What happens on your property, day by day
- Days 1-3: excavation and blinding. The pit is dug 25 centimetres oversize on every face to leave room for formwork and insulation, then a 50-millimetre blinding slab is poured so the reinforcement never touches soil.
- Days 4-6: steel and formwork. Ten-millimetre deformed bar in two layers at 150-millimetre centres, 40 millimetres of cover held with spacers, and the 1.5-inch drain and 2-inch suction lines cast in now. Nothing is chased into this concrete afterwards, ever.
- Day 7: one pour. Walls and floor go in on the same day, so there is no horizontal cold joint anywhere below the waterline. A 900-litre tub takes roughly 2.4 cubic metres.
- Days 8-17: curing and waterproofing. Concrete is kept wet for ten days, the formwork strikes on day ten, and two coats of crystalline cement membrane are brushed onto the inside. That membrane reacts with moisture in the concrete and self-seals hairline cracks up to 0.4 millimetres.
- Days 18-25: microcement and external insulation. Four coats totalling 3 millimetres, each sanded before the next, closed with two coats of two-part polyurethane. The outside of the walls is wrapped in 50-millimetre XPS before the ground is backfilled.
Why the insulation goes outside the concrete
Soil around the tub sits at 26 to 28 degrees all year in Java and Bali. That is cooler than afternoon air and enormously warmer than 4-degree water, so without a break the concrete wall becomes a heat fin drawing warmth in continuously. Fifty millimetres of XPS on the outside cuts that path, and the concrete inside it becomes part of the cold mass instead. A 900-litre sunken tub built this way holds at about 890 watts in Jakarta — close to the same volume standing above ground, despite far more surface in contact with its surroundings.
Concrete gradeK-300, 12 cm slump
Cover to steel40 mm all faces
Membranecrystalline, self-seals cracks to 0.4 mm
Microcement thickness3 mm total in four coats
Top coattwo-part polyurethane, two coats
External insulationXPS 50 mm, R-value 1.7
Holding load, 900 L sunken, Jakartaabout 890 W with the lid on
Four coats of microcement, not one thick one
Microcement is a polymer-modified cement applied about a millimetre at a time. Forced on thick in one go, the inside dries far slower than the face and the result is crazing within months. Four thin coats with sanding between them fuse into one layer with no joints and no grout lines, which is the whole reason people choose it for a sunken tub. Pigment goes into the third and fourth coats, so a small scuff does not reveal a different colour underneath. Nine standard colours run from pale grey to graphite plus cream and sand; a custom colour needs a 40 by 40 centimetre test panel first, because wet microcement reads two shades darker than sealed and dry.
What the owner has to have ready before we arrive
- Access for a mixer truck, or room for a site mixer. If the lane is narrower than 2.2 metres the concrete has to be mixed by hand and the programme grows by three days.
- A working water point and a 1,300 VA supply near the pit for the wet sander and pumps.
- Certainty about what is underground to 1.2 metres: city drains, cables, and large tree roots. Banyan and ketapang roots will lift a tub wall inside five years.
- Somewhere for 900 litres to go on a water change, able to take it in about ten minutes.
- A chiller position under cover, within six metres of hose run, not facing a blank wall.
Leases, pools and the rest of the garden
Two questions come up almost only in English. The first is the lease: a cast tub cannot be moved, cannot be resold and cannot be taken with you, so on a 15 or 25-year leasehold we ask you to think about the remaining term before you ask us about colour. The second is the pool. If a swimming pool is being built or renovated at the same time, the plant room, drainage and electrical work should be shared, and we will coordinate directly with your pool contractor. When there is a sauna, a shower and a deck in the same plan, the sensible route is a single programme under custom wellness areas rather than three trades arriving in three different months.
When casting in place is the wrong decisionThis tub cannot be moved, cannot be sold on, and removing it means demolishing reinforced concrete. For a rented villa, or land whose five-year future is unsettled, buy a freestanding shell. The work is also noisy and dusty in your garden for three weeks, so neighbours sharing a wall deserve warning. If you want the sunken look without a month of site work, a terrazzo ice bath dropped into a pit needs only a hole and a base slab — three days.
Price, and what is deliberately excluded
The range is Rp 35-65 million for 400 to 1,600 litres, covering excavation in normal ground, concrete, steel, membrane, microcement, external insulation, pipework, the lid and the crew's time on site. Excluded: breaking out existing tiles or slabs, diverting existing drainage, rock excavation, and the chiller. For a first estimate of the cooling, put your intended volume into the chiller calculator; a 900-litre concrete tub usually lands on 3 PK in hot lowland. All component prices are on the pricing page, and what our crew does and does not do is set out under installation and cold test.
One more point about time. Microcement is not fully cured for 28 days. During the first four weeks we ask you to run plain water with no high ozone dose and no cleaning products at all, because an immature surface still absorbs. After that date it stops caring.
Do in-ground concrete tubs start leaking after a few years?
Almost every leak in a concrete tub is a pipe penetration installed after the concrete set, not the concrete itself. That is why every line is cast in before the pour and not one hole is drilled afterwards. The crystalline membrane closes hairline cracks up to 0.4 millimetres on its own while any moisture remains in the slab.
Can it be built indoors, in a ground-floor wet room?
Yes, provided the existing floor can be broken out and whatever is underneath can carry about 1.6 tonnes for a 900-litre tub. On an upper floor it almost never makes sense; the point load exceeds nearly every residential slab.
How long before it can be filled?
Seven days after the final polyurethane coat. The first fill is plain water, left 48 hours so we can watch the level before the chiller runs. If the level drops more than 3 millimetres in 48 hours we find out why before going any further.
Can we add a sauna later?
Tell us now even if the build is next year. We will cast empty conduit and a cable route to your future sauna position while the concrete is being poured. About Rp 2.5 million today, against demolishing a finished terrace later — the combined layout is described under sauna and ice bath packages.
Who does the landscaping around it?
Your own landscaper, and we prefer it that way. We hand over the finished tub, the levels and the drainage falls; paving, planting and lighting are cheaper and better done by someone already working on your garden.
