A thick south-facing thermal-mass wall sits behind double glazing, soaks up the sun by day and releases the heat slowly into the room at night. Tune the wall material, thickness, glazing, air gap, solar irradiance and outdoor temperature to see net heating and solar fraction update in real time.
Parameters
Wall area
m²
Wall thickness
cm
25-40 cm gives 6-12 hr thermal lag
Thermal mass
Sets density, specific heat, conductivity, absorptance
Glazing
Sets U-value (thermal transmittance)
Air gap
mm
Air gap between wall and glazing (drives vent flow)
Solar irradiance
W/m²
Outdoor temp.
°C
Indoor setpoint
°C
Glazing transmittance
%
Results
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Wall capacity (MJ/K)
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Solar gain (kWh/day)
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Heat to room (W)
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Glazing loss (W)
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Net heating (kWh/day)
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Solar fraction (%)
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Trombe wall cross-section animation
A thick thermal-mass wall behind south-facing glazing absorbs sunlight, drives a vent thermosyphon during the day and releases stored heat to the room at night.
Room temperature vs time (24 h)
Thermal capacity by material
Theory & Key Formulas
$$Q_{net} = G \cdot A \cdot \eta \cdot \alpha - U_g A (T_i - T_o),\quad \tau = \frac{\rho c_p V}{hA}$$
G: solar irradiance (W/m²), η: glazing transmittance, α: wall absorptance, U_g: glazing U-value (W/m²K), τ: wall time constant (thermal mass / convective transfer).
Trombe Wall Passive Solar Heating — Storage and Convection
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"Trombe wall" — first time I've heard the name. Is it basically a wall that hoards the sun's heat?
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Exactly. Roughly speaking, it's just "a thick south-facing concrete wall with double glazing in front". French physicist Felix Trombe and architect Jacques Michel published the design in 1957, and it was first built in 1967 at Saint-Félix-de-l'Aude in the Pyrenees. No pumps, no electricity — it is a purely passive system.
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I can't picture heating without electricity though — how does the heat actually reach the room?
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Two stages. During the day, sunlight passes through the glazing and warms the wall surface to 50-70°C. Because concrete has a large heat capacity, the heat seeps slowly toward the inner face — that "thermal lag" is the τ in this tool. For a 30 cm concrete wall the lag is roughly 6-12 hours, so the inner face peaks right when it gets cold at night. At the same time, small openings (vents) at the top and bottom of the wall drive a thermosyphon: warm air rises in the air gap and pours into the room.
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When I picked the water tank, the numbers jumped right up. Is water that good as a storage material?
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Water's specific heat is 4186 J/kg/K — about 5x that of concrete, so the same volume stores roughly 4x as much energy. In the 1970s, test houses at NREL and Sandia National Lab lined whole walls with 55-gallon drums of water — the "Water Wall". Leakage, corrosion and algae held back commercial uptake, so concrete and brick still dominate. Adobe (rammed earth) is another tough traditional choice, used for sun-tempered design on the China loess plateau and in the Anasazi pueblos of the US Southwest since about AD 1000.
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Switching glazing from single to double cut the loss by more than half. That matters most in cold climates, right?
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Right. Q_loss = U_g · A · ΔT, so the colder it is outside (larger ΔT) the more the glazing's U-value matters. Stick with single glazing (U=5.8) in northern Japan or Scandinavia and night loss can exceed daytime gain — "negative heating". Double glazing (U=2.8) is the realistic baseline; for Passivhaus-grade homes go to triple Low-E (U=1.0) and the night-time loss almost disappears. Try the slider and feel the difference.
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It also shows the CO2 saving — over 1000 kg per year looks pretty big.
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With the defaults (25 m² wall, 30 cm concrete, double glazing, 0°C outside) you get 30.7 kWh/day net heating, about 5,500 kWh per year and roughly 1,188 kg of CO2 avoided — equivalent to driving a mid-size car 5,000-6,000 km. Trombe walls show up in Passivhaus, LEED v4.1 and Japanese environmental design standards, and a Modified Trombe (Sankey Trombe with a selective surface) can add another 30% efficiency.
Frequently Asked Questions
A Trombe wall is a passive solar heating system proposed by Felix Trombe and Jacques Michel in 1957. A massive south-facing wall (concrete, brick, adobe or a water tank) is placed behind double glazing on the north-hemisphere side of a building. The glazing captures solar radiation, the wall stores it, and at night the heat is released into the room through a long thermal lag. Top and bottom vents drive a natural convection loop (thermosyphon) that covers 30-60% of the heating load.
Concrete (rho=2300 kg/m³, cp=880 J/kg/K, k=1.7 W/m/K) is the most common choice for its balance of availability and storage performance. Adobe (rammed earth) has been used for over a thousand years on the China loess plateau and in the Anasazi pueblos of the US Southwest. A water tank has a very large specific heat (cp=4186 J/kg/K) and stores about 4x more energy per unit volume, used in modified Sankey Trombe designs. Brick sits in between for both storage and conductivity.
