Part V: Passive Solar Homes and SunspacesChapter 9. Sunspaces, Greenhouses, and Solar Rooms

Solar Greenhouse with Heat Storage

Water and masonry storage for a greenhouse that must stay above freezing after sunset.

Difficulty: Advanced
Cost: Varies

published

A solar greenhouse has to still be a greenhouse after the sun goes down. The glazing that collected the afternoon will dump it overnight unless something in the room is holding heat and giving it back. That something is usually water, because water stores more heat per cubic foot than soil or concrete, and you can put it where the sun hits it.

This is a different design from the low-mass heating porch. If you fan the greenhouse empty at sunset, you have stolen the charge the plants needed. Vent extra afternoon heat outdoors, or into the house only when the house wants it and the barrels have already had their sun. Then close up and leave the mass with the plants.

A string of cloudy days or a night far below the design temperature will still beat a passive barrel bank. Have a listed backup heater and a cold-night plan before you put crops in that you cannot afford to lose.

Difficulty

Level: Advanced
Time: A building, not a weekend, plus a winter of watching the thermometer
Skills: Structure for a wet, heavy room, and the honesty to add backup heat

Cost estimate

Varies

Glazing, a foundation, and barrels of water dominate. A small greenhouse is often several thousand dollars before a backup heater. The sketch below sizes water for glazing loss only. It is not a bid.

Piece	Why it is there
Glazing you can look up a U-factor for	The overnight leak
Water in dark, sunlit, closed containers	The storage
Listed backup heat	The week the sun does not show

Materials

Item	Qty	Notes
Glazing with a known U-factor	The south and roof faces	Twinwall is the usual compromise. Film loses heat faster. Read the glazing comparison.
Water containers the sun can see	From the sketch, then more for leaks	Closed barrels or tanks rated for water. Not pesticide drums. Not glass carboys.
A floor that can be wet and can carry the weight	1	Water is about 8.3 pounds per gallon. A few hundred gallons is a structural load, not a decor choice.
Night insulation or a solid north wall	As designed	Every face that is not earning sun is a place to put a real wall instead of more glazing.
Listed heater and a thermometer	The backup	Electric heat on a proper circuit, or a heater rated for a greenhouse. No unvented combustion in a closed glazed room.

How much water the glazing asks for

Overnight, the glazing loses heat in proportion to its area, its U-factor, how cold it is outside, and how long the night is. Water replaces that heat by cooling a little. Divide the heat by about 8.34 BTU per gallon per degree, and by the drop you will tolerate in the barrels, and you have gallons.

The sketch uses those inputs and nothing else. A leaky door, a north wall of film, and a windy site add loss the sketch does not see. Soil in beds helps, slowly, and it does not replace sunlit water. Paint or choose dark containers so the sun charges them instead of only the air. A barrel in the shade stays cold.

If the outdoors is not colder than your target, the sketch shows no glazing loss. The room may still not be safe; this particular arithmetic simply has nothing to calculate.

The barrels stay inside with the plants. Afternoon surplus can leave. The overnight charge should not.

Backup, humidity, and weight

Set a thermometer at plant height and another on a barrel. If the air is near freezing while the barrels are still warm, you are not coupled: the barrels are in the wrong place, or the air is leaking faster than they can answer. If both are falling on a clear night and the sketch said you had enough water, believe the thermometer over the sketch and add backup or insulation.

Humidity will be high, as in any greenhouse. Do not duct that air into bedrooms. Condensation on the glazing is normal. Condensation inside a house wall is not. Keep the shared wall able to dry, and keep irrigation off the sill.

The floor and the footing have to be designed for wet weight. A plastic stock tank of water on a wood platform that was sized for people is how floors fail. Anchor containers so they cannot tip onto a person or a child. Leave aisles that stay aisles.

A backup heater that burns fuel inside the greenhouse needs a listed vent and combustion air, or you do not use it. Carbon monoxide and oxygen depletion in a tight glazed room are fatal. Electric heat still needs a ground-fault protected circuit in a wet building, installed by someone qualified if that work is not yours. An extension cord under a bench is not a winter wiring method.

Decide the coldest night you are designing for
A normal winter night and a brutal one are different projects. Write down the target at plant height, often above freezing for tender crops and warmer for some of them.
Run the glazing sketch, then add the leaks
Use a U-factor from the product, not from memory. Add area for a door you will open and for any single-film wall.
Put dark water in the sun
Closed containers, on a floor designed for the weight, with aisles left clear. A shaded barrel is only dead weight.
Close the room at night
Do not fan it into the house at sunset. Vent only the extra afternoon heat.
Watch one clear night and one cloudy stretch
Thermometers on the air and the water. If the air loses, fix coupling or add the listed backup before a crop depends on it.

Solar Greenhouse with Heat Storage

Difficulty

Cost estimate

Materials

How much water the glazing asks for

Glazing loss into gallons of water

Backup, humidity, and weight

Decide the coldest night you are designing for

Run the glazing sketch, then add the leaks

Put dark water in the sun

Close the room at night

Watch one clear night and one cloudy stretch

Further reading