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?
- 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.
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.
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 (e.g.), 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.
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!
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!