Passive solar greenhouse
A greenhouse that heats itself: a heavy, insulated wall on the side away from the sun, film on the sunny side, and a blanket over the film at night.
Build
Tier 5 · Second industrial revolution
Food
Hard — a masonry wall and roof, and a shape set out for one latitude
Weeks; the wall is most of the work
Crew 4+
Home: Protected growing, Level 5 · Current, level 2 of 3 in its track. Serves 1 step.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
Northern China grows winter vegetables in about 930,000 ha of these greenhouses, over 100 million tonnes a year, without active heating (Fan et al., 2021). The wall on the side away from the sun takes in the day's sunlight and gives it back at night; the back roof is insulated; and before sunset a blanket is rolled down over the film so that the heat stays in. The shape is set by the sun's height at noon on the shortest day, so it must be worked out for the latitude where it stands. The dimensions below are for about 41° N, after a measured greenhouse at Shenyang (Fan et al., 2021).
Passive solar greenhouse at 41° N, in section at noon on the shortest day
Fig 1
Earth / subsoilMasonry / brickTimberLoose fill / grainGravel / aggregate
Day and night: where the heat goes
Fig 2
Masonry / brickTimber
Keyed parts
| 1 | Heat-storing wall | Brick 370 mm thick, 2.7 m high, on the side away from the sun; the same for the end walls. |
| 2 | Wall insulation | 100 mm of rigid insulation on the outside of the brick, so the brick stays on the warm side. |
| 3 | Back roof | Rafters from the wall top to the ridge beam, boards, 100 mm of insulation and a waterproof skin. |
| 4 | Front arches | Steel tube or bamboo from the front footing to the ridge beam, 1.2 m apart. |
| 5 | Film | Greenhouse film about 0.1 mm thick, UV-stabilised, over the arches. |
| 6 | Night blanket | A quilt about 40 mm thick, rolled up at the ridge by day and down over the film at night. |
| 7 | Top vent | A gap opened below the ridge by rolling the film down. |
| 8 | Bottom vent | The film lifted along the front footing. |
| 9 | Beds | Growing beds on the floor, running across the house from front to back. |
Specifications
| Span | 8.0 m | From the front footing to the inside face of the wall. |
| Ridge | 4.0 m | 1.5 m in front of the wall. |
| Wall | 2.7 m high | 370 mm brick plus 100 mm insulation outside. |
| Back roof | 41° | Steeper than the midwinter noon sun, 25.6° at 41° N. |
| Front roof | about 32° on average | Noon sun meets it 33° off square; keep under 40° from 10:00 to 14:00. |
| Length | 30 m, east–west | Fan et al. measured one 60 m long. |
| Film | about 0.1 mm, 9.5 m wide | From footing to ridge with spare at both edges. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Brick and mortar | about 47 m³ | 30 m³ for the back wall, 17 m³ for the end walls. Or rammed earth, built thicker. |
| Rigid insulation, 100 mm | about 190 m² | Outside the back and end walls, and on the back roof. |
| Footings | 1 set | Under the back and end walls and along the front edge, below frost depth. |
| Ridge beam, rafters, boards | 30 m | A ridge beam 30 m long, rafters every 1.2 m, boards and a waterproof skin for the back roof. |
| Arches, steel tube or bamboo | 26 | One every 1.2 m, footing to ridge beam. |
| Greenhouse film, about 0.1 mm, UV-stabilised | 9.5 × 32 m | Plus strips for the vents. |
| Night blanket | about 30 × 9.5 m | Cotton or felt quilt about 40 mm thick, with a waterproof cover. |
| Roller shaft and rope, or a motor | 1 set | To roll the blanket up and down the roof. |
Tools: String lines, pegs, tape and a 3-4-5 triangle · Level · Trowels and a mortar board · Saw, drill and driver · Pipe bender, if the arches are steel · Ladders and a crew
Construction
Work out the angles for your latitude
The sun's height at noon on the shortest day is 90° − latitude − 23.4°: 25.6° at 41° N, 20.6° at 46° N. Then:
- Back roof: steeper than that, so its underside does not shade the wall. Here 41°.
- Front roof: steep enough that the sun meets the film within 40° of square from 10:00 to 14:00 on the shortest day (Frontiers in Energy Research, 2023). Here about 32° on average, as on the measured house at 40.8° N (30°).
Further from the equator, make both steeper.
Set out east–west
Mark the house 30 × 8 m with its long side facing the equator, square it with 3-4-5 and equal diagonals (set out a block), and dig footings below frost depth under the back and end walls and along the front edge. Keep neighbouring buildings and trees out of the midwinter sun.
Build the heat-storing walls
Lay the back wall 370 mm thick and 2.7 m high, and the two end walls to the roof profile. Leave a door in one end wall. Fix 100 mm of rigid insulation to the outside of all three and render over it: on the outside, it keeps the brick on the warm side where it can store heat (Fan et al. recommend at least 80 mm on the back and end walls).
Build the back roof
Set the ridge beam at 4.0 m, 1.5 m in front of the wall, on posts or the end walls. Run rafters from the wall top to the beam at 41°, board them, lay 100 mm of insulation and a waterproof skin, and carry it over the top of the wall so rain runs off behind.
Raise the front arches
Fix arches 1.2 m apart from the front footing to the ridge beam: steep near the ground, flattening toward the ridge. Tie them together along their length, like the purlins of a high tunnel.
Put on the film and the vents
On a calm day, pull UV-stabilised film about 0.1 mm thick over the arches from the footing to the ridge and fasten it at both ends and along the gables. Arrange it so it can be rolled down below the ridge (top vent) and lifted along the footing (bottom vent).
Fit the night blanket
Lay a quilt about 40 mm thick with a waterproof cover along the top of the back roof, fixed at the ridge, with a roller shaft and ropes, or a motor, to roll it down over the film and back up. Weigh or tie its lower edge against wind.
Run it
Roll the blanket up once the morning sun is on the film, and down before sunset. Open the top vent when the air passes the crop's maximum and the bottom vent too on hot days; shut both in the afternoon to hold heat for the night. Grow in beds on the floor.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Back roof | steeper than the midwinter noon sun | Shallower, and it shades the top of the wall that should be storing heat. |
| Front roof | sun within 40° of square, 10:00–14:00, shortest day | At steeper angles of incidence more light reflects off the film. |
| Insulation | on the outside, continuous, no gaps | Inside, it would cut the brick off from the air it must warm; gaps leak heat all night. |
| Blanket | down before sunset, dry, edges closed | An open edge or a late start loses the heat the wall has stored. |
How this fails
Built for the wrong latitude: a back roof that shades the wall, or a front roof so flat that the winter sun glances off it.
Insulation inside the wall, or none: the brick cannot store heat, or loses it outward all night.
The blanket rolled down late or up early, or wet and sagging.
Winter shade from a building, tree or the next greenhouse.
A ventless heater lit on the coldest night in a tightly closed house: carbon monoxide for the grower, ethylene for the crop.
How you know it worked
VerificationAt noon on the shortest day the sun lights the whole face of the back wall, and on clear winter nights the air inside at dawn stays well above the air outside without any heater.
Scaling and variants
Longer: the measured house at Shenyang is 60 m. Wider: the house studied at 40.8° N is 9 m across, 4.3 m to the ridge, with a 3.2 m wall and a 30° front roof (Frontiers in Energy Research, 2023). Colder places: steeper roofs, thicker insulation. Sunk into the ground: a walipini or pit greenhouse uses the earth around it as the heat store.