A Greenhouse Front That Heats the House: How Passive Solar Really Works

Photo by Dominic Alves, CC BY 2.0

On the coldest January nights in Alberta, the greenhouse along the front of the Kinney family's earthship drops to a few degrees above freezing. By mid-morning on a sunny day it is back in the mid-teens, and the peppers and tomatoes inside are still ripening on the vine. The house behind the glass sits at about 22 °C on those sunny days with no furnace running. On cloudy days, the family lights a small wood stove in the morning and gets on with things.

That gap, between the sunny-day version and the cloudy-day version, is the whole subject of passive solar. The brochures print the first half. Builders and owners who have lived with these houses print the second. This article follows the second group, and it counts everything it can: the sun angles, the heat in a barrel of water, and the share of a heating bill a wall of glass can honestly carry.

The idea itself is simple enough to say in one breath. Put a room of glass on the sun-facing side of a heavy house. Sunlight comes through the glass, lands on something massive, and turns to heat. The mass gives the heat back slowly, after the sun is gone. Everything else, the vents, the overhangs, the arguments, is machinery for keeping that one exchange under control.

Three ways sunlight becomes heat

Passive solar comes in three basic arrangements, and they are not equally good at the same job.

In direct gain, sunlight falls straight through south windows onto a mass floor or wall inside the living space. It is the most efficient of the three: about 60 to 75 percent of the solar energy striking the glass ends up as useful heat. In indirect gain, the mass sits right behind the glass and the room sits behind the mass. The classic version is the Trombe wall, a masonry wall a hand's breadth behind the glazing. It delivers about 30 to 45 percent. In isolated gain, the sun heats a separate room, the sunspace or greenhouse front, and the house takes heat from that room through doors and vents when it is offered. Its efficiency varies more than the other two, because it depends on how the room is run day by day.

So the greenhouse front is, on paper, the least efficient heater of the three. People choose it anyway, for two reasons no efficiency table captures. The first is control. A sunspace can be closed off. At night, on a cloudy stretch, or in a summer heat wave, the doors shut and the house keeps its own weather. Direct gain cannot be shut off; the sun is in the room whether the room wants it or not. The second reason is that the space works twice. It is a heater when the sun shines and a garden the rest of the time.

There is a third benefit, quieter than either. Even on a day when the sunspace is too cold to give the house any heat, it is still warmer than the outdoors. The wall it covers is a wall the house is not losing heat through. Researchers who measure sunspaces carefully count part of their value as exactly that: not heat delivered, but heat never lost.

The earthship version, stated plainly

The greenhouse front reached its widest audience through the earthship, the design Michael Reynolds and Earthship Biotecture have built out of Taos, New Mexico since the 1970s. The houses are horseshoe-shaped, opening toward the sun; the original Taos prototype faces its opening 10 to 15 degrees east of due south. The long front wall is glass, and in current designs it is a double greenhouse: an outer angled glass wall, a corridor of planting beds, then an inner glass wall before the rooms. The planting beds do double duty as the house's greywater treatment, watering the plants with filtered sink and shower water on its way through.

The claim attached to these houses is famous: an earthship holds about 70 °F, around 21 °C, whatever the weather does, with no heating system at all. Stated that way, it does not survive contact with the owners. It is a design aim from the high desert of New Mexico, where the sun is strong and the sky is clear most of the year. Earthship Biotecture designs for a steady indoor temperature using mass and sun, and the best desert builds come close to it. Owners in cloudier, colder country report something else, as the problem accounts below show. The design travels. The guarantee does not.

The sun-angle arithmetic anyone can redo

Every passive solar rule of thumb rests on one piece of geometry, and it takes thirty seconds to check. At solar noon, the sun's height in the sky is 90 degrees, minus your latitude, plus the sun's declination, a tilt figure that runs from +23.44 degrees on June 21 to −23.44 degrees on December 21.

