ICF Block Hot Tub? Absolutely! Case Study

ICF Block Hot Tub

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

ICF Block Hot Tub? Yes please! In this Case Study, we will be following John from the UK and his DIY Hot Tub build using ICF Blocks.

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 or Insulated Concrete Forms 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?

According to Wikipedia,Insulating concrete form or insulated concrete form (ICF) is a system of formwork for reinforced concrete usually made with a rigid thermal insulation that stays in place as a permanent interior and exterior substrate for walls, floors, and roofs.

The forms are interlocking modular units that are dry-stacked (without mortar) and filled with concrete. The units lock together somewhat like Lego bricks and create a form for the structural walls or floors of a building. ICF construction has become commonplace for both low rise commercial and high performance residential construction as more stringent energy efficiency and natural disaster resistant building codes are adopted.”

Why should you use an ICF Block?

Energy efficiency

  • Minimal, if any, air leaks, which improves comfort and reduces heat loss compared to walls without a solid air barrier
  • High thermal resistance (R-value) typically above 3 K·m²/W (in American customary units: R-17); this results in saving energy compared with uninsulated masonry
  • Continuous insulation without thermal bridges or “insulation gaps”, as is common in framed construction
  • Thermal mass, when used well and combined with passive solar design, can play an important role in further reductions in energy use, especially in climates where it is common to have outside temperatures swing above inside temperatures during the day and below at night.


  • Insulating concrete forms create a structural concrete wall, either monolithic or post and beam, that is up to ten times stronger than wood framed structures.
  • Structural integrity for better resistance to forces of nature, compared to framed walls.
  • The components of ICF systems — both the poured concrete and the material used to make the ICF — do not rot when they get wet.

Sound absorption

ICF walls have much lower rates of acoustic transmission. Standard thickness ICF walls have shown sound transmission coefficients (STC) between 46 and 72 compared to 36 for standard fiberglass insulation and drywall. The level of sound attenuation achieved is a function of wall thickness, mass, component materials and air tightness.

Fire protection

ICF walls can have four- to six-hour fire resistance rating and negligible surface burning properties. The International Building Code: 2603.5.2 requires plastic foam insulation (e.g. Polystyrene foam, Polyurethane foam) to be separated from the building interior by a thermal barrier (e.g. drywall), regardless of the fire barrier provided by the central concrete. Forms made from cement bonded – wood fibers (eg), polystyrene beads , or air (i.e. cellular concrete – e.g) have a fire rating inherently.

Indoor air quality

Because they are generally constructed without a sheet plastic vapor barrier, ICF walls can regulate humidity levels, mitigate the potential for mold and facilitate a more comfortable interior while maintaining high thermal performance. Foams, however, can give off gasses, something that is not well studied.

Environmental sensitivity

ICF walls can be made with a variety of recycled materials that can minimize the environmental impact of the building. The large volume of concrete used in ICF walls has been criticized, as concrete production is a large contributor to greenhouse gas emissions.


Because the entire interior space of ICF walls is continuously occupied (no gaps as can occur between blown or fiberglass insulation and a wood frame wall) they pose more difficulty for casual transit by insects and vermin. Additionally, while plastic foam forms can occasionally be tunneled through, interior concrete wall, and the Portland cement of cement-bonded type forms create a much more challenging barrier to insects and vermin than do walls made of wood.

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!

ICF Block

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.

ICF Block Hot Tub

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

Hot Tub Membrane

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.

Braced ICF Blocks

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 😉

ICF Block Concrete Pour

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!

Project Complete

John got there in the end and how good does his ICF Block, DIY-built Hot Tub look. Ok, it’s the size of a small pool, but it really does look the part.

The project didn’t go to plan and as we have seen, John had problems with the waterproofing layer coming unstuck from the surface of the ICF Blocks.

John has gone with a mosaic tiled finish and I think it is a beautiful colour and contrasts perfectly with the red colour of his English Country House and the beautiful leafy green countryside.

Job well done John. You definitely have graduated from my online course with honours – that is where it started back in December 2021 and here we are.

Hope you and the family enjoy the tub for many years to come.

Happy Hot Tubbin’

ICF Hot Tub

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

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.

Today, I've helped over 1000 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

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