XPS insulation between concrete slab and hot tub?

[Ian H]

I can’t quite decide if this is a good idea. I’ve laid a 125mm thick concrete slab for the hot tub and was thinking to put Celotex under it to retain the heat. I e-mailed them and they said it wasn’t suitable and to use something with a higher compressive strength.

Now I’m thinking 20mm of this but not sure if the numbers add up:

And the spec:

There will also be plastic grass between the tub and insulation but if it would be better I could cut that around it.

Any advice would be much appreciated.

 

[SpecialK]

Is it a proper hot tub or an inflatable one? If its a proper hot tub then ask the manufacturer if its worth it?

 

[Ian H]

Good idea, it’s a proper one so I’ve e-mailed the distributor to ask, although, they have the staff pictures on the website and they look like a load of car salesmen.

 

[Ian H]

E-mails to their sales and info address’s both bounced back

 

[SpecialK]

Eeek! Have you paid a deposit? Who makes the hot tub?

E-mails to their sales and info address’s both bounced back

 

[Ian H]

It’s a Platinum Spas but I’ve bought it through a local shop.

 

[freddiemercurystwin]

Would think something like this would be more suitable - design to take higher loads. https://www.jablite.co.uk/application/jablite-fillmaster/

 

[Ian H]

Thanks for that

I don’t understand those figures either so I googled the difference between EPS and XPS and there doesn’t seem to be much.

I’m going to give it a try and if it fails for whatever reason I can always get rid of it or swap it for something else.

 

[endecotp]

How much does the hot tub weigh, and over what area is that weight distributed?

As an example, if it has a flat bottom, then tonne over 1 sq metre applies an even pressure of 10 kPa. Compare with the quoted 300 kPa for 10% compression of that XPS board.

On the other hand if it puts all the weight on four feet each of 5cm x 5cm, that’s 1000 kPa under those feet.

 

[Ian H]

It’s spread through a flat bottom that measures 2.1x2.1mtrs. It weighs 300kg dry but must hold at least a ton of water, I’m not sure of the exact volume.

 

[Ian H]

It weighs 1595kg when full.

https://miamispas.co.uk/wp-content/uploads/2018/10/spec-sheet-SANTORINI.pdf

 

[endecotp]

1600 kg / 2x2m = 4 kPa.

 

[Ian H]

Thanks endecotp

Does the 300kpa mean that the 1595kg could sit on as little as 53sq/cm?

 

[bsr]

Hello, why do you want to insulate under the tub? Any decent tub has insulation sprayed around and under at manufacture. Heat rises so you would do better buying a thicker cover.

Tubs I've seen have a void under the insulation for the pump, controls etc. You wouldn't want to insulate that area as the pump could over heat.

 

[Ian H]

I think the heat will travel all ways so don’t want to be heating the concrete slab.