XPS insulation between concrete slab and hot tub?

Ian H said:

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

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53 square cm = 0.0053 square m.

Multiply the mass in kg by 9.8 (say 10) to get the weight in Newtons. Say 16,000 N.

Pressure in Pascals = weight / area = 16,000 / 0.0053 = 3,000,000 Pa.

So no, that’s off by a factor of 10. It needs about 500 sq cm.

(But I think these numbers for 10% compression are really the destructive loads; I think these materials don’t compress at all and then crush irreversibly. If the specs quoted “recommended” loads or something, that would be more useful. But anyway, you’re certainly OK with 1600 kg over 4 square m.)

Mmm. I have friends who installed a premium hot tub 10 years ago. They have had to do maintenance and as others point out, there is usually polystyrene insulation inside the outer cladding. But maybe not underneath.

If it was an option I may have insulated under the slab. However, you've poured that and now talking about the upper layer? Even if you were to insulate the upper layer, with concerns about concentrated pressure points, there would still be some airflow under the hot tub wouldn't there?

If so, the insulation under is of very limited value. I wouldn't bother with the hassle.......IMO

I too would not bother, but if you feel you have to, why not construct some sort of wooden frame on the slab out of 4x2 - kind of like a decking base with joists spaced appropriately to support the tub and fill the gaps with 100mm eps.

GoodDIYjob said:

If it was an option I may have insulated under the slab. However, you've poured that and now talking about the upper layer? Even if you were to insulate the upper layer, with concerns about concentrated pressure points, there would still be some airflow under the hot tub wouldn't there?

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I’m not concerned about the pressure point now I (sort of) understand the figures.

There will be some airflow but I think with the insulation there won’t be a mass of concrete trying to absorb the heat.

pilsbury said:

I too would not bother, but if you feel you have to, why not construct some sort of wooden frame on the slab out of 4x2 - kind of like a decking base with joists spaced appropriately to support the tub and fill the gaps with 100mm eps.

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I’ve ordered the 20mm stuff now. I reckon in 10 years everyone will be using the Howarth Technique and insulating under them