How it works
Most of what makes a Talpa house work is not a product you can buy. It is the way the parts cooperate. Concrete that seals itself. Earth that stores heat. A roof that holds its own rain. Light that reaches the back of the house. Air that moves without a fan. Water that runs in a loop. And a structure that can be rearranged long after we have gone. Here is each part, and why it is built the way it is.
01Shell
One pour, no seams, no membrane.
Walls and roof are cast as a single continuous piece of watertight concrete. The concrete itself is the waterproofing, so there is no separate waterproofing membrane and no separate radon membrane.
- No waterproofing membrane
- Around €30,000 saved
- Radon-tight without a membrane
5A conventional underground wall
- 01Structure
- 02Insulate
- 03Waterproofing membrane
- 04Protection layer
- 05Drainage
1A Talpa wall
- 01Watertight concrete
Five layers against one. Leaving out the membrane saves around €30,000.
- 01Crystalline sealing
- 02Bentonite joints
- 03A mix tuned to the day
- 04No membrane to age
That matters for two reasons. The membrane is the most common long-term point of failure in underground building, and a leak below ground is not a repair job: it means digging the house out. Leaving out the membrane also saves around 30,000 euros on the build.
The concrete carries a crystalline admixture, so when a hairline crack forms, the material grows crystals on contact with water and closes it. Construction joints are sealed with bentonite, which swells the same way. And because the mix is tuned to the actual weather on the day of the pour, it cures without cracking.
Radon is dealt with by measurement and by the airtight concrete structure, not by a membrane that ages and tears.
02Earth
The ground does the heating and the cooling.
The soil around and above the house is not just backfill. It is a battery that stores the sun's heat for months.
- Soil at the house 8–24 °C
- About 40% less insulation
- About 90 heating days
Weekly lowest and highest. Earth: three sensors by one of our houses near Banská Štiavnica, read every hour from January 2024 to March 2025; in weeks the logger was not running, the earth temperature is estimated from the air temperature. Air: Sliač weather station, data from Meteostat.
Three-hour means, 10 Dec 2024 – 6 Feb 2025. Earth and room: sensors in one of our houses near Trnava, the earth 0.5 m from the buried wall under 1.7 m of fill. Outside: Bratislava-Ivanka weather station, data from Meteostat. The house was lived in and heated.
Heating days a year, house at around 500 m above sea level
We measure it. At one of our houses, over 14 months the soil stayed between 8 and 24 °C while the air swung from −12 to +35 °C, and the warmest weeks arrived in the soil six to seven weeks later than in the air. At another house, the soil just behind the backfilled wall held 13 to 17 °C from late November to early February instead of freezing.
Because the earth carries much of the load, we use roughly half the insulation a standard passive house needs: on every build, about 40 per cent less material and less embodied carbon.
The same mass that stores heat also makes the house quiet. Most of the walls and the whole roof are underground, and traffic noise stays outside with them.
03Roof
The land you build on comes back on the roof.
Every house takes up a piece of land. A green roof gives most of it back, as meadow, as vegetable beds, or as a garden with trees in a deeper layer of soil.
- Rain held on your plot
- Meadow or garden
- No roof covering to replace
- A
- Meadow: 8 to 15 cm of soil, sedum and grasses, no watering, nothing to maintain.
- B
- Garden: 40 cm of soil and more, beds and small trees, watered from the house's own water.
It holds the rain on your own plot instead of sending it into the public drains, which more and more authorities now require rather than merely prefer. It shades the concrete from the summer sun and buffers it against frost, so the shell spends its whole life at a mild temperature. And because it is soil rather than a covering, there is no roof material to ever replace.
04Light
Light that reaches the back of the house.
The layout is based on Socrates’ megaron, refined further. Living spaces sit at the front behind full-height south glass. The ceiling is higher at the front and lower at the back, which throws daylight 6 to 8 metres deep, strongest in the morning and the evening, when you are actually home.
- 6–8 m of daylight into the house
- Skylights from 80 × 80 cm
- No north windows
A covered terrace shades the high summer sun and lets the low winter sun pour straight in. Skylights and light wells, at least 80 by 80 centimetres, bring daylight down to the back rooms. Every habitable room is checked against local daylighting regulations, room by room, because that is what actually decides a permit for an underground house.
There are no windows on the north side, because a north window loses energy and gives back very little light. Leaving it out is not a compromise. It is the reason the south side can be as generous as it is. One of our homes has a single south window 17.5 metres long.
05Air
Fresh air without a machine.
Air enters through the windows and trickle vents, passes through the house and leaves through a duct at the back of the roof, drawn by the stack effect. No fan, no electricity.
- No fan
- No electricity
- No servicing
Talpa house
- 01Windows and micro-ventilation
- 02Pipes at the rear of the roof
- 03The chimney effect
- 04Nothing to service
- 05Nothing to replace
A conventional passive house
- 01A heat-recovery unit, bought and installed
- 02Electricity to run it, always
- 03Filters and servicing on a schedule
- 04Ducting through the building
- 05Replacement after 15 to 20 years
A standard passive house relies on a heat-recovery unit. We do without one, because the mass of earth around the house does the tempering that heat-recovery ventilation exists to provide.
