DIY Hot Tub Insulation - Everything you need to know!

DIY Hot Tub Insulation is essential to keeping those running costs down on your tub. As we see fossil fuel prices increasing all over the Word, more now than ever the insulation on your tub is really important.
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TubBlox Kits - buildahottub.c...
What about TubBlox Insulated Concrete Forms?
TubBlox Kits from Buildahottub.com are Insulated Concrete Forms or ICF Blocks. These Insulated Concrete Forms as the name suggests comes ready-made with the insulation.
The way that the TubBlox work is that you put them together rather like “lego for adults”. They are lightweight so perfect for the individual DIYer building their own insulated DIY Hot Tub.
DIY Hot Tub Insulation
DIY Hot Tub Insulation
Once you have the structure ready, you then pour in the concrete. The concrete gives the TubBlox their strength and the expanded polystyrene that are used in their construction gives them their insulation or R-value credentials.
This is a great way to build an Insulated DIY Hot Tub. You can read more about TubBlox Kits here
DIY Hot Tub Insulation for Block Built Hot Tubs
If you have read other articles on my site, you will know that I actually made my hot tub out of block. What I didn’t do however, was insulate it. Yes, you read that correct - I did not put any insulation in it.
The thing was, that when I built my hot tub - I didn’t really know what I was doing. As I have said many times over, this journey to helping over 600 DIYers like you means I am very well qualified to help - I made all the mistakes.
On, so how do we insulate a hot tub that is built of blocks?
Really, there are two ways for insulating block built tubs. If you are building a double skinned cavity, then inside, with the plumbing and the rebar, you can add some of the PIR boards - they come from various manufactures like Celotex - they all do the same job.
These foil backed boards can then fit inside of the cavity, adding insulation to your DIY Hot Tub then you will fill the cavity with concrete.
If you are doing a single skin of block, then you have two options. You can either put the PIR boards onto the structure before you fit the plumbing, or you can add the plumbing, then add the Pic boards around the plumbing. There is no right or wrong way of doing this.
Just be mindful that you are creating a dry warm place which will be very attractive to any rodents that you might have in your back yard - I must say I’ve picked a few out that have passed on from my control room over the years!
DIY Hot Tub Insulation
DIY Hot Tub Insulation
Insulate Blocks
DIY Hot Tub Insulation for Poured Concrete Forms
Adding insulation to your DIY Hot Tub formed of poured concrete is actually quite straight forward.
In terms of the base itself, this one is always open to debate. Personally, I prefer not to insulate the base itself as I am always a little concerned about the structural integrity. However, if the insulation is placed underneath the crushed aggregate and then the concrete pad is poured on top, arguably, the structural integrity would be just fine.
However, for me the question is always whether this approach is actually worth it when most heat is lost vertically. I leave you to decide this one on your own.
Adding insulation into your poured concrete form walls just means leaving a little extra space. If you plan on having the plumbing inside your walls, they are going to be 10” at a minimum thick to accommodate the Gunite bodies.
This is fine and you can use PIR board around the plumbing before you pour. Make sure your rebar structure is good and that there is concrete on either side of the PIR for structural integrity and you will be just fine.
DIY Hot Tub Insulation for Stock Tank Hot Tubs & Pools
Stock Tank Hot Tubs if you are not familiar with these are the DIY Hot Tubs that are made from the steel animal feeders. They can also be plastic, but generally, these are the galvanised steel tanks.
Very popular and very trendy, these kinds of DIY Hot Tubs present a slightly different challenge when it comes to insulation. Firstly, there are different challenges if they are in ground or above ground.
Above ground, quite often, the jets do not go all the way around to give a more rustic look. Therefore, it would be pretty much impossible to create an insulated tank that looked good as there was be exposed insulation. However, if these are surrounded in part by a deck for example, spray foam as well as the wool loft insulation is always popular. Again, especially with the wool this makes a good nest for rodents so make sure it is well sealed.
The spray foam you can pick up in any good hardware store. This is pretty much the method of insulation that is used on regular plastic shell tubs underneath in the cabinet. Foam is sprayed in.

[@davidianwalsh]

Hiya, good video - I'm a civil engineer - if you're trying to keep something warm (or cold) ALWAYS insulate under the concrete slab - It's now mandatory for houses and even more important if you have underfloor heating where you have to add even more insulation above the slab. So the same logic applies to hot tubs.

[@Buildahottub]

Thanks for the comment Dave - really good to know! Where would you put the insulation for the slab, under the ballast or physically inside the slab with the rebar and just make it thicker so the strengh is not compromised?

[@davidianwalsh]

@@Buildahottub lay and compact the sub-base (ballast) then lay a Damp Proof Membrane (DPM) then the insulation - 100mm minimum of PIR, then more DPM so that the insulation is encapsulated and cannot become wet. The concrete base can then be poured on top of that.

[@leereynolds8999]

Our pools are always insulated under the slab even if there is a sub floor

[@jaredvdg9]

I work for a general contractor and I've never heard of dampproofing on both sides of the insulation for underslab insulation. Is that because you are using Polyiso insulation? We would typically use XPS insulation (which can get wet) under slabs with no damproofing on either side - Just 6 Mil or 10Mil Poly above the insulation . @@davidianwalsh

[@superandy89]

Sit on a concrete bench in the winter for 10 minutes, then revisit the idea of insulating the slab 🥶

[@R32SWEET]

Thanks for showing the different styles. Can foam be the inside layer with a liner over that?

[@Buildahottub]

Thanks for the comment. Potentially yes, although the foam is quite soft so you have the potential for dents in your liner prety quickly that would not look great. Also, for hot tubs I dont recommend liners at all as it is very hard to find jet bodies that work with liners - as one of my case studies "Brandon" on www.buildahottub.com if you check it out explains. - hope that helps Andi

[@R32SWEET]

@@Buildahottub makes Sense, thanks 👍

[@jaredvdg9]

Hi Andy, I am wondering about the heat loss from the jets and plumbing? Is it possible to put all the plumbing in an 8" poured concrete wall and then insulate outside of that, so that the plumbing is also all insulated? Or is this a bad idea, since you can not access the plumbing in the event there is a leak?

[@Buildahottub]

Quite the question Jared - there are two schools of thought on this as you point out. Depends how confident you are on your plumbing - no reason it cant go inside of a wall - it is designed to do so. However, access is quite handy, especially if you spring a leak - what I dont suggest is doing 50/50 - I did on my own and it was a right pain when i needed to do some modifications! Thanks for the comment - Andi

[@SolarGuy605]

I live in a coldish climate. I want to use a block tub that will be insulated daily, what temperature would you recommend keeping the tub at?

[@Buildahottub]

Hi Andrew, a lot of that depends on the type of heating you have. I've found with my own block tub that the most economical way of running it is holding at 30C / 86F and then taking it up to 39C / 102F when i want to use it which for me is weekends only - with my air source, that takes about 90mins on average, a little longer if it is cold out. After use, i drop it down again. If you plan to use it daily, it might be worth putting in a system where you can schedule the heat (Control My Spa from Balaboa can do it) In that way, you could drop your temp down overnight to reduce running costs, if you were well insulated it would not loose too much, then heat it back up ready for use at the times you would be using it. All possible - depends on your heating method as I said (electric is slow for example) and the times you plan to use it - hope that helps! - Andi

[@SolarGuy605]

@@Buildahottub huge help! Thanks for the thoughtful response. I plan on heavily insulting. I don't think the air source heat pump will be a good idea here in the Pacific Northwest since when I am going to want to use the tub the most will be in the winter.