Passive Solar Heating for Homes: Trombe Walls, Direct Gain, Sunspaces and Solar Furnaces
Passive solar heating is the oldest and most cost-effective solar technology available. No panels, no pumps, no electricity, no maintenance. Just the right combination of south-facing glass, thermal mass, and insulation — and your building heats itself for free whenever the sun shines.
This guide covers everything from simple retrofits anyone can do this weekend to whole-building passive solar design principles.
In this guide, you'll learn:
How Passive Solar Heating Works
The physics are simple. Solar radiation passes through south-facing glass and is absorbed by dark or dense surfaces inside the building (floors, walls, water containers). Those surfaces convert light to heat. At night, they release that stored heat slowly back into the room.
Three elements are required for passive solar to work well:
- South-facing glazing — glass that admits low-angle winter sun (in the northern hemisphere, south; in the southern hemisphere, north). Winter sun is low in the sky, so vertical or near-vertical south-facing glass captures it well. Summer sun is high, so it clears a properly designed overhang.
- Thermal mass — dense material (concrete, stone, brick, water) that absorbs heat during the day and releases it at night. Thermal mass without south-facing glass does nothing. Glass without thermal mass overheats during the day and loses the heat immediately after sunset.
- Insulation — the rest of the building envelope must be well-insulated. A passive solar building with poor wall insulation simply heats the outside world.
Direct Gain: The Simplest Approach
Direct gain is the most common passive solar strategy — south-facing windows admit sunlight directly into the living space, which is absorbed by a dark, dense floor or internal wall.
How much heat does direct gain provide?
A standard double-glazed south-facing window in a temperate climate (UK, northern US, central Europe) admits approximately 200–400 kWh per m² of glazing over a heating season (October–March). At current electricity prices of €0.25/kWh, that is €50–€100 per m² of window per year in free heat — assuming adequate thermal mass to prevent overheating.
| South glazing area | Seasonal heat gain | Equivalent electricity saving |
|---|---|---|
| 2 m² (two standard windows) | 400–800 kWh | €100–€200/year |
| 4 m² (patio door + window) | 800–1,600 kWh | €200–€400/year |
| 8 m² (full south wall glazing) | 1,600–3,200 kWh | €400–€800/year |
Direct gain works best when the floor is a dark, dense material — dark slate, stone tile, or polished concrete absorbs and stores far more heat than carpet or wood. The rule of thumb: 5–8kg of thermal mass per Watt of south-facing glazing area.
Trombe Wall: The Most Effective Passive Solar Retrofit
A Trombe wall is a south-facing masonry or concrete wall glazed on the outside, with a narrow air gap between the glass and the wall. The wall absorbs solar radiation, heats up to 40–60°C, and then conducts heat slowly through to the room interior over 6–10 hours — meaning it warms the room in the evening rather than during the day, when solar gain would cause overheating.
Building a Trombe wall
The basic construction:
- An existing south-facing masonry wall (200–300mm brick or concrete) painted flat black or coated with a selective solar absorber coating.
- A pane of glass or polycarbonate mounted 50–100mm in front of the wall, creating a sealed air gap.
- Optionally: vents at the top and bottom of the air gap, connecting to the interior room. These allow daytime thermosiphon circulation — hot air from the gap rises into the room through the top vent while cooler room air enters through the bottom vent, providing immediate daytime heating alongside the delayed overnight conduction.
A 1m² Trombe wall in a temperate climate provides 80–120 kWh per m² per heating season — roughly 50–70% of what a direct-gain south window provides, but delivered at the most useful time (evening).
Material cost for a DIY Trombe wall: glass or polycarbonate (£10–£20/m²), frame timber (£5–£10/m²), flat black paint (£5). Total: £20–£40 per m² of wall. A 4m² section produces £80–£120 worth of heat per year — payback under one season.
Sunspace and Solar Greenhouse
A sunspace (also called a solar conservatory or attached greenhouse) is an unheated or lightly heated glazed space attached to the south face of a building. It acts as a thermal buffer — warming during the day to 15–25°C from solar gain — and this warm air then enters the main building through doors, windows or vents.
The key advantage over a Trombe wall: a sunspace is multifunctional. It extends the growing season for vegetables and herbs, provides a temperate outdoor living space in spring and autumn, and preheats incoming air for the main building at zero running cost.