Run it at 40 degrees north, a common reference latitude. The June noon sun stands 73.4 degrees up. At the equinoxes it is 50 degrees. On December 21 it reaches only 26.6 degrees. Run it again at 43.5 degrees north, roughly the latitude of southern Ontario. June gives 69.9 degrees, the equinoxes 46.5, and December just 23.1. That December sun is low enough to shine under a porch roof and across a whole room, which is exactly what a designer wants it to do.

The 47-degree gap between the June and December noon suns is what makes a fixed overhang work. The arithmetic: to shade a window of height H completely at June noon, the overhang depth is H divided by the tangent of the June altitude. At 40 degrees north, shading five feet of glass at June noon takes about a foot and a half of overhang. That same ledge at December noon shades only about the top nine inches of the glass. One fixed piece of wood or roof does both jobs, because the sun itself changes height.

Two honest caveats belong with the trick. It is noon geometry. Morning and afternoon sun comes in at slant angles the ledge does not stop, which is why real designs add vents and shade cloth instead of trusting the overhang alone. And builders usually pick their shading dates, not the solstice: a house at higher latitude might be shaded from late April to mid-September, the local overheating season, and left open to the sun the rest of the year. The US Department of Energy's siting rule is blunter than any of this. Keep the glass within 30 degrees of true south, and keep it unshaded between 9 a.m. and 3 p.m. through the heating season.

Thermal mass, counted

Mass is where the promises get vague, so count it. Take the standard 55-gallon drum of water, painted dark and stood in the sunspace. Water holds 1 BTU of heat per pound for every degree Fahrenheit it warms, and a gallon of water weighs 8.34 pounds. One barrel therefore stores about 459 BTU per degree. Across a good sunny day's swing of 20 degrees, that is roughly 9,200 BTU, about 2.7 kilowatt-hours of heat. Real, but modest: a small plug-in heater makes the same amount in under three hours. Barrels earn their place in rows along the back wall, not as single drums. The famous precedent is Steve Baer's Zomehouse in Corrales, New Mexico, which stacked a wall of water-filled drums behind its glass in the 1970s.

Measured by volume, water stores about twice what masonry does: roughly 62.4 BTU per cubic foot per degree, against about 30 for concrete. The slab still does most of the work in most sunspaces, for the simple reason that it is already there, being the floor. An earthship's earth-packed tire walls sit in the masonry range, not the water range. They are bulk, and bulk is what they contribute.

Mass also has a speed, and the speed is slow. Heat moves through concrete at roughly an hour per inch. Sun absorbed by the face of an 8-inch wall at noon arrives at the room side at about 8 in the evening. That delay is the point: mass sized for the daily cycle takes the noon surplus and spends it in the evening. Walls far beyond 8 to 16 inches add little to a daily cycle. The extra thickness is a battery the day cannot charge.

The last number in this section is the glass itself. A single pane insulates at about R-1. Standard double glazing reaches about R-2 to R-2.5, roughly half the heat loss. Double glazing with a low-E coating and argon fill gets to about R-3.3 to R-4. Even the best of those is nowhere near an insulated wall, which is why serious designs add night insulation over the glass, insulating shades or panels rated around R-4 or better, and why the doors between sunspace and house close at sundown.

Where it fights back

Left to itself, a greenhouse front does not hold a steady temperature. It swings. A conventional sunspace with little mass and no venting can pass 100 °F on a sunny winter day and fall below freezing the same night. Mass, vents, and night insulation narrow the swing. Nothing removes it.

The failure accounts are specific, and they rhyme. A Montana earthship owner, writing about his own house, reports fall sun driving surfaces near the glass past 140 °F on an infrared thermometer, even with overhangs, shades, and vents working. In overcast winter weather at 10 °F, the front of that house barely reached 55 °F with a 30,000 BTU propane furnace running at full duty. The earth-sheltered back rooms held 65 to 68 °F and no more. His conclusion was about geography: the design grew up under the Taos sun, and a wall of glass answers to local cloud whether the drawings mention it or not.

