A greenhouse is bought by footprint and used by volume. The floor area tells you how many plants fit; the cubic capacity tells you how much air has to be heated, cooled and exchanged, and it is the figure that decides whether a small fan heater is enough in March or whether the temperature will swing fifteen degrees between noon and night. This works out both, takes the mean height properly from the eaves and the ridge rather than assuming a box, and tells you how much growing medium the beds inside will need.
Structure, beds and compost
Volume is not floor area. A greenhouse is sized by its air, not by its footprint.
Half the floor in beds, half as a path, is the usual layout.
Result
Why the mean height is not the eaves height
A greenhouse with a pitched roof is not a box, and treating it as one understates the volume by a quarter or more. The eaves might be 1.6 metres and the ridge 2.4, and the average height of the air inside is halfway between: two metres. Multiply the floor area by that and you get the real volume, which is what a heater has to warm and what a vent has to exchange.
It matters most at the extremes of the year. In March, a small greenhouse has to be warmed from near freezing to fifteen degrees or more overnight, and the energy that takes scales with volume — not with floor area. A house that is taller than it needs to be is a house that costs more to heat every night of the spring, for growing space that is above head height and unused.
Ventilation scales with volume too
The rule of thumb for greenhouse ventilation is that the vent area should be about a fifth of the floor area, split between ridge and side vents so that air can move through rather than in and out of the same opening. Ridge vents alone, which is what most small houses have, are the reason a greenhouse overheats on a still sunny day even with the door open: hot air collects at the ridge and cannot get out without cooler air coming in low down to push it.
Automatic vent openers are the best-value thing you can fit to a greenhouse, because they solve the problem that actually kills plants — a house left shut on a sunny morning reaches forty degrees by ten o'clock and everything in it is dead by lunch. They need no power and no attention, and they cost less than the plants they save.
A still, hot, humid greenhouse is a powdery mildew and red spider mite factory, whatever the thermometer says.
A through draught on a warm day — vents open at both ridge and side — does more for plant health than any amount of heating or shading.
Beds, paths and how much compost
The usual layout is beds down both sides and a path up the middle, which puts about half the floor in production and half in circulation. In a 2.4-metre house that means two beds of roughly 0.9 metres and a path of 0.6, which is tight but workable; in a three-metre house it is comfortable.
Bed depth is worth being honest about. Thirty centimetres is enough for almost everything grown under glass, and more than that is expensive compost sitting below the root zone. Where the house is built on good ground, the beds can be dug out and the native soil used, with bought compost only for the top; where it is on slabs, the whole depth has to be brought in.
| Size | Floor m² | Air m³ | Beds m³ at 30 cm | Plants at 45 cm |
|---|---|---|---|---|
| 1.8 x 1.8 m | 3.24 | 6.5 | 0.49 | 16 |
| 2.4 x 3.0 m | 7.2 | 14.4 | 1.08 | 35 |
| 2.4 x 3.6 m | 8.64 | 17.3 | 1.30 | 42 |
| 3.0 x 4.5 m | 13.5 | 27.0 | 2.03 | 66 |
| 3.6 x 6.0 m | 21.6 | 43.2 | 3.24 | 106 |
A worked example
- Mean heightEaves 1.6 and ridge 2.4 gives a mean of 2.0 m.
- Floor and volume2.4 by 3.6 is 8.64 m² of floor, and at 2 m mean height the house holds 17.28 m³ of air.
- BedsHalf the floor is 4.32 m², which at 30 cm deep needs 1.30 m³ of growing medium.
- BagsThat is 1,296 litres, or 33 bags of forty litres.
- CapacityAt 45 cm spacing the beds hold about 42 plants.
Thirty-three bags is a real amount of compost, and it is the argument for digging out the beds and using the ground underneath rather than filling a raised frame on slabs. Where the house is on a slab, the raised bed figure prices it properly.
Siting and what follows
Site the greenhouse with its ridge running east to west so the long side faces south and the house catches the low winter and spring sun. Avoid a position in the shade of a tree or a wall between about ten in the morning and four in the afternoon, and avoid a frost pocket at the bottom of a slope, where cold air settles.
Water is the next thing. A greenhouse needs a supply at the door, and a water butt off the guttering is the cheapest answer. Plants under glass need watering far more often than the same plants outside, and the irrigation run time turns the bed area into minutes, while the drip line figure sizes the system.
Do
- Take the mean height from eaves and ridge, not from the eaves alone.
- Fit automatic vent openers, and vents at ridge and side.
- Use about thirty centimetres of growing medium, not more.
- Run the ridge east to west, long side facing south.
- Put a water butt on the guttering.
Avoid
- Sizing a heater from the floor area rather than the volume.
- Relying on a ridge vent alone, which cannot draw air through.
- Filling beds deeper than the roots will use.
- Siting it where a tree or a wall shades it through the middle of the day.
- Siting it at the bottom of a slope, where frost settles.
Where the numbers stop being true
The mean-height approach assumes a simple pitched roof with a straight slope from eaves to ridge. A house with a steep pitch, a Mansard profile or a curved roof has a different mean, and the error can be ten percent either way. For a lean-to against a wall, the volume is roughly half the equivalent freestanding house.
Nothing here accounts for the contents. A house full of staging, pots and mature plants contains far less free air than the empty volume suggests, which is why it heats faster than the calculation implies — and why it also holds heat longer.
Heat loss is not modelled at all. Single-glazed glass loses heat roughly twice as fast as twin-wall polycarbonate, and the difference in running cost over a heating season is larger than any difference in volume. Insulating the north side, and using bubble wrap inside the glazing over winter, changes the picture more than the geometry does.
How the figures are produced
The mean height is the average of the eaves and the ridge, which is the correct average for a straight pitched slope. Floor area is width times length and the air volume is that times the mean height. Bed area is the floor area times the share you entered, growing medium is bed area times bed depth, and bags are litres divided by bag size. The plant count divides bed area by 0.2025 square metres, which is a 45-centimetre square per plant.
Questions people ask
Why not just use the eaves height?
Because the roof adds a lot of air above it. A 1.6 m eaves with a 2.4 m ridge has a mean height of 2.0 m, which is a quarter more volume than using 1.6 would give.
How much ventilation do I need?
Vent area of about a fifth of the floor area, split between ridge and side so air actually moves through.
How deep should greenhouse beds be?
About thirty centimetres. More is expensive compost below the root zone; less dries out too fast.
Should I heat it?
Frost protection from late winter onwards, and warmth in March and April for sowing. Size the heater from the volume, and insulate before you buy a bigger one.
Glass or polycarbonate?
Polycarbonate loses heat at about half the rate and is safer with children, but it lets in slightly less light and it yellows over fifteen years or so.
How many tomatoes fit?
About five plants per square metre of bed at 45 cm spacing, so just over forty in a 2.4 by 3.6 m house with half the floor in beds.