School · September 2023 – May 2024

Solar heater for a tiny house

As part of our curriculum at Icam Toulouse, 30 engineering students came together to design and build a tiny house for a homeless person from the outskirts of Toulouse. The finished house was donated to a local association. The guiding constraint: every solution had to be low-tech.

Within this larger effort, I led the 7-student team responsible for heating and cooling. With an electric heater off the table, we needed a creative alternative, and thankfully Toulouse's generous sunshine pointed us straight at the answer.

A "crazy" idea

Browsing the Low-Tech Lab wiki, I stumbled on a slate-based solar air heater concept by Guy Isabel. Honestly, we thought the idea was a bit crazy and doubted it would really work. But we wanted to try it out.

Working principle

The system relies on a simple but effective chain of physics. A sheet of glass sits on the front face of the panel and lets solar radiation pass through while trapping heat inside, more commonly known as the greenhouse effect. Behind the glass, a layer of slate absorbs the incoming energy. Slate is an ideal material here: its dark colour gives it high solar absorptance, and its density provides excellent thermal inertia, meaning it stores heat and releases it gradually. It's no wonder it's been used for centuries for roofing in the Pyrenees.

Behind the slate, a shallow air gap forms the circulation channel. As the slate heats the air in contact with it, that air becomes less dense and naturally rises, a phenomenon known as natural convection. Cooler room air is drawn in through an opening at the bottom of the panel to replace the rising warm air, creating a continuous passive loop with no fan or pump required. The warm air exits through a hole at the top of the wall and flows directly into the living space.

Proving the concept

Before committing to a full-scale build, I wanted a quick proof of concept. Following our low-tech philosophy, we aimed for a Minimum Viable Product, just enough to answer three questions: how fast does the air heat up, what peak temperature can we reach on a sunny spring day, and how long does the slate retain heat once the sun disappears?

Browsing LeBonCoin for building materials, we found a 50 × 60 cm slate slab that was perfect for a small-scale prototype. With a budget of less than €50 we sourced everything we needed, and built the frame. The setup was straightforward:

• A thick 50 × 60 cm slate slab sourced from LeBonCoin
• A frame and air circuit made from wood
• A glass pane on top to trap heat
• An Arduino board with a thermocouple at the air exhaust, wired to a laptop for live data logging

We oriented the prototype at a 154° SE heading, the same angle as the façade of the future tiny house in Naubalette, to really get representative results.

Results

Within about an hour, with the early sun hitting the panel at a ~45° angle, the exhaust air reached 27 °C. As the sun climbed and shone more directly, the heating rate increased: the temperature hit 42 °C by 11:10, just two hours after the start of the experiment. We then moved the prototype into the shade to study thermal inertia. The air stayed above 35 °C for another half hour showing a slow, gradual decline confirming that the slate would keep delivering warm air even during cloud cover or at the end of the day.

I was thrilled with the results and confident the concept would scale. The final panel for the house would be roughly 4× larger than the prototype, promising even better efficiency. Beyond validating the physics, building the prototype taught me practical manufacturing insights that directly informed the full-scale build.

Full-scale build

For the real version we switched from a single thick slab to slate roof tiles. Thinner and lighter, but still highly effective. Slate was the ideal choice given its thermal inertia and wide availability in the South of France. The finished panel measured 2.5 m high × 1.5 m wide.

Installation

The panel was firmly fixed onto the wood cladding of the tiny house. We then cut two holes through the wall: one for air intake at the bottom, one for warm air output at the top.

To give the openings a clean finish, I 3D-printed two covers for the holes. I also designed and printed a manual damper piece that lets the occupant block the incoming airflow when heating isn't needed, preventing unwanted heat gain in summer.

In the press

Our tiny house project was featured in the local media: La Dépêche.