DIY Hot Tub Insulation: What to Use and Where It Goes

Updated September 2026. I’ve changed my advice on insulating under the floor (I now recommend it), added which boards to use below ground and which to keep dry, and put the cover where it belongs, at the top of the list.

Insulation is essential if you want to keep the running costs of your DIY hot tub down. Energy prices have made this more important than ever. On this blog and my YouTube channel I talk a lot about air source heat pumps as a way of cutting running costs, but a heat pump can only do so much if the heat is leaking straight into the ground. Insulation is the other half of the job.

In this post I’ll go through the options for each type of DIY hot tub: ICF, block, poured concrete and stock tank.

Where does a DIY hot tub lose its heat?

• The surface. Hot water loses heat to the air and by evaporation, and this is by far the biggest loss when the tub is uncovered. It’s why the cover is number one. See The ultimate guide to hot tub covers, or make your own.
• The walls. An in-ground tub sits against cold ground all year round. Concrete and block conduct heat well, so an uninsulated wall is a big radiator into the soil.
• The floor. This is the same story, and it never gets a break from the cold ground.
• The pipework and equipment, especially long runs to a control room.

Which insulation material?

Material	Typical thermal conductivity	Best for	Watch out for
PIR board (e.g. Celotex, foil-faced)	0.022 W/mK (Celotex GA4000)	Dry places: cavities above ground, behind cladding, control rooms	Not my choice for wet ground or backfill
XPS (extruded polystyrene)	0.029–0.039 W/mK	Below ground and under slabs. Products like Owens Corning FOAMULAR 250 are sold for under-slab and foundation use	Choose a compression-rated grade under the slab
EPS (expanded polystyrene, as in ICF blocks)	0.032–0.038 W/mK	ICF walls, where it’s part of the formwork	Can soak up water over time if left permanently wet
Spray foam	Varies by product	Odd shapes: around stock tanks and plumbing. It’s what shop-bought shell tubs use in the cabinet	Use a closed-cell foam anywhere it could get wet
Mineral or wool insulation	Varies	Dry, enclosed spaces above ground only	Loses its insulation value when damp, and rodents love it

The polystyrene figures are from Wikipedia’s polystyrene article. It also notes that both EPS and XPS can become waterlogged over long periods below ground, and recommends an impermeable layer on the outside. So wrap or face below-ground boards with a membrane where you can, and make sure the ground drains.

ICF blocks: walls and insulation in one

ICF (insulated concrete form) blocks come with the insulation built in. They go together like Lego for adults, and they’re light, which makes them perfect for one person building their own tub. Once the structure is up, you pour in the concrete. The concrete gives the walls their strength, and the expanded polystyrene gives the insulation.

TubBlox are ICF blocks made for hot tubs and pools. Their published spec is a 1000 x 250 x 300mm block with a U-value of 0.32 W/m²K. This is the approach I’d take if I built my tub again. There’s more in TubBlox kits and What are the best ICF blocks for building a hot tub?

Insulating a block-built hot tub

If you’ve read my other articles, you’ll know I built my hot tub out of block. What I didn’t do was insulate it. Yes, you read that correctly. I didn’t put any insulation in it at all. When I built it, I didn’t really know what I was doing. I made all the mistakes, and that’s a big part of why I’m qualified to help the 1,500-plus DIYers I’ve worked with since.

So how do you insulate a block-built tub? There are two ways, depending on how you build the walls.

Double skin with a cavity

If you’re building two skins of block with a cavity for the plumbing and rebar, fix insulation boards against the outer skin, then fill the cavity on the tub side with concrete around the rebar and plumbing. That keeps a solid, reinforced concrete core next to the water. Above ground, foil-faced PIR (Celotex and similar) is great here. Below ground, I’d use XPS.

Single skin

With a single skin of block, the insulation goes on the outside of the wall. You can fix the boards before you fit the plumbing and cut them around the pipes, or fit the plumbing first and fit the boards around it. There’s no right or wrong way. For an in-ground tub, use XPS, then backfill carefully so you don’t crush the boards or the pipework. There’s more in Cinder block hot tub: a DIYer’s guide.