A wall thickness of 25-40 cm gives a thermal lag of 6-12 hours, ideal for shifting daytime solar gain into nighttime heating. A wall that is too thin overheats during the day and stores nothing for the night; a wall that is too thick traps heat inside and never releases it. Concrete at 30 cm is the standard design value and the default in this tool.
The balance between solar gain and glazing loss is critical. Single glazing (U=5.8 W/m²K) is cheap but loses a lot of heat at night, often turning net heating negative in cold climates. Double glazing (U=2.8 W/m²K) is the practical baseline and the default in this tool. Triple Low-E glazing (U=1.0 W/m²K) suits Passivhaus-grade homes and almost eliminates night-time loss at the cost of some transmittance.
Real-World Applications
Detached-house heating boost: A 20-40 m² south-facing Trombe wall can deliver 1,000-2,000 W of room heating even at near-freezing outdoor temperatures, covering 30-60% of a well-insulated home's heating load. Enter your own south-wall area and design outdoor temperature into the tool to estimate heat-pump energy and CO2 savings for your climate.
Passivhaus and LEED-certified buildings: Trombe walls are recognised by Passivhaus (Europe) and the US LEED v4.1 rating system as a credible energy-reduction measure. Combined with triple Low-E glazing and a selective absorber surface, night-time loss is practically eliminated, making them an excellent match for super-insulated houses.
Modern revival of vernacular building: The yaodong cave dwellings of the China loess plateau and the Anasazi pueblos of the US Southwest (around AD 1000) have used the same "south-facing thick earth wall + solar gain" principle long before Trombe gave it his name. Modern adobe houses in New Mexico and Mexico now quantify that traditional performance with tools like this one and pair it with contemporary HVAC.
Research labs and agricultural greenhouses: NREL, PNNL and Sandia National Lab ran broad measurement campaigns on Trombe walls in the 1970s-1990s and produced variants such as the Solar Air Heater (Sankey Trombe). In agricultural greenhouses, water-tank Trombe walls are widely used as night-time heat reservoirs and can cut fuel costs by 40-70%.
Common Misconceptions and Pitfalls
The biggest pitfall is summer overheating. The Trombe wall is optimised for winter, but it captures sunlight in summer too. Without countermeasures the room can climb above 35°C. Real designs add external awnings or roller shades in summer, close the vents to stop convection, or vent the air gap straight to the outside to bypass the room. South of about Kanto in Japan, summer mitigation is mandatory. This simulator assumes winter operation only.
Next, "thicker is always better" is wrong. A 60 cm concrete wall has a thermal lag of more than 24 hours, so most of the solar gain stays locked inside and never reaches the room. The optimum is 25-40 cm; beyond that, performance plateaus or even drops. Try setting the thickness to 60 cm in this tool — net heating does not improve. The optimum thickness depends on the thermal diffusivity α = k/(ρcp); for water tanks even 15-20 cm is enough.
Finally, do not chase a solar fraction of 100%. Pushing it above 80% requires huge wall area (most of the south facade) and enormous storage, sacrificing cost and architectural freedom. In practice, aim for a solar fraction of 30-60% and cover the rest with a heat pump or a high-efficiency gas backup — a hybrid design gives the best cost-performance. Cloudy stretches and cold snaps always require backup capacity.
How to Use
Enter wall area in m² (typical range 10–40 m² for residential south-facing walls)
Set wall thickness in cm (150–300 cm concrete or masonry provides optimal thermal mass; thinner walls charge faster, thicker walls release heat longer)
Specify air gap in mm between glazing and wall surface (50–100 mm balances convection efficiency and stagnant dead space)
Input solar irradiance in W/m² (use 800 W/m² for clear winter midday, 500 W/m² for cloudy conditions, or measured local data)
Read outputs: wall thermal capacity, daily solar energy collected, convective heat transfer to room, glazing conduction loss, and net heating contribution to building load
Worked Example
25 m² concrete Trombe wall (ρ=2400 kg/m³, c=840 J/kg·K), 200 cm thick, 80 mm air gap, 700 W/m² winter irradiance. Wall capacity = 10.08 MJ/K. Solar gain = 4.2 kWh/day. Convective heat transfer to room averages 850 W during peak afternoon hours. Double-glazing loss ≈ 180 W (U-value 2.8 W/m²·K). Net heating = 3.1 kWh/day. For 20 kWh daily building load, solar fraction = 15.5%.
Practical Notes
Concrete density and specific heat dominate capacity; brick (ρ=1800 kg/m³) stores 25% less than concrete for same thickness
Air gap convection coefficient peaks at 60–80 mm; gaps below 40 mm create thermal resistance, above 120 mm allow dead-air stratification and reduced heat transfer
Winter performance dominates passive solar contribution; summer overheating requires vents at top and bottom of wall to exhaust hot air and prevent interior temperatures exceeding 28°C
South-facing orientation in Northern Hemisphere; east–west walls or diffuse climates reduce irradiance by 30–50% and lower payback period