Build a lean-to greenhouse against a south-facing wall in cold climates or small plots, because the wall gives structural support, wind shelter and thermal mass and shares heat with the building. Build a freestanding greenhouse where you need more light, a bigger footprint or an independent orientation, and accept that it loses more heat.
The first greenhouse decision is whether to attach to an existing building or stand alone, and each choice trades light against heat.
The Shared Wall Is What Makes A Lean-To Cheaper And Warmer
A lean-to is built against the south wall of an existing building in the Northern Hemisphere (the north wall in the Southern). The wall carries part of the frame, blocks wind, and stores daytime heat. A door or window between the greenhouse and the building lets warm air flow both ways, and the building gains passive solar heat from the greenhouse. The shared wall means less material and less heat loss. It suits cold climates, small to medium scale, and a home where daily access matters.
A freestanding greenhouse stands apart, with glazing on all or most sides. It receives more total light and can be any size or orientation, but every wall is an exposed surface that loses heat. It suits larger operations, mild climates and community growing.
What You Need
- For a lean-to: a south-facing wall with sound masonry or timber, and no shade from taller buildings or trees to the south
- For a freestanding house: open ground with winter sun from the south and a windbreak on the windward side
- For both: a frame and glazing (see Frame Materials: Wood, Bamboo, Salvage), plus vents
Method: Choose And Site
Step 1: Find true south. Establish the sun’s direction with Finding True South. A lean-to needs the wall itself to face roughly south.
Step 2: Check the shared wall. A damp, crumbling or shaded wall defeats the purpose. Moisture from the greenhouse will also reach the wall, so see Wall Moisture Barriers before attaching a roof edge or growing against the masonry.
Step 3: Weigh climate against scale. In a cold climate, pick the lean-to for its shared heat. In a mild climate with space, pick freestanding for its extra light.
Step 4: Plan the connecting door. If you will open a door between building and greenhouse, remember that warm, humid greenhouse air will enter the room. Never connect a greenhouse to a room containing a stove or fire without a way to keep combustion fumes out of the growing space and humidity out of the living space.
Worked Example
Comparing wall surfaces. A greenhouse 3 m wide, 6 m long and 2.4 m to the eave has exposed wall and end area. A freestanding house exposes both 6 m long walls (2 × 6 × 2.4 = 28.8 m²) and both 3 m ends (2 × 3 × 2.4 = 14.4 m²), a total of 43.2 m² of wall. A lean-to of the same size, with one long wall shared with the building, exposes one long wall (6 × 2.4 = 14.4 m²) and two ends (14.4 m²), a total of 28.8 m². The shared wall removes 14.4 m² of exposed wall, one third of the 43.2 m² (14.4 / 43.2 = 0.33), before counting the roof.
Common Mistakes
- A shaded lean-to — a wall in the shadow of other buildings starves the plants.
- Leaving the wall unprotected — greenhouse humidity soaks a timber or earth wall and rots it.
- A freestanding house without a windbreak — it loses heat quickly in exposed positions.
Safety
Open flames and fuel-burning heaters
Do not attach a greenhouse to a room with a combustion appliance unless smoke and carbon monoxide cannot reach the growing space. A heater inside a closed greenhouse produces carbon monoxide and humidity; see Carbon Monoxide Risk.
What’s Next
- Learn next: Frame Materials: Wood, Bamboo, Salvage — building the frame
- Combine with: Thermal Mass for Night Warmth — making use of the shared wall