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Greenhouse Thermal Mass: Water Barrels Explained
Water holds about twice as much heat per volume as concrete — and it is the cheapest mass most people can get.
Direct answer: Water barrels absorb heat during the day and release it at night, moderating greenhouse temperature. Use 2–3 gallons of water per square foot of glazing, place barrels where they get winter sun, paint them black, and accept that they slow temperature swings rather than eliminating frost.
How thermal mass stores day heat
Water has a high specific heat: about 1 Btu per pound per °F, and a cubic foot weighs 62 lb. A 55-gallon drum holds roughly 460 lb of water, so raising it 20°F stores about 9,200 Btu. That is a meaningful buffer overnight — roughly the heat from burning a pound of propane — but it is not unlimited.
How many barrels, where to place them
- Count: 2–3 gallons per square foot of glazing. A 10x12 greenhouse with 120 sq ft of glazing needs 240–360 gallons — four to six drums.
- Placement: along the north wall and the inner east/west edges, where they catch low winter sun but do not shade summer beds.
- Colour: black or dark blue absorbs more solar gain. Clear barrels grow algae.
- Elevation: on a stable base, never on bare soil that can settle.
Worked example
Five drums, 275 gallons, 2,290 lb of water. A sunny day raises the water 15°F: 2,290 × 15 = 34,350 Btu stored. Overnight, as the greenhouse loses heat, that water releases the same amount as it cools — roughly equivalent to 0.4 gallons of propane. Helpful, not a furnace.
Limits
- Two cloudy days in a row and the mass is depleted.
- Mass inside the glazing envelope loses heat outward like everything else.
- Barrels take floor space that could grow plants.
- They moderate swings; they do not raise the average temperature.
Pair mass with insulation and a night curtain. Mass without insulation is a bucket of warm water in a cold room.
Frequently asked questions
Do the barrels need to be in direct sun?
Ideally yes in winter. Barrels in permanent shade still add mass but gain far less heat.
Can I use milk jugs instead?
Yes for small greenhouses. They are cheap and work, but you need many more of them and they degrade in UV.
Does the water freeze?
In a well-insulated greenhouse, rarely. If it does, the barrels are damaged and the heat buffer is gone — drain and refill in spring.
Part of the self-heating greenhouse guide; see also heating without electricity.
Frequently asked questions
Do the barrels need to be in direct sun?
Ideally yes in winter. Barrels in permanent shade still add mass but gain far less heat.
Can I use milk jugs instead?
Yes for small greenhouses. They are cheap but need many more of them and degrade in UV.
Does the water freeze?
In a well-insulated greenhouse, rarely. If it does, the barrels can be damaged and the heat buffer is gone.