As the air keeps moving, no condensation forms, not even where the insulation is thinner than in a conventional build. Radon and CO2 leave with the air instead of building up.
If local rules insist on a mechanical unit, we fit one. It changes almost nothing about how the house works.
06Water
A house that keeps its own water.
Rain filters slowly through the roof soil, a journey of about 24 hours, and collects in a layer of 3 to 7 cubic metres beneath the house.
- 24 hours through the soil
- 99% cleaned
- 90% back to the ground
- 01Rain lands on the roof.
- 02It sinks through the roof soil in about 24 hours.
- 03It gathers in a 3 to 7 m³ layer beneath the house.
- 04The household’s wastewater joins it.
- 05Both pass through the reed bed and come out 99% clean.
- 06It waters the roof and serves the house, and 90% returns to the ground.
From there it passes, together with the household’s wastewater, through a reed-bed treatment system that removes about 99 per cent of impurities. Plants and gravel do the work that a treatment plant would otherwise do with pumps and power.
The cleaned water irrigates the roof garden and can serve the washing machine and the toilets. Roughly 90 per cent of the water returns to the ground, and only about 10 per cent is truly used up.
The rain stays on your land, which is exactly what more and more municipalities now require rather than merely encourage.
That is the house. What would yours cost?
Shape a home in the calculator and see an honest estimate in a few minutes. The rest of this page is for the reader who wants the engineering behind it.
07Materials
Recycled concrete, and a build-up where every layer does two jobs.
We were the first in Slovakia to build with recycled-aggregate concrete, replacing about 55 per cent of the natural aggregate while still meeting our demands for waterproofing, workability and strength.
- 55% recycled aggregate
- About 0.5% of the build cost
- 100+ tonnes of material saved
- 01Watertight recycled concrete
The structure and the waterproofing, in one layer.
- 02Foam glass
Insulation made from recycled glass, and drainage.
- 03Reduced insulation
Roughly half that of a passive house, and only where the earth cannot reach.
- 04Root barrier
A barrier against roots, not a waterproofing layer.
- 05Earth
Insulation, thermal battery, sound barrier and fire cover.
- 06Green roof
Rain held, life returned, and the land given back.
It was developed and validated with the Faculty of Civil Engineering at STU Bratislava, with doc. Ing. Ivan Hollý, PhD., and published in the concrete industry's own literature rather than in a brochure.
For about half a per cent of the build cost, one house saves well over a hundred tonnes of new material and keeps a similar amount of construction waste out of the landscape. If concrete could be reused even twice, stone quarried once would serve for 100 to 160 years, four to seven generations.
Inside, where owners choose them, clay plaster and wood do quiet work of their own: clay keeps humidity steady, softens sound and keeps the air clean, which people with asthma tend to notice first.
08The build
What actually happens on site.
The shell covers foundations, walls, roof, waterproofing, insulation, retaining walls and backfill. Together that is about 65 per cent of the price, with insulation and waterproofing already included. Earthworks costs depend far more on your plot than on the house itself.
- 01/06
Excavate
The cut is shaped to the slope and the plan.
- 02/06
Cast the shell
Walls and roof in one continuous pour.
- 03/06
Seal and test
Joints, openings, and the water test.
- 04/06
Insulate
Foam glass and a reduced layer, where earth cannot reach.
- 05/06
Backfill
As early as the structure allows, above all on a slope.
- 06/06
Plant
The roof becomes ground again.
On a slope we keep the excavation open for as short a time as possible, because a waterlogged slope can move. As soon as the shell and the retaining walls are finished, we backfill. This is the part of the process that separates a specialist from a main contractor tackling an underground build for the first time.
The savings come later, and they keep coming: less insulation, no heat pump, no ventilation unit, no façade to repaint, no gutters to clean.
09What runs it
Fewer machines, on purpose.
The energy calculation required by building regulations assumes a machine recovers the heat. It does not recognise heat stored in the ground and in the green roof, so on paper it asks for a heat-recovery unit. We assess our houses with the Passive House Planning Package instead, which follows measured reality far more closely, and in it the unit turns out to help only minimally.
- About 90 heating days
- No heat pump
- No ventilation unit
In a Talpa house
- 01The earth, storing the heat
- 02A small heat source for the coldest weeks
- 03Solar panels, if you want them
In a house full of machines
- 01A heat pump
- 02A heat-recovery ventilation unit
- 03Air conditioning
- 04Replacing all three, every 15 to 20 years
So a Talpa house has no heat pump, no ventilation unit and no air conditioning. For the coldest weeks there is a small heat source, and solar panels if you want them. The earth does the storing that a heat pump would otherwise be bought to do.
Fewer machines are cheaper to install, more reliable, longer lasting and easier to repair. After years in the houses, our owners do not miss them.
And if you want to go further
Because the house needs so little to begin with, going off-grid is arithmetic rather than ambition. A small 2 to 3 kWp solar array covers most of the year. Full independence for a house this size takes about 12 to 16 kWp with a battery, against 29 to 38 kWp for a house with a heat pump and a heat-recovery unit.
None of this has to be decided now.
- 2–3 kWp for most of the year
- 12–16 kWp for full independence
Next
Now put it on your land.
Every number on this page describes a house in general. A feasibility study describes yours: your plot, your slope, your water table, your budget.