Hot TubDESIGN & CONSTRUCTION REVIEW
Coordination study · not for constructionEarlier review PDF
Our preferred V2 plan · 100 mm FOAMGLAS inside walls and floorSupporting wall outside → FOAMGLAS inside → waterproofing → mosaics.
See the simple plan

VERSION 2 / OUR PREFERRED PLAN

10 cm FOAMGLAS on the inside.

The supporting walls are outside the insulation. The mosaics and water are on the inside. This is the V2 arrangement we’re developing.

Your wall, from outside to waterConcept section · not to scale · supporting wall and finish thicknesses still to be designed
  1. OUTSIDESupporting wallConcrete / Ytong option*
  2. 100 mm / 10 cmFOAMGLASInside the wall
  3. SYSTEM TO CONFIRMSupporting coatSuitable base for the finish
  4. CONTINUOUS SEALPool sealWaterproof coating, including corners and jet fittings
  5. VISIBLE FINISHMosaicsCompatible adhesive + grout
  6. INSIDEWarm waterThe bathing space

FOAMGLAS is fully supported and bonded to the wall using the specified system. The 100 mm is your proposed design thickness; the boards in storage still need measuring.

The idea, in three steps.

  1. The outer structure supports the tub.Reinforced concrete is the working structural option. Ytong is an alternative to investigate with the engineer, including its ability to resist the water pushing sideways.
  2. 10 cm FOAMGLAS lines the inside.On the walls and, as agreed, the floor. The floor proposal also uses 100 mm above the structural concrete base.
  3. Charlotte’s mosaics finish it.Over a suitable supporting layer and waterproofing—not directly onto the boards.

In this proposal, there is no insulation underneath the tub’s concrete base. The floor insulation sits above it, inside the tub.

*Concrete and Ytong are not interchangeable specifications. Wall thickness, restraint, joints and connection to the base need a structural design. The insulation and coating do not supply that design.

The shape you liked stays.

We’re keeping the organic bench, two massage seats, 12 planned jets, stairs and walking glass as the design intent.

The 3D model now shows V2: concrete outside and 100 mm FOAMGLAS inside both walls and floor. The PDF remains an earlier reference; its outside-insulation layers are superseded.

Explore the V2 exploded model
Preferred plan—not ready to build yet.

Two checks remain: an engineer must confirm the structure above the kitchen can carry the tub, and the suppliers must confirm that your exact FOAMGLAS boards work with the waterproof mosaic lining.

Technical details and earlier options Only open this if you want the full construction study.

No pouring, filling or insulation removal from this study. The approximate 10-tonne weight remains an unverified scenario. The 100 mm insulation is a proposed design thickness, not a measured stock thickness. Permanent support, construction loads and the complete waterproof lining need approval.

01 / COMPARE THE LAYERS

Compare with the earlier options.

These temporary comparisons do not change our preferred plan.
V2B · preferred planTop → bottom / water → kitchen · not to scale

Select a layer to inspect its job and unresolved details. Wall layers are stacked for readability, not shown in perspective.

Thermal insulation above the RC base only—not beneath it

MY PREFERRED INVESTIGATION

V2B: keep a thermal break inside the tank.

Ask the engineer and pool-system supplier to investigate an internally insulated floor as well as walls. The RC shell can bear through a designed non-insulating interface, while the thermal lining sits inside it.

This separates the shell’s own weight from the insulation, but not all loads: water pressure, people, tile carrier and any seats supported on the lining still load the floor insulation. Full support, local loads (including empty-tub maintenance) and the complete assembly must be checked. It may also reduce the available water depth. The shell’s weight bypasses the insulation only with its own engineered bearing path to the supporting structure.

EARLIER WALL-ONLY OPTION

A warm kitchen is not zero heat loss.

For example, water at 38 °C above a room at 21 °C still has a 17 °C temperature difference. Heat can travel downward by conduction. The resulting ceiling heat may help in winter but be unwanted in summer. HT

If you choose V2A, first quantify the whole floor / edge heat loss, warm-up behaviour and summer comfort. Keep the insulated lid in either option. Do not remove the surrounding roof insulation or compromise the kitchen’s weatherproof roof.

02 / SMALL THERMAL EXPLORER

Thickness and temperature—not a running-cost forecast.

Insulation layer only · approximate R2.44 m²·K/W
Water minus kitchen temperature17.0 °C

Thermal drive from tub toward kitchen.

R ≈ thickness (m) / 0.041 W/(m·K), using the manufacturer’s declared conductivity limit. This arithmetic is not a certified assembly R-value; published product R-values are rounded. It excludes all other layers, thermal bridges and surface effects. The value applies only where this insulation is present—not the V2A floor. The input range does not certify any temperature as safe for users or materials. FG

03 / KEEP CHARLOTTE’S MOSAIC

One agreed system, from concrete to water.