New Mexico itself is not exempt. An architect who consults for frustrated earthship owners there reports south galleries at 95 °F and humid through the summer, north ends of rooms sitting near 60 °F in winter, and at least one owner who opened every door and window in blowing snow to escape winter overheating. Ventilation researcher Rachel Preston Prinz, summarising earthship owners in cold climates, found the standard skylight vents buried by snow and dug out as often as three times a day, and houses whose winter and summer winds arrive from directions the vent diagrams never assumed. A vent that has to be shovelled out is a chore wearing the word "passive" as a costume.

The working fixes are unglamorous and they are known. Paired vents between greenhouse and house, high on the wall to let warm air out and low to let cool air return, move heat with no fan at all. One long-running attached greenhouse, 38 feet by 13, runs exactly that loop, staples shade cloth over its clear ceiling panels each summer, and grows a living screen of sunflowers and morning glories across the front glass. Shade cloth and a green wall look like decoration in photographs. In the accounts, they are load-bearing.

The greenhouse also exports trouble the brochures do not mention: moisture. A planted room is a humid room, and humidity above roughly 80 to 85 percent, with condensation settling on leaves, is the trigger condition for botrytis, the grey mould every greenhouse grower knows. Whitefly, aphids, and fungus gnats do well in warm, still, wet air. The controls from commercial greenhouse practice are routine rather than technical. Vent in the morning to dump the night's moist air. Keep air moving horizontally. Space the plants. Water early in the day. Hang sticky traps and read them. And keep the door to the house shut in winter, because an open door is a humidity pipe into rooms that were never built to be wet.

Underneath every one of these accounts sits the same limit. Mass sized for a day stores a day. Nothing in a wall stores a grey week. Every serious treatment of passive solar pairs it with a backup heat source, and the cold-climate owners use theirs. Passive solar shrinks the heating bill and the backup's workload. In a northern winter, it does not delete the backup.

What the growing side really gives

The second job of the greenhouse front is food, and it needs the same honest counting. University extension services measure unheated season extenders in weeks, and their numbers agree with each other. Oregon State's teaching puts an unheated structure at about three to four weeks of extra season at each end. Wisconsin Extension and Cornell-cited guidance credit hoop houses with six to eight weeks. Illinois Extension credits cold frames with one to three months, depending on the crop and the winter.

An attached greenhouse against a heated house does better than a freestanding tunnel at protecting tender plants, because one of its walls is a warm house. The fair expectation: roughly a month or more of extra growing at each end of the season for cold-hardy greens, more in the sheltered strip along the house wall, and a genuinely kind microclimate for starting plants early. The Alberta family's January peppers and tomatoes are real, and they come from a massive earth-sheltered build in a sunny continental climate. They are the strong end of the range, not a promise that transfers to a lean-to in a cloudy one.

The honest bottom line

So what can a greenhouse front actually carry? A whole passive solar design, orientation, glass, mass, insulation, and control working together, is commonly credited with covering about 30 to 70 percent of a home's heating demand in a suitable climate. The sunspace alone is the least predictable part of that design. On sunny days it contributes a substantial share. Under cloud its contribution shrinks fast, and left connected to the house through a grey stretch it can become a net heat loser, a glass room the furnace ends up heating. Anyone who promises that the greenhouse alone heats the whole house is selling, not measuring.

None of that makes the idea a failure. It makes it a tool with a known size. A greenhouse front, built with mass under it, vents that work in the local wind, shade for the summer, doors that close, and a backup heat source behind it, gives a house a warm room in March, greens weeks before the garden wakes, and a heating bill with a real dent in it.

The Kinneys in Alberta still light their stove on cloudy mornings. They also pick ripe peppers in January, in a house the sun heats to 22 °C on a clear winter day, a few degrees of frost on the other side of the glass. That is not the brochure's version. It is better, because it is the one a family is actually living.