For a full guide on sunspaces in a greenhouse context — including SHCS climate battery integration — see the SHCS greenhouse heating guide →
Thermal Mass: The Key to Making It Work
Passive solar without adequate thermal mass causes overheating during the day and rapid heat loss at night — worse than no south glazing at all in some cases. Thermal mass is non-negotiable.
| Material | Heat capacity (kJ/kg·°C) | Density (kg/m³) | Heat stored per m³ per °C rise | Notes |
|---|---|---|---|---|
| Water | 4.18 | 1,000 | 4,180 kJ | Best heat capacity by far |
| Concrete | 0.88 | 2,300 | 2,024 kJ | Common, free in existing slabs |
| Brick | 0.84 | 1,800 | 1,512 kJ | Good for Trombe walls |
| Stone (granite) | 0.79 | 2,700 | 2,133 kJ | Dense, excellent thermal mass |
| Sand | 0.84 | 1,700 | 1,428 kJ | Cheap for sand battery builds |
| Timber | 1.70 | 600 | 1,020 kJ | Poor — low density offsets OK specific heat |
Water barrels painted black and placed in south-facing windows are the cheapest way to add thermal mass to an existing building. For a full comparison of thermal mass options including sand batteries, water tanks and phase-change materials, see the solar thermal storage comparison → A 200-litre black barrel in a sunny south window absorbs 8–14 kWh of heat on a clear winter day — equivalent to a 1kW electric heater running for 8–14 hours, for free.
Solar Furnace for Home Heating: What It Actually Means
The term "solar furnace" is used loosely online to describe two distinct things:
1. A DIY solar air heater — sometimes called a solar furnace when scaled up to 4–8m² of collector area and fed into a home's existing ductwork. In this usage it is simply a large solar air collector. A 4m² unit can produce 800–1,400W on a clear winter day — enough to meaningfully supplement a home's forced-air heating system. For build details: DIY solar air heater guide →
2. A concentrated solar collector — parabolic reflectors or Fresnel lenses focusing sunlight to a point, generating temperatures of 300–1,000°C+. True solar furnaces are research and industrial tools. They are not practical for residential home heating — the precision tracking mechanisms, safety requirements, and complexity make them unsuitable for DIY use in any normal home.
If you have encountered "solar furnace" content online claiming you can heat your home with a DIY parabolic mirror, that content is either describing a solar air heater (reasonable DIY project), a solar cooker (legitimate low-tech tool for cooking outdoors), or a misunderstanding of industrial technology. Stick to solar air collectors for home heating supplementation.
Retrofitting Passive Solar to an Existing Home
Most homes cannot be fully redesigned for passive solar, but several retrofits are worthwhile:
- South-facing window enlargement: If you are already replacing a window, upsizing from a standard 0.8m² window to a 1.5m² patio section adds 100–200 kWh/year of free heat and costs little extra at replacement time.
- Dark thermal mass floor in south rooms: When replacing carpet or wooden flooring in a south-facing room, use dark slate, stone tile or polished concrete instead. No extra cost if timed with a renovation, significant passive solar benefit.
- Water barrels: 200-litre black barrels in south-facing windows, as above. Cost: £10–£25 per barrel. Immediate benefit.
- Trombe wall panel: Mount a glazed polycarbonate panel on a south-facing brick or concrete wall. As described above — £20–£40/m², payback in one season.
- Solar air heater: For active supplemental heating beyond what passive elements provide: DIY solar air heater →
- Window insulation: Passive solar works far better in a well-insulated building. Bubble wrap window insulation, secondary glazing, and draught proofing all reduce overnight heat loss from charged thermal mass.
Combine with window insulation for best results. Every passive solar technique works significantly better in a well-insulated building. Bubble wrap window insulation and draught proofing both reduce overnight heat loss from the thermal mass you have just charged during the day — the two approaches multiply each other's effect.
Frequently Asked Questions
What is passive solar heating?
Passive solar heating uses south-facing glass, thermal mass (concrete, stone, water) and insulation to collect, store and release solar heat without any mechanical systems. The building itself is the heating system — sunlight enters through glass, charges the thermal mass during the day, and the mass releases heat slowly overnight. No pumps, no fans, no running costs.
What is a Trombe wall?
A Trombe wall is a south-facing masonry wall with a pane of glass mounted in front of it, creating a sealed air gap. The wall absorbs solar heat during the day, reaching 40–60°C, and conducts that heat slowly through to the room interior over 6–10 hours — delivering useful heat in the evening rather than the middle of the day. It is one of the most effective passive solar retrofits for an existing masonry home.
How much does passive solar heating save?
Direct gain from 4m² of south-facing double glazing saves 800–1,600 kWh per heating season — worth €200–€400/year at current European electricity prices. A Trombe wall adds another 320–480 kWh per 4m² of wall. Together, on a well-insulated 100m² home, passive solar measures can realistically reduce heating energy demand by 20–40%.
What is the difference between passive solar and a solar air heater?
Passive solar uses the building structure itself — glass, mass, orientation — to collect and store heat with no moving parts. A solar air heater is an active (though unpowered) system: a separate panel mounted on the wall that moves air through a heated absorber. Passive solar works 24 hours a day using stored heat; a solar air heater only produces heat while the sun shines. Both approaches complement each other — passive solar for overnight warmth, active air heater for supplemental daytime heat boost.