REAL PRECEDENT · NOT A RECIPE

Internal cellular glass + waterproofing

SCHOMBURG’s Seesteg reference uses FOAMGLAS inside the pool, with UNIFIX-S3 fixing/coating, AQUAFIN-2K/M-PLUS waterproofing and a natural-stone finish. It proves that the concept has been built—not that those products are approved on your board facing or for your mosaic spa. SP

SIKA ROUTE · CONFIRM SUBSTRATE

Mineral carrier + under-tile membrane

Sikalastic-1K ES is a Belgian-market under-tile pool-waterproofing candidate on its specified mineral substrates. It is not verified here for direct application to BOARD T4+. Ask Sika to specify the entire carrier-to-mosaic assembly for the actual heated, treated water. SK

FINISH CANDIDATE · NOT THE TANK SEAL

Mosaic + compatible epoxy system

SikaCeram-880 Easy Epoxy lists glass mosaics, pools and thermal baths among its applications. It is a candidate adhesive / grout, not a substitute for waterproofing. Confirm the chosen mosaic backing, chemistry and complete-system compatibility. Do not mix the Sika and SCHOMBURG routes without written agreement. SE

What makes it watertight?

The continuous waterproofing below the tile, its designed floor / wall / rim joints, and its sealed connections to every fitting—not the mosaic, grout or water-resistant insulation alone. Use an approved immersion-rated assembly and its curing, inspection and test-fill procedure. PL

04 / HOW WE WOULD DEVELOP IT

Build the support first. Resolve every interface before pouring.

  1. Survey and structural design. Establish the existing kitchen slab, reinforcement, supports, GL1 opening and roof layers. Confirm the full load path and fresh-concrete / temporary-works loads. The 10-tonne scenario is not a measured final weight. BW
  2. Agree the new-concrete interface. Engineer bearing and the tank shell. Specify whether the new base is separated or structurally connected; simply pouring against old concrete does not create a verified composite slab. No improvised dowels, concrete pins through insulation, or loading of the glass frame.
  3. Freeze the liner and fittings. Have FOAMGLAS and one pool-waterproofing supplier confirm the exact board, facing, bond, carrier, joints and spa service conditions. Cast the selected fitting bodies / sleeves in coordinated locations; verify all required flange extensions for the new internal lining depth.
  4. Form the organic seating on a verified support. Keep the flowing, wall-to-wall shape and two massage seats plus bench. Develop it as a supported mineral shell or another fully approved pool assembly; do not rely on a cosmetic skim over loosely stacked boards. A solid extra concrete seat/inner shell adds weight and must be counted.
  5. Make the sealed bowl, then tile. Install the approved internal lining, waterproof the complete basin and seat surfaces, connect the fitting flanges, and follow the system’s inspection / test-fill sequence. Tile only when the substrate and membrane have been released. Any water test requires structural release first.

Keep all 12 massage positions—but redesign the penetrations.

The visual intent remains M1 × 4, M2 × 4 and bench × 4. Moving the waterproof face inward changes jet-body lengths, flange locations and service access. Also coordinate suction, circulation inlets / returns, drains and any lights. Bond the membrane to suitable fitting flanges; sealant around a bare pipe is not the complete assembly. PL

The coloured piping view now uses the V2 tank layers. Its routes and jet locations are retained concept lines, not sized pipes or detailed penetrations. Equipment replacement through H1 and safe pump suction still need specialist checks.

Outer footprint and seating retained; finish allowance still needed.

The V2 model assigns the outer 100 mm band to concrete and the inner 100 mm band to FOAMGLAS. This retains the existing wet outline and the GL1 relationship. Concrete thickness is a study assumption; carrier, adhesive and mosaic thicknesses are still to be allowed for in the final dimensions.

Recheck seat depth, knee space, water depth, jet centres, coping, lid seal and landing levels after the complete build-up is chosen. Existing 100 mm RC dimensions and the old PDF must not be treated as V2 construction dimensions.

05 / YOUR MATERIAL ON SITE

The three reference photos.

The label identifies BOARD T4+. Nominal thickness, condition, batch and facing compatibility still need confirmation. The board is not the same as a bare T4+ slab. Do not peel off its facing based on this study.

Stored FOAMGLAS board
Board / factory-facing photograph · Open original
Cellular-glass edge beside an angled tape measure
Edge / thickness to verify from label · Open original
Black and green FOAMGLAS BOARD T4+ label
BOARD T4+ identification · Open original

The manufacturer lists λD ≤ 0.041 W/(m·K) and declared compressive strength ≥ 600 kPa. The latter is a test declaration, not the allowable pressure for this project or proof of the stored boards’ condition. Confirm design stresses, local loading and the complete supported assembly.FG

What we need before detailing the preferred plan

Our preferred direction is V2B: insulation inside the walls and floor. If the stored faced board cannot be approved for an internal pool system, compare an approved bare-cellular-glass system or keep an engineered V1 support solution. Do not force the existing stock into an unsuitable wet assembly.

Information to obtain: the board’s nominal thickness / batch label; proposed maximum spa temperature and water treatment; exact mosaic and backing; and the engineer’s survey of the kitchen slab. Then request one written, compatible layer schedule including all 12 jets and the organic seats.

MANUFACTURER & TECHNICAL REFERENCES

Evidence, and where it stops.

SP

SCHOMBURG · Seesteg pool precedent

Real internal FOAMGLAS pool insulation with waterproofing and a stone finish. Exact BOARD T4+, hot-spa conditions and mosaic are not established.

PL

SCHÖNOX / Sika · Pool Line

Fitting-flange connections, underwater mosaics, bedding and movement-joint guidance. Not a FOAMGLAS-board system approval.