TubBlox and ICF Hot Tub Walls: Insulated Hot Tub and Pool Walls Made Easy

Updated September 2026. TubBlox kits aren’t currently listed in my shop, so I’ve rewritten this post as a guide to building hot tub and pool walls with TubBlox-style ICF blocks. I’ve kept the original specifications and step-by-step photos, and removed some claims I can’t back up. I’ve also corrected what I said about pests.

If you’ve read much on this blog, you’ll know I’d build my own hot tub from ICF (insulated concrete form) if I did it again, not the concrete block I used first time round. TubBlox were my take on that: an ICF kit sized for hot tubs and small pools. Easier to lay than blocks, insulated as you go, and quick to put together. That makes them perfect for the DIYer.

This guide explains what TubBlox are, how they compare with block, how to build with them, and the things to watch out for. Everything here applies to any good ICF block, so it’s still useful whichever brand you end up with.

Are TubBlox still available?

At the time of this update (September 2026), TubBlox kits aren’t listed in my shop. If you’re interested in them, get in touch and I’ll tell you what I can do. If not, don’t worry. The principles below apply to any ICF block suitable for below-ground walls, and I’ve written about what to look for in an ICF block for a hot tub. My advice there still stands: keep it simple. A regular ICF foam form and regular rebar is all a hot tub needs.

What are TubBlox?

TubBlox are formwork made from expanded polystyrene. You build the basic shape of the hot tub or pool from simple interlocking blocks, reinforce it with steel rebar, and fill the cores with concrete. Think of it as Lego for adults.

Accessory pieces let you create almost any shape of tub or pool. Because the foam stays in place, the finished wall is insulated on both faces. With energy prices where they are, I think it’s really important that our DIY hot tubs and pools are properly insulated to keep the running costs down.

The specification

These are the specifications of the blocks as I originally supplied them:

Specification	Metric	Imperial (approx.)
Block size (L x W x H)	1000 x 250 x 300mm	39.4 x 9.8 x 11.8in
Blocks per area of wall	3.33 per m²	about 0.31 per ft²
Concrete to fill	0.14m³ per m² of wall	about 0.46ft³ per ft²
Thermal rating (U-value)	0.32 W/m²K	roughly R-18 (US units)

For comparison, the US Department of Energy says ICF walls typically give “about R-20”, so these sit right in the normal range for ICF. A plain block wall has only a fraction of that.

How many will I need? A worked example

Take an 8ft x 8ft (2.44m) internal hot tub with walls four courses high (1.2m, just under 4ft). The wall centreline runs about 10.8m (35ft) around, so you’ve got roughly 13m² (140ft²) of wall. That works out at about 43 blocks and 1.8m³ (about 2.4 cubic yards) of concrete for the walls. Add extra for waste, cuts, seats and steps, and the slab is separate. Treat this as a ballpark figure. Your plans give you the real numbers.

Advantages over block

Quick and simple to build

The blocks are made of very light EPS and they’re hollow in the middle. They have interlocking grooves, so they stack easily. They’re glued together course by course, so there’s no mortar to mix. If you’re a DIYer like me, you don’t lay blocks very often, and it takes forever to get right. I know, I’ve done it. ICF blocks are much bigger than bricks or concrete blocks too, so there are far fewer to lay.

No timber formwork

Forming up a poured concrete wall takes a lot of time and a lot of timber, and then it all has to be stripped and cleaned. With ICF, the formwork stays in place and becomes the insulation. There’s nothing to strip. (You do still need some temporary bracing to keep the walls straight during the pour. More on that below.)

Plumbing is easy (this bit is good!)

If you build with block, every pipe through the wall means coring a hole through solid masonry with a diamond core drill. With ICF, you cut a hole in the foam with a knife or a hole saw, push your pipe or gunite jet body through, and the concrete locks it in place when you pour. Doesn’t quite fit? Open the hole out a little. Your pipes and gunite bodies sit neatly inside the wall, insulated. It doesn’t get much better than that. Seal any gaps around the pipes with a little expanding foam before the pour, so the concrete doesn’t leak out. For setting the jets at the right depth and position, see my gunite jet body installation guide.

Insulation built in

An in-ground tub loses heat into the ground all year round. With ICF, the walls are insulated on both faces from day one, without fixing separate boards. Add rigid insulation under the slab and a good cover, and you’ve dealt with most of the heat loss. There’s more in DIY hot tub insulation, and it’s a big help if you’re heating with an air source heat pump.

Strong and durable

Once filled, you have a reinforced concrete wall. The rebar goes into the slots in the interlocked blocks and ties into the starter bars in your slab. That makes the floor and walls one structure. Concrete and EPS don’t rot, and they aren’t affected by moisture the way timber is. See my rebar guide for bar sizes and spacing.

The honest downsides

When I first wrote this post, I was pretty enthusiastic. I still am, but there are a few things you should know, and one thing I said that I’d correct.

