Hot Tub Wall Construction: Block, CMU, ICF and Poured Concrete Compared

Last updated: September 2026

There are several ways to build the walls of a DIY hot tub. The plans in my shop work with each of the construction methods below, so this article focuses on the walls themselves: poured concrete, block cavity, CMU block, ICF and the in-ground cavity method.

Disclaimer: I’m not a structural engineer. This article is based on practical experience, not formal qualifications. If you’re unsure about the structural integrity of your build, or you need a permit, consult a professional.

Poured form concrete walls

Poured form construction means building a wooden mould (formwork) and filling it with concrete to make the walls and seating. It’s popular with DIY builders because it gives a clean, seamless finish and integrated plumbing, but it comes with challenges you need to plan for.

• Bracing is critical. The pressure of wet concrete on the formwork is enormous. If it isn’t braced properly, the walls can bulge or curve and ruin the final shape.
• Use strong timber and plenty of supports along the outside of the form to stop movement while the concrete sets.
• The result is a solid, custom-cast tub that’s very durable and can be tailored to your design.
• You can embed plumbing lines and fittings directly into the walls during the pour, which often seals better around jets and pipes and lowers the risk of future leaks.

If you’re comfortable with concrete, or willing to learn, it can be one of the most rewarding methods.

Concrete block cavity

Getting enough wall strength is one of the main challenges of a DIY tub or plunge pool. Concrete block is often thought the easiest method for DIYers, but it needs a cavity. The cavity does three jobs:

• It houses the plumbing.
• It allows for insulation.
• It contains the rebar for strength.

The downside is a fairly wide wall. For example, a 4in (100mm) block, plus a 6in (150mm) cavity, plus another 4in block gives a minimum of 14in (about 350mm) in total, before any capstone. Inside the cavity you place the plumbing, insulation and vertical and horizontal rebar. Then the cavity is filled with vibrated concrete so there are no air pockets.

CMU block

CMU (concrete masonry unit) blocks are the hollow blocks used in general construction (in the UK, hollow dense concrete blocks are the equivalent). They’re cost-effective, readily available and strong when properly reinforced.

• Rebar goes through the hollow cores, tied into the concrete base and running vertically up the wall.
• You typically reinforce every other core with vertical rebar, and can add horizontal ties for extra stability.
• Once the walls are up and the rebar is in, the cores are filled with concrete.

Have a look at cinder block hot tub and rebar for detail.

ICF block

Insulating concrete forms (ICF) are interlocking modular units of rigid insulation. They stack like Lego bricks, need no mortar, and are then filled with concrete to make reinforced, insulated walls.

• Advantages: fast and straightforward, excellent insulation and strength, suitable for in-ground and above-ground designs.
• Disadvantage: a higher cost than traditional methods.

If I built my own again, I’d use ICF. The insulation matters a lot for heating costs, especially with a heat pump. Read ICF hot tub, the best ICF blocks for a hot tub and DIY hot tub insulation.

In-ground cavity

This method suits reusing an existing hole, for example replacing a sunken plastic shell hot tub with a permanent one.

• Start with a minimum 6in (150mm) cavity around your interior wall space.
• The cavity holds the gunite body fixtures (which need space), insulation and the rebar framework.
• Once the plumbing and insulation are in, the cavity is filled with concrete.

A 4in (100mm) concrete block is typically used for the inner wall, giving a strong, insulated structure that fits your existing space. See gunite jet body installation.

Comparing the options

Method	Strength	Insulation	Cost	Best for
Poured form concrete	Very high	Add separately	Moderate, plus formwork	Custom shapes, embedded plumbing
Block cavity	High when filled	Add in the cavity	Low to moderate	DIYers happy laying block
CMU block	High when reinforced	Add separately	Low	Budget builds
ICF	High	Built in	Higher	Fast, well insulated walls
In-ground cavity	High	Add in the cavity	Moderate	Reusing an existing hole

Whichever you choose

• Every wall sits on a reinforced slab that’s tied in with rebar. See how to build a concrete slab or base and my in-ground hot tub build guide for the build-up.
• Waterproof the finished walls properly before tiling. See waterproofing, grout and tile adhesive.
• Fit a hydrostatic relief valve if there’s any chance of groundwater: hydrostatic release valve.
• Plan where the plumbing goes before the walls go up.

Which construction method should you choose?

The right method depends on your preference, materials, skill level and budget. Each can give a fantastic result, and each has its own challenges. The best choice fits your location, tools and materials, and how confident you feel. Whatever you choose, the aim is a reliable, well-insulated tub that stands the test of time. For alternatives such as liners or wood, read can I use a liner on my DIY hot tub?, how to build a DIY wooden hot tub and alternatives to gunite.

Want the walls detailed for you?

My DIY hot tub plans include architect-drafted structural blueprints suitable for permit submissions, and CAD designs you can adapt to your wall type. For something bespoke, there’s the architectural CAD and 3D plumbing design service.

FAQs

What is the best wall construction for a DIY hot tub?

There isn’t one answer. Block is the easiest, poured concrete gives the best shape, and ICF gives the best insulation. Choose by skill, budget and location.

How thick are hot tub walls?

A block cavity wall of 4in block, 6in cavity and 4in block comes to about 14in (350mm). ICF and single block walls are thinner. Your plan sets the exact figure.

Do I need rebar in hot tub walls?

Yes. Rebar reinforcement, tied into the base and filled with concrete, gives the walls their strength.

Is ICF worth the extra cost?

I’d say so. It’s quick, strong and insulated in one go, which can help your running costs.

Can I build a hot tub in an existing hole?

Yes, using the in-ground cavity method with at least a 6in (150mm) cavity for the insulation, rebar and fittings.

Got a question about your own build? 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