Cold nights, rising bills, and a simple question. Could a home stay warm without radiators, even in January, and still feel cosy?
Across France, one coastal block has done just that, turning heads with a fabric-first upgrade and a smart use of winter sun. The results read like a promise, not a gimmick: warm rooms, lower demand, fewer drafts. The method rests on insulation, airtightness, and a glass buffer that traps free heat.
What sits behind the 23°C headline
A west-coast residence near the Atlantic now holds steady at around 23°C indoors while the heating stays off. Residents report stable comfort, fewer cold spots, and a calmer soundscape. A reworked building envelope lies at the centre of the change. It cuts heat loss, tames air leaks, and turns daylight into energy rather than glare.
23°C indoors with radiators off. Heat losses down by more than 70%. Energy label up from D to B.
The project team sealed the thermal shell, upgraded the roof, and pulled old balconies into the living space with high‑performance curtain walls. Those glass fronts act as a suntrap by day, then the insulated layers hold onto those gains. The result is not magic. It is physics and detailing.
How the greenhouse effect helps, without a greenhouse
The enclosed balcony behaves like a winter garden or sunspace. Cold outside air does not sweep straight into the flat. Sunlight enters and warms the buffered zone. The warmth then bleeds gently inward through well‑insulated, airtight layers. The envelope keeps that energy in the building rather than letting it leak out through joints, frames, or thin walls.
On bright winter days, the sunspace gathers plenty of heat. On cloudy days, the improved insulation keeps the home steady by relying on internal gains from people and appliances. The maritime climate in north‑west France also helps, because winter averages hover well above freezing, which reduces heat demand.
What changed in the building
- Continuous insulation added to façades to eliminate thermal bridges.
- Roof rebuilt and upgraded to reduce upward heat loss and stop condensation risks.
- Old open balconies enclosed and coupled to living rooms via high‑performance curtain walls.
- Measured reduction in heat loss above 70%, slashing the building’s heat demand.
- Energy rating stepped up from D to B after the works and commissioning.
Funding came from several public sources. The total bill reached €1.58 million, and grants covered around 45% through the Brittany region, Brest Métropole and Anah, the national housing agency. A local public service, Tinergie, coordinated the technical path and the finance, guiding owners’ associations through design, procurement and works. Regional authorities set a clear pace too, with a target to renovate 45,000 homes each year.
€1.58m spent, 45% subsidised, a local one‑stop service guiding design and delivery. From drafty to tight, in one programme.
Could you do this where you live
Many homes can borrow key parts of this approach. The principle is simple. Stop heat escaping. Harness light. Manage air on your terms, not through cracks. The detail depends on your building, your climate and the rules where you live.
Five fabric‑first moves that shift the dial
- External wall insulation to remove cold bridges at slabs and columns.
- A properly insulated and airtight roof or loft, with moisture‑safe detailing.
- High‑performance windows with warm frames, tidy installation, and careful airtight tapes.
- Draught‑proofing combined with controlled ventilation, ideally with heat recovery.
- Solar‑shading to keep summer gains in check while allowing winter light.
If you have a balcony, a sunspace can act as a thermal buffer. It needs robust glazing, trickle ventilation, and safe drainage. It also needs approvals. Enclosing a balcony changes the façade and can affect fire safety and escape routes. A structural engineer and the local authority must sign off the design. In a shared block, the owners’ association needs to agree.
Will it always hit 23°C without heating
Not every home will reach that figure every day. Orientation and weather matter. A south or west‑facing façade gains more winter sun. A mild winter helps. Heavy thermal mass moderates swings. Good airtightness keeps night‑time temperatures stable. In harsher cold snaps, even a well‑insulated flat may still need a small top‑up heater. The goal is not perfection. It is to cut demand so far that free gains carry most of the load most days.
What to ask before you start
- Where does my home leak heat. A thermal camera survey can show bridges and drafts.
- Can the balcony or loggia become a code‑compliant sunspace. Ask a qualified designer early.
- How will I ventilate after sealing cracks. Plan for continuous, quiet ventilation with filters.
- What about moisture. Specify vapour‑open external insulation and check risk at junctions.
- Who funds what. Map grants, 0% loans, and collective schemes with your local energy office.
The numbers that make it plausible
A renovated block reduces its heat‑loss coefficient sharply. That means every watt of internal or solar gain lifts indoor temperature more than before. Two people, appliances and lighting can provide several hundred watts through the evening. A sunspace on a bright winter day can deliver useful gains as well. When the envelope traps those gains, room temperatures stay high with no radiators running. The project near the Atlantic shows the effect at scale, with measurements rather than wishful thinking.
Risks to manage, benefits you feel
Moisture and air quality
Tight buildings need planned ventilation. Fit quiet fans or a heat‑recovery unit. Keep filters clean. Match extraction to kitchens and baths. Moisture will then leave the home without chilling it.
Summer comfort
Sunspaces can run hot in July. Fit external blinds or screens. Add vents high and low for purge flow. Configure summer settings so the buffer zone does not overheat the flat behind it.
Fire and structure
Façade works must comply with current fire regulations. Choose tested systems. Keep cavity barriers continuous. Use certified installers. A structural engineer should check loads from new glazing and anchors.
A roadmap for residents and building managers
- Commission an energy audit that maps heat loss and ventilation paths.
- Set a collective target for airtightness, heat loss and indoor comfort.
- Bundle measures into one contract so trades do not work at cross‑purposes.
- Track grants and sequence paperwork early. Public programmes often unlock 30–50%.
- Monitor temperatures and humidity after works. Fine‑tune ventilation and shading.
Why this matters beyond one block
Heating takes a large bite out of household budgets. A fabric‑first plan trims that bite without asking anyone to shiver. It also cuts peak demand on the grid. Cities gain quieter streets and cleaner air when old fans and boilers run less. For residents, the benefits go beyond the meter. Fewer drafts, warmer surfaces, and quieter rooms improve daily life.
If you manage a block or own a flat, you can assess your odds quickly. Map exposure and glazing. Check roof and wall build‑ups. Note balcony types. Ask a designer to test a sunspace concept alongside insulation and ventilation. A short simulation can show likely winter temperatures and the extra hours you can spend at 21–23°C without switching the heating on. That gives a clear case for investment and a path to funding.
Love the fabric‑first approach. Insualtion + airtightness + a sunspace that buffers the wind makes sense—it’s physics, not magic. The Atlantic case is mild, sure, but cutting heat loss ~70% means internal gains actually count. How did they handle trickle vents without re‑introducing draughts, and what airtightness was measured?
23°C with no heating? Try that in Aberdeen in January and tell me how it goes. Orientation matters; so does mass.