• Pests: a correction. I originally said ICF walls were one of the best ways to get a rodent-free tub, because the foam contains nothing for insects to feed on. That’s too strong. The concrete core can’t be chewed through, but the foam can be tunnelled. The US Department of Energy notes that the foam webbing around the concrete cores “can provide easy access for insects and groundwater”. Protect exposed foam above ground (render, board or cladding), keep the control room sealed and tidy, and in termite areas check your local code. Some US jurisdictions restrict foam insulation below ground or require termite protection.
• Blowouts. Wet concrete is heavy. If you pour too fast or too high in one go, a block can burst. Brace the walls, pour in lifts, and have someone watching for bulges.
• Cost. ICF blocks usually cost more than plain concrete blocks. You save on time, labour, timber formwork and separate insulation, but compare real quotes for your own build.
• The inside face. Your waterproof membrane has to bond to something sound. How you prepare the inside face depends on the block system, so follow the block supplier’s and the membrane maker’s instructions to the letter. John’s ICF block hot tub is a useful lesson here: his first membrane, applied over a primer, delaminated, and he had to redo it with a product that could go straight onto the shell.

In the original post I also quoted figures for concrete savings (around 10% less than a straight wall and up to 25% lower cost) and a 65% labour saving. I can’t verify them, so I’ve taken them out. The time saving over laying block is real, as anyone who’s built both will tell you, but it varies from build to build.

TubBlox step-by-step installation

Here’s the build sequence. It’s the same for most ICF systems.

1. Dig the hole

Carry out the excavation. Essentially, dig your big hole. Allow for the sub-base, insulation, slab and the full wall thickness. My in-ground build guide covers the dig depth in detail.

2. Prepare the base

Prepare the base just as you would for a block build: a compacted sub-base, a DPM, ideally insulation, and a reinforced concrete slab with starter bars left sticking up where the walls go. Fit the main drain, the hydrostatic relief valve and any sleeves before you pour. See installing a main drain and installing a hydrostatic relief valve.

3. Lay the first course on a mortar bed

Get this course level and square. Check the diagonals. Everything above follows the first course, so take your time here.

4. Install the rebar

Tie the vertical bars to the starter bars in the slab, and lay horizontal bars in the slots as you go up.

5. Glue the following courses together

Stagger the joints like brickwork and glue each course to the one below.

6. Add the plumbing

Fit the jets, skimmer, returns and suction lines through the foam as you build. Check the heights against your plans before you pour, because after that they’re set in concrete.

7. Brace, then pour the concrete

Brace and align the walls. You can have the concrete pumped in, which I’d recommend, or mix it yourself. Originally I said the forms can be filled up to 1.75m (5ft 9in) high. Whatever the height, pour in lifts around the tub rather than filling one spot to the top, and consolidate each lift well (vibrate or rod it) so there are no voids around the pipes and rebar.

For the mix, I recommend buying ready-mix in: 4,000psi concrete with fibres added in the USA, or C25/C30 in the UK, with fibres if your supplier offers them. The mix needs to be fluid enough to flow into the cores. Tell the supplier it’s for ICF. If you must mix it yourself by hand or with a mixer, use a proper concrete mix with aggregate and consolidate it thoroughly.

8. Waterproof, then finish

Once the concrete has cured, prepare the inside face, apply a pool-rated membrane, flood test, and then tile. My waterproofing, grout and tile adhesive guide and hot tub tiles guide take it from there.

See ICF in real builds

Plenty of customers have now built with ICF. A few worth reading:

• ICF block hot tub? Absolutely! (case study)
• Building a hot tub with ICF: Tennessee DIY project
• Out with the cedar, in with an ICF in-ground hot tub
• Adam’s DIY hot tub with ICF blocks

If you’re still deciding on wall construction, see Hot tub wall construction: what are the options? and my cinder block hot tub 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 or pool 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

What is an ICF hot tub?

It’s a hot tub whose walls are built from insulated concrete forms: hollow foam blocks stacked, reinforced with rebar and filled with concrete. You end up with a reinforced concrete shell that’s insulated on both faces.

Is ICF better than concrete block for a hot tub?

For most DIY builds, I think so. It’s quicker to build, easier to plumb and far better insulated. Block is cheaper to buy and perfectly usable, but you’ll need to insulate it separately.

How many courses of ICF does a hot tub need?

Usually around 3.5 to 4 courses. With 300mm (12in) high blocks, that’s about 1.05–1.2m (3ft 5in–4ft) of wall.

Can I waterproof and tile an ICF hot tub?

Yes. The inside is finished and waterproofed with a pool-rated membrane, then tiled, just like a block tub. Follow the block and membrane makers’ instructions for preparing the inside face.

Do mice or insects get into ICF?

They can’t get through the concrete, but they can tunnel through exposed foam. Cover any foam that’s above ground and, in termite areas, check local building rules.

Got a question about building your walls with ICF? 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