Just be mindful that you’re creating a warm, dry place that’s very attractive to any rodents in your garden or backyard. I’ve picked a few out of my control room over the years that have passed on! Seal the gaps and use mesh over any openings.

Insulating under the floor

This is where I’ve changed my mind. When I first wrote this, I said I preferred not to insulate the base, because I was worried about its structural integrity. I also questioned whether it was worth it when most heat is lost from the top.

Today I’d insulate under the slab of any in-ground tub. Most heat does go from the surface, but that’s what the cover is for. Once the cover’s on, the floor is sat on cold ground 24 hours a day, every day. The structural worry is solved by using the right board. A compression-rated XPS board made for under-slab use goes on the DPM, and the reinforced slab is poured on top, just as in a house floor. The full build-up is sub-base, sand blinding, DPM, insulation, then a reinforced C25/C30 slab. You can see it layer by layer in my DIY in-ground hot tub build guide. If you’re unsure about loads, ask your engineer or follow your plans.

Insulating a poured concrete hot tub

Adding insulation to a poured concrete tub is quite straightforward. If you’re planning to have the plumbing inside the walls, they’ll need to be at least 10in (250mm) thick to fit the gunite bodies. You can fit insulation boards around the plumbing inside the formwork before you pour, but make sure the rebar cage is good and there’s enough concrete on the water side of the board for structural strength. The easier option is usually to put the boards on the outside face of the wall, against the ground. If your plans don’t show where the insulation goes, check with your engineer before you pour. See DIY hot tub construction methods for more on poured walls.

Insulating a stock tank hot tub

If you’re not familiar with them, stock tank hot tubs are DIY hot tubs made from galvanised steel (or sometimes plastic) animal feeding troughs. They’re very popular, very trendy, and a slightly different challenge to insulate. A thin steel wall loses heat quickly, and whether the tank is in-ground or above ground makes a difference.

• Above ground: the jets often don’t go all the way round, to keep the rustic look, so there’s no way to insulate the outside without the insulation showing. If the tank is partly surrounded by a deck, spray foam or wool insulation behind the decking works well. Seal it properly, especially wool, because it makes a lovely nest for rodents.
• In-ground: spray foam is an option, or scored rigid board that bends around the curve of the tank. I’d use XPS for that, not PIR. Don’t use wool in the ground. It soaks up water, and once it’s damp it loses its insulating properties.

There’s more in How to build the ultimate DIY stock tank hot tub.

Common insulation mistakes

• Spending on a heat pump and not on a decent cover.
• Leaving the walls and floor bare because “concrete holds heat”. Thermal mass stores heat, but it isn’t insulation.
• Using wool or unprotected foil-faced boards in wet ground.
• Crushing the boards or pipes when backfilling.
• Leaving gaps that rodents can get into.

If you want the bigger picture on running costs, read How I saved 66% on my hot tub electricity cost and The guide to air source heat pumps for hot tubs.

Planning your own build?

My DIY hot tub plans show you the full build-up, including where the insulation goes, alongside the CAD drawings, plumbing layout, shopping list and video course. For heat pumps and other parts, visit parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

FAQs

What is the best insulation for a DIY hot tub?

For the walls of a new build, ICF blocks, because they’re structure and insulation in one. For boards behind block walls and under the slab in the ground, compression-rated XPS. And whatever you build, a good insulated cover.

Should I insulate under my hot tub slab?

For an in-ground tub, yes. Lay compression-rated XPS on the DPM and pour the reinforced slab on top. It’s very hard to add later.

Can I use Celotex or PIR boards below ground?

PIR works best where it stays dry. For boards in contact with wet ground or backfill, I’d use XPS, which is made for below-ground and under-slab use.

How do I insulate a stock tank hot tub?

Use closed-cell spray foam, or scored rigid board bent around the tank, then seal it. Avoid wool in the ground, because it loses its insulating value when damp.

Does insulating a hot tub really save money?

Yes. Less heat lost means the heater or heat pump runs less. How much you save depends on your climate, tariff and how you use the tub, but the cover and the ground-contact surfaces make the biggest difference.

How are you insulating your build? Let me know in the comments below. I read them all.

Cheers,
Andi

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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