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

Attached Sunspace Planning: Heating Room vs. Growing Room

Why a room meant to heat the house is detailed differently from a room meant to grow plants.

Difficulty
Intermediate
Cost
Planning only
published

An attached sunspace is a glazed room on the south side of a house. People ask it to do two jobs that fight each other. A heating room should get hot on a sunny day and give that heat to the house, then close up so it does not give the house's heat back to the sky at night. A growing room should stay moderate, hold enough warmth overnight that plants do not freeze, and keep its humidity out of the bedrooms.

You can compromise, and the compromise should be written down before you pour a footing. A room detailed for tomatoes will be a poor furnace. A room detailed as a furnace will kill the tomatoes the first night you harvest its heat. Make that decision first, and the details follow from it. None of this is a structural plan for an addition.

Difficulty

Level
Intermediate
Time
Planning, then a construction project that is beyond a weekend
Skills
Reading sun angles and being willing to pick one primary job

Cost estimate

Planning only

A small attached sunspace is a foundation, a frame, glazing, and doors. Materials alone are often a few thousand dollars and can pass ten thousand once glazing and a proper footing are included. Get local prices before you treat a magazine width as a budget.

Materials

ItemQtyNotes
Site survey of the south wall1Winter sun, summer overhang, and a door location that does not wreck a room.
A written job statementOne sentenceEither heat the house by day, or keep plants alive. Secondary wishes come after.
Permit questionsA listFooting depth, glazing safety, egress, smoke alarms, and whether the room counts as living space.

What a heating room wants

The glazing faces true south, or close to it. East or west glazing adds summer heat you then have to throw away and does less for a January afternoon. Size the glass for the winter day, not for a feeling of living in a greenhouse. A common planning range is a glazed area that the house can actually absorb during the day without driving the adjacent room past comfort. If you cannot move the heat indoors, extra glass only overheats the sunspace.

Keep the mass low if the goal is daytime heat delivery. A light floor and light walls warm up quickly, the air gets hot, and a fan or an open doorway moves that air into the house while the sun is out. At night you close a solid door, or you close an insulated panel, and you stop the fan. The sunspace is allowed to go cold, so the house is no longer trying to heat the outdoors through a plastic wall.

A backdraft damper belongs on any fan opening, for the same reason it belongs on a window heater. A hole high in the wall will dump warm house air into the sunspace as soon as the sunspace is colder, unless the damper or a door stops it.

An overhang or a deciduous vine keeps the high summer sun off the glass. Without that, a heating room becomes a cooling load in July. The shade guide is the geometry. Sketch the overhang with the summer noon altitude before you build the roof.

What a growing room wants

Plants want light, water, and a temperature that does not swing from 90 degrees F in the afternoon to 25 degrees F at dawn. The tools for that are different. Water in dark barrels is thermal mass: it absorbs heat during the day and gives it back at night. The room is allowed to run cooler in the day than a heating sunspace, because some of the sun is going into the barrels instead of into a blast of air for the kitchen.

You do not strip that room with a fan at sunset. If you do, the mass never charged, or you stole the charge, and the plants see the night sky through the glazing with nothing to help them. A growing room can still vent excess afternoon heat outdoors, or into the house when the house actually wants it, but the night position is closed and the mass stays with the plants.

Humidity is the cost. Soil and watering put moisture in the air. Blowing that air into a cold bedroom condenses it on the first chilly surface and invites mold. A growing room wants its own vents to the outdoors, a floor that can get wet, and a door that stays shut in damp weather. Treat it as a greenhouse that happens to share a wall with the house.

Glazing for plants can include some east exposure so the room wakes up, and it should still avoid a huge unshaded west face. Summer shading matters even more than in a heating room, because you cannot simply let the space overheat and then dump it.

Heating room and growing roomTwo attached sunspaces. The heating room is lightly furnished and can send air to the house by day, then close off at night. The growing room holds water barrels and plants and stays separated when the house would steal its heat.Heating roomDaytime air to the houseClose it at nightGrowing roomWater barrels for the plantsDo not strip this room at night
Left: a heating room sends daytime air to the house and is closed at night. Right: a growing room keeps water barrels with the plants and does not give that heat away at sunset.

The details people swap by accident

ChoiceHeating roomGrowing room
Night connection to the houseClosed, insulatedClosed, so humidity stays put
Thermal massLittle, so the air heats fastWater barrels or masonry, for overnight
Where afternoon heat goesInto the houseOutdoors first, into the house only if it is welcome
FloorLight and dryAble to get wet
Summer shadeOverhang sized for JuneOverhang plus vents
Failure you are avoidingA night-time hole in the houseFrozen plants or a moldy wall

A hybrid is possible if you say which job wins. Example: plants in containers that you accept will not see a hard freeze protection, and a heating room that is allowed to drop to near outdoor temperature on clear nights. That is a heating room with a few tough plants, not a greenhouse. The reverse hybrid, a growing room that sometimes shares a sunny afternoon with the house through a door you open on purpose, is also coherent. The version that fails is a door left open from November to March.

Structure, code, and the fact that this is an addition

An attached room needs a foundation that goes below frost where the climate requires it, a connection to the house that does not rot the existing wall, and glazing that meets the safety rules for the location. Tempered or laminated glazing is commonly required in doors, in sidelights, and near the floor. Ordinary annealed glass is not a bargain if someone can fall through it.

If the room is big enough or conditioned enough to count as living space, egress, smoke alarms, and energy codes may apply. A fan and a backdraft damper do not turn a sunspace into a legal bedroom. Ask the building department what they will call the room before you design a wall of glass you cannot permit.

Water barrels are hundreds of pounds. The floor and the footing have to be designed for that load if you chose the growing-room path. A light heating room should not gain a slate floor "for mass" at the last minute without someone checking the structure.

Fire separation, if the sunspace is on a garage wall or near a property line, is a local rule. So is the distance to a lot line for a glazed wall. The site survey's note about easements matters here.

  1. Write one job in a sentence

    Heat the house on sunny days, or grow plants through the winter. Put the sentence at the top of the sketch.

  2. Place it on a real south wall

    Use the site survey. If that wall is shaded at noