A greenhouse in a cold climate loses heat faster than it gains it. Heat escapes through the glazing and the structure at night, and the winter sun is low and short for much of the day. Heating with fuel or electricity can close the gap, but the running cost is high. Passive thermal mass offers a different approach: the greenhouse captures some of the daytime sun and holds it in a material that releases the heat slowly after dark.
Water is the most practical material for this job. This article explains why water is so effective, how to estimate how much storage a greenhouse needs, how to place a water wall without shading the crop, and where passive storage stops being enough. The figures are worked examples to help you reason about your own building, not guarantees for a specific site.
1. Why Water Stores So Much Heat
Every material stores heat according to its specific heat capacity, the energy needed to raise a kilogram of it by one degree. Water has a specific heat capacity of about 4,184 joules per kilogram per kelvin, one of the highest of any common material. Air is far lower, at roughly 1,000 joules per kilogram per kelvin. The comparison that matters for a greenhouse, however, is per unit volume, because that is how much space the storage occupies. Air at ordinary conditions has a density of about 1.2 kilograms per cubic metre, so a cubic metre of air stores only about 1,200 joules per kelvin. A cubic metre of water stores about 4.2 million joules per kelvin. Water therefore holds roughly 3,500 times more heat than the same volume of air.
This is why a greenhouse with a few containers of water can keep its night temperature noticeably higher than the same greenhouse without them. The water does not generate heat. It simply absorbs surplus daytime heat when the air is warm, and gives it back when the air cools.
Figure 2.1 (Illustrative): Night Temperature With and Without a Water Wall
Illustrative sketch of the general pattern, not measured data. The water wall slows the night-time fall in temperature, so the minimum stays higher. The real difference depends on glazing, insulation, wind, and how much water is stored.
2. Sizing a Water Wall
The energy a water store can release is its mass multiplied by its specific heat and by the temperature drop it undergoes. Consider a store of 1,000 litres of water, which weighs about 1,000 kilograms. If it cools by 10 kelvin overnight, it releases roughly 41.8 million joules, which is about 11.6 kilowatt hours. That is a useful amount of heat for a small greenhouse on a cold night, but it only applies if the water actually cools by that amount, and it does not tell you how much heat the greenhouse loses in the same period.
The practical sizing question is therefore whether the stored energy covers the night's heat loss without the water cooling below the frost threshold you need. The calculation requires your greenhouse's heat loss rate, which depends on glazing area, glazing type, and the local winter temperature. Measuring that loss on a cold night, before installing the water wall, is more reliable than estimating it from a table.
Winter sun enters at a low angle. Placement of storage should be checked against the shadow it casts at midwinter.
3. Placement and Colour
Water storage works best when it absorbs sunlight during the day. Dark containers absorb far more solar energy than clear or pale ones, which is why black-painted water totes are common. In the northern hemisphere, a wall along the north side of the greenhouse receives the least direct sun and so casts the least shade on the crop. Placing the store there is a common choice, but the exact position should be checked against your own site, because the shadow pattern depends on the building's orientation and the surrounding terrain.
The water store should not shade the plants it is meant to protect. A tall, dark wall in the wrong place can reduce light to the growing beds enough to slow growth, so check the shadow path at midwinter, when the sun is lowest.
Figure 2.2 (Illustrative): Solar Gain and Storage Through a Winter Day
Illustrative shape only, not measured. Solar gain peaks around midday, and the stored heat peaks later as the water absorbs it. The stored heat then releases through the evening and night. Exact timing depends on latitude, season and cloud cover.
4. Glazing, Orientation and Losses
Thermal mass can only store heat that reaches it, so the greenhouse envelope matters as much as the store. Double-walled polycarbonate loses less heat than single glazing and lets through enough light for growing, which makes it a common choice for cold-climate greenhouses. A south-facing roof or wall gains the most sun in winter in the northern hemisphere, because the low sun enters at a shallow angle.
Night insulation is often the most cost-effective addition. Thermal screens or insulating curtains drawn over the glazing after dark can reduce heat loss substantially, and they work alongside the water store rather than competing with it. A greenhouse with good night insulation needs far less stored energy than one without.
5. Where Passive Storage Stops
Passive thermal mass buffers temperature. It slows the fall, but it does not create heat from nothing. In a prolonged cold snap with overcast days, the store may not recharge fully, and the greenhouse can still drop below the frost threshold. Passive storage works well for the typical swings of a cold season and for protecting against brief frosts. For extended severe cold, it should be combined with supplementary heating and a clear alarm system.
The most useful habit is measurement. Log the temperature inside and outside the greenhouse across several cold nights, before and after installing a water wall. The comparison will show what your building actually achieves, which is the figure that should guide the next design decision.