have made it so much easier (but would have cost $$$)

The concrete has rebar, fibres and waterproofer in it. Probably overkill but John likes to make sure of things 😉

Let the Concrete Cure, then Remove the Bracing

After several days of allowing the concrete to cure, it was time for John to remove the bracing to reveal his filled ICF Block form – wow! How good does this look?

John’s current status as of early March 2022 is that it’s poured and looks good. He is waiting on Ardex S7 to complete the job but there seems to be a delay at the moment for some reason.

What John found when he was doing his research is that there didn’t seem to be much information on whether you can go straight onto the ICF or not. He played around with a few ideas but ultimately decided that it’s fairly stable so is going to put a rendering primer on first and then S7 onto that and then we should be into more familiar territory ready for tiles.

Watch this space!

April 2022, so looking at the last update I think John had red gritted the interior and he was about to apply Ardex S7 and build the seats.

He decided to apply waterproofing to the entire outer shell prior to building the seats. Realistically this probably isn’t necessary but he’s tried to be as belt and braces everywhere as possible so this should be no exception.

John used concrete blocks for the seats. In hindsight he’d probably try using thermalite or similar as the blocks burnt through two diamond corers pretty quickly.

Internal pipework then started to go in which was fairly fiddly but am assuming that’s pretty standard.

He did a basic test of the internal pipework which seems to be sound so concreted the seats after packing with a few sheets of Celotex we had left over from some other projects. He used a weakish concrete mix so that if he ever need to get to the pipes it should break up more easily. He also poured in a few stages so that there were cold joints around the pipes that would break more easily if needed.  This was then red gritted and S7 was applied  – all looking good at this point.

John then test filled for water tightness and fired up some of the jets. The water looks a bit manky because of a combination of the S7 grey and the fact I filled it with well water as John tries to be water conscious and knew this was only temporary. The water is clean but has a bit of fine sediment in it.

John then heated to 33degrees using the heat pump which seemed to operate at minimum power/max efficiency – drawing ~1KW. Heat up speed was fine but given it’s a huge beast of a thing (24KW) I’d like to know how to tell it to sacrifice efficiency for speed. We later found that John was on the silent setting which decreases the power output. They had some good fun messing about in it for a couple of days. (sorry no pictures of John in his speedos ;))

Unfortunately, that is where the good times ended…

While John was in it he noticed that part of it felt a bit odd – almost like a large bubble. Further investigation showed that in certain areas the S7 had delaminated from the shell. Nightmare… It’s possible he could have stripped back and patched and tiled but in his mind there would have been a real possibility that it would continue as the months went by and we would have to junk the tiles too. Consequently he decided that it all needed to be pulled out. If you want to see what hundreds of pounds of junked waterproofing looks like then watch the video 🙁

John spoke to Ardex and they said that we shouldn’t have used a primer and that caused the weak point as the product needs to bind directly with the surface. This is probably correct but everyone always blames the other manufacturer so we spoke to the people who supplied the ICF blocks.

According to them they have people using a product called Vandex BB75 which can be applied directly to the blocks. Given they recommended it we decided to give it a go (the Ardex would have probably been fine but once bitten…).

They didn’t stock it atm but you can buy it from the distributors under the rebranded name of drybase. The price looks cheap(ish) at ~£50 per 25kg bag but you actually need to use an elasticiser with it that adds more that £100 a bag onto it… John did manage to get a bit of a deal but it was expensive. Also, you need to protect it from wind and rain for 5 days after applying which adds additional complications in the UK.

And so John starts again. He’s covered the tub with a tent and has applied the waterproofing to the shell. It’s currently sat under a load of plastic. I’ll share pictures as we move forward but I need to wait a few weeks for it to fully go off before we tile etc

Watch this space!

Final Thoughts

John’s ICF block DIY hot tub turned out fantastic! Sure, it’s more like a small pool, but it looks absolutely stunning.

The project had its challenges, especially with the waterproofing layer not adhering as planned. But John overcame the hurdles and achieved an incredible result. His choice of mosaic tiles adds a beautiful contrast to the red tones of his English country house, blending perfectly with the lush green surroundings.

John, you’ve done an amazing job—graduating with honors from my online course, where this journey began back in December 2021.

Wishing you and your family years of relaxation and enjoyment in your new hot tub.

Happy Hot Tubbin’!

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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