Trombe Wall: How It Works and Why It Matters

Passive solar systems are a set of technologies and construction practices useful for absorbing, retaining, and distributing heat within buildings during the winter period and protecting against solar radiation while increasing ventilation during the summer period. These systems are divided into 3 categories: direct gain, indirect, and mixed.

Among the most widespread indirect gain systems is the trombe wall, a solution that utilizes a massive wall combined with a glazed surface to accumulate solar heat and slowly release it indoors. An indirect gain system consists of a transparent element and a storage collecting element. The storage element is an integral part of the building’s envelope and prevents solar radiation from reaching the indoor space directly. The collecting element, on the other hand, intercepts solar radiation before it reaches the interior environment, accumulating heat and slowly returning it.

The trombe wall is primarily used to reduce winter energy needs through the free energy of the sun. When optimized, it can significantly contribute to energy savings, potentially leading to the design of passive houses without heating systems.

What is the trombe wall and how does it work?

The trombe wall is an indirect passive solar system designed to accumulate and release heat by exploiting solar radiation, falling among the main passive solar systems used in bioclimatic design. It is named after the French engineer Félix Trombe, who popularized its application in the 1960s, but the physical principle it is based on is much older: thermal mass accumulation in sun-exposed masonry.

The system consists of three main elements:

• a massive wall with high thermal inertia (concrete, solid brick, stone, adobe);
• an air gap;
• an external glazed surface that serves to collect solar energy and protect from the wind.

The wall is generally oriented to the south (in the northern hemisphere) and has a dark surface finish to maximize solar radiation absorption. The air gap, located between the glass and the masonry, creates a greenhouse effect that raises the temperature of the air and the collecting surface.

The operation combines three physical phenomena:

• irradiation, for the absorption of solar energy;
• conduction, for the transfer of heat within the mass;
• natural convection, for the circulation of air through adjustable openings.

The combination of these mechanisms allows the wall to behave like a true “thermal battery”, capable of storing energy during irradiation hours and gradually releasing it when external conditions become less favorable.

Winter operation: controlled accumulation and release of heat

During the winter season, solar radiation passes through the glass and is absorbed by the massive wall, which gradually heats up. The air in the gap increases in temperature, generating a natural convective motion.

With the opening of the upper and lower vents:

• the warm air rises and enters the internal environment;
• the cooler air from the environment is drawn into the gap;
• the cycle continues until sunset.

At the same time, the heat accumulated in the masonry is transmitted to the interior by conduction, with a time delay (thermal lag) that can vary from 6 to 10 hours depending on thickness and material.

During the night, the openings are closed. At this stage, the system operates exclusively through irradiation and conduction, gradually releasing the accumulated heat and helping to stabilize the internal temperature.

Trombe wall: winter daytime and nighttime operation

Summer operation: passive cooling strategies

In the summer, the management of the trombe wall changes configuration to avoid overheating phenomena.

During the day:

• a shading system (awnings, movable screens, or curtains) is activated;
• the upper openings of the glass are opened to favor the expulsion of hot air outside;
• the direct input of radiation to the masonry is limited.

At night, however, it is possible to take advantage of natural cooling:

• the vents of the massive wall are opened;
• the internal hot air is expelled;
• the cooler night air passes through the cavity cooling the mass.

In this way, the system can contribute, if correctly designed, to natural ventilation and the reduction of cooling loads.

Trombe wall: summer daytime and nighttime operation

How to Create a Trombe Wall: Construction Techniques and Materials

The correct design of a trombe wall requires careful consideration of orientation, materials, thickness, ventilation, and glass characteristics. It is not simply about adding a glazing in front of an existing wall, but rather integrating a passive system within the architectural project, taking into account the climate, intended use, and energy behavior of the building.

Inaccurate design can lead to summer overheating, nighttime heat loss, or performance below expectations. Conversely, a well-sized system can significantly contribute to reducing winter energy needs.

Thickness, Thermal Inertia, and Materials

The central element of the system is the high thermal inertia wall, responsible for the gradual accumulation and release of heat.

The most commonly used materials are:

• reinforced concrete;
• solid brick;
• natural stone;
• adobe or rammed earth;
• prefabricated panels with phase change materials (PCM).

The thickness generally varies between 30 and 40 cm for solutions in concrete or solid masonry. Thinner walls reduce heat storage capacity and thermal lag; excessive thickness, on the other hand, can delay heat release too much, compromising evening comfort.

A key parameter is the volumetric heat capacity of the material: the higher the density and specific heat, the more energy can be stored.

The outer surface of the wall, facing the gap, is generally treated with:

• dark finishes (black or highly absorbent colors);
• selective coatings with high solar absorptance and low emissivity (in more advanced solutions).

Glass, Gap, and Adjustable Openings

The glazed surface serves a dual purpose:

• solar capture, thanks to the greenhouse effect;
• protection from wind, reducing losses due to convection.

In contemporary applications, the following are used:

• double or triple low-emissivity glass;
• thermal break frames;
• systems with laminated safety glass.

The distance between the glass and the wall (the gap) is generally between 5 and 15 cm.

A gap that is too narrow restricts convective motion; a gap that is too wide can lose heat or create instability in airflow.

Ventilation openings are a determining element. Normally, it’s planned to have:

• two lower openings;
• two upper openings;
• shutters or hatches that can be adjusted manually or automatically.

In advanced configurations, the control can be entrusted to:

• temperature sensors;
• home automation systems;
• motorized actuators.

Orientation and Integration into the Architectural Project

The ideal orientation is south ±15° in the northern hemisphere, to maximize winter irradiation.

It’s essential to check:

• the absence of permanent shading;
• seasonal solar height;
• integration with summer shading systems (sunshades, overhangs, external curtains).

From a construction standpoint, the trombe wall is most effective when planned in the initial design phase. Later insertion into existing buildings may, in fact, lead to structural difficulties, thermal bridge issues, and higher costs. For this reason, the system should be assessed within a comprehensive bioclimatic design strategy, together with insulation, natural ventilation, and spatial distribution.

Diagram and construction detail of a Trombe wall

From a design perspective, the trombe wall is a layered system where each element influences the overall performance. A typical section, from the inside to the outside, includes:

• interior finish;
• solid wall with high thermal inertia (25–45 cm);
• dark absorbing surface on the cavity side;
• air gap (5–15 cm);
• external glazing with thermal break frame;
• possible solar shading.

The solid wall constitutes the heart of the system: materials such as concrete, solid brick, or stone ensure high thermal capacity and a useful time lag for the evening release of heat.

The air gap must be balanced in size: too small limits convective motion, too large reduces its effectiveness. In practical design, values between 8 and 12 cm often prove optimal.

The ventilation openings – generally two lower and two upper – allow for natural air circulation in winter and management of summer cooling. It is essential that they are equipped with sealed shutters to avoid nighttime losses.

From an execution perspective, particular attention should be paid to:

• continuity of insulation on the reveals;
• air tightness of the glass-frame junction;
• condensation control in the gap;
• integration with external shading systems.

Trombe Wall: the most innovative configurations

Since its earliest applications, this system has been employed in numerous buildings and has undergone various experiments. These continuous research and adaptations have led over time to the creation of modern and advanced configurations designed to improve efficiency. Among the most innovative configurations of the trombe wall, which involve both the use of an innovative design in the system’s shape and the use of new generation materials and technologies, are:

• in zig zag;
• with phase change materials;
• with aerogel insulation.

Zig Zag Trombe Wall

The zig zag trombe wall, developed by the team at the “Center for Buildings and Thermal Systems” of NREL (National Renewable Energy Laboratory), was designed to mitigate some problems associated with traditional trombe walls, such as excessive heat accumulation and glare on summer days.

This type of wall consists of three sections: one facing south and the other two angled inward, forming a V-shaped structure.

The southeast-oriented section includes a window that provides light and heat in the cold mornings when an immediate heat source is needed. The opposite section of the V wall is a classic trombe wall, intended for heat accumulation to be redistributed during the night.

The design of the zig zag trombe wall exploits the movement of the sun, which rises in the east and sets in the west. The south-east facing windows capture early morning sunlight, providing natural lighting and heating. In the afternoon, in addition to the south-oriented storage wall which collects heat throughout the day, the south-west exposed wall comes into play, absorbing the sun’s heat for use in nighttime heating.

Trombe Wall with Phase Change Materials

The trombe wall with phase change materials (PCM) represents an innovation that employs materials such as eutectic salts or hydrated salts, instead of traditional materials like concrete, brick, or adobe.

The use of PCM allows for increased efficiency of the wall, as these materials can store more energy in a smaller and lighter volume compared to traditional materials.

An example of a trombe wall with phase change materials can be found in the SimbCity House, built in Versailles by the team from the Universities of Alcalá and Castilla la Mancha for the Solar Decathlon Europe 2014.

To maintain a contained mass for thermal storage yet with high thermal inertia, the Spanish designers created a prefabricated panel adaptable to the wooden structure of the building. The interior finish of the panel includes a mix of natural clay and a phase change material, consisting of microcapsules with a wax core, which act as a latent heat accumulator.

Trombe Wall with Aerogel Insulation

Another innovative system involves the use of translucent materials, such as aerogel, inserted between two protective glass panels. This material has the capacity to store heat up to four times more than conventional insulating materials, while maintaining a high level of translucency.

The translucent insulation made of silica aerogel is applied directly to the black plaster of the massive wall.

To prevent summer overheating of the wall, it is necessary to shade the transparent material externally. This can be achieved by inserting a rollable cover between the glass and the thermal insulating layer, or by installing an external shading system over the glass. Numerous studies have shown that the incorporation of translucent insulation (TIM) in a trombe wall system offers significant energy savings.

The largest trombe wall with aerogel insulation in the world is located in Glasgow, Scotland, at Strathclyde University. Here, there are 1040 m² of translucent insulation applied to the glazing, and the trombe wall is installed in four buildings that house accommodations for a total of 376 students.

Case Study: Water Wall / Hybrid PV-TW Systems

A less common but technically interesting variant is the water trombe wall, where the wall mass is replaced or supplemented by containers filled with water.

Water has a thermal capacity superior to that of most building materials, allowing for:

• high storage with reduced thicknesses;
• more even heat release;
• greater speed of thermal response.

The main limitations concern:

• construction complexity;
• need for perfect sealing;
• architectural integration not always simple.

Despite this, in experimental contexts or in low-energy buildings, it represents a high-performance solution.

Schematic diagram of the trombe wall

Pros and cons of a trombe wall: advantages and disadvantages for energy-efficient design

The trombe wall is one of the most studied and tested passive solar systems. However, like any technological solution, it presents significant advantages as well as design limitations that need to be carefully evaluated.

Advantages

Among the main strengths, we can highlight:

• reduction of winter energy consumption, thanks to passive solar contribution;
• internal thermal stability, favored by the high inertia of the masonry mass;
• simplicity of systems, as it is primarily a passive system;
• durability over time, with limited maintenance compared to mechanical systems;
• consistent integration into bioclimatic architecture projects.

Disadvantages and critical issues

Alongside the benefits, some critical issues must be considered:

• risk of summer overheating if not adequately shaded;
• dependence on climatic conditions and actual irradiation;
• need for careful design of construction details;
• limited effectiveness in hot-humid climates or shaded contexts;
• possible increase in costs if integrated into existing buildings.

Another often underestimated aspect concerns user management: in manual configurations, incorrect opening or closing of the vents can reduce the effectiveness of the system.

In a broader approach oriented towards passive house and sustainable design, the trombe wall represents one of the most established strategies for reducing energy consumption.

The trombe wall still represents an effective passive solar design solution, capable of improving internal comfort and reducing energy consumption when properly sized and integrated within the building envelope.

Like any bioclimatic strategy, it requires careful analysis of the climatic context, materials, and seasonal behavior of the building.

To accurately evaluate the energy contribution of passive systems and verify the performance of the envelope, it is essential to rely on dedicated calculation tools.

• design and verification of the energy performance of buildings,
• energy certification APE,
• energy diagnosis,
• renovation and efficiency improvement interventions.

FAQ – Trombe Wall

What is a trombe wall and what is it used for?

The trombe wall is a passive solar system with indirect gain that uses a massive wall and a glazed surface to accumulate solar heat and gradually release it inside buildings. It is primarily used to reduce winter energy needs by harnessing the free energy of the sun.

How does the trombe wall work?

The system exploits three physical phenomena: irradiation, conduction, and natural convection. Solar radiation passes through the glass and is absorbed by the massive wall, which accumulates heat. This heat is then slowly released indoors, both through conduction through the wall and via the movement of air in the cavity.

What are the main elements that make up a trombe wall?

A trombe wall consists of three main elements:

• a massive wall with high thermal inertia;
• an air cavity;
• an external glazed surface.

These elements work together to capture, store, and distribute heat within the building.

How does the operation change between winter and summer?

In winter, the system accumulates solar heat during the day and gradually releases it during the evening and night. In summer, however, it is managed to avoid overheating, using solar shading and promoting natural ventilation to expel warm air.

What materials are used to build a trombe wall?

The most commonly used materials are those with high thermal inertia, such as concrete, solid brick, stone, and adobe. In more advanced solutions, phase change materials (PCM) may also be employed to enhance thermal storage capacity.

What is the ideal orientation for a trombe wall?

The optimal orientation is south (±15°) in the northern hemisphere, to maximize winter solar irradiation. It is important to avoid permanent shading and to integrate shading systems for the summer period.

What are the advantages of a trombe wall?

The main advantages are:

• reduction of energy consumption for heating;
• increased internal thermal stability;
• simplicity of the system;
• long lifespan and low maintenance;
• integration into bioclimatic architecture projects.

What are the disadvantages or limitations of the system?

Among the main criticalities, the following are highlighted:

• risk of summer overheating if not properly shaded;
• dependence on climatic conditions;
• need for careful design;
• less effectiveness in hot-humid climates or in the presence of shading;
• possible increase in costs for interventions on existing buildings.

Can a trombe wall contribute to the design of passive buildings?

If properly designed and sized, the trombe wall can significantly contribute to reducing energy needs, potentially being integrated into low-consumption buildings or advanced bioclimatic design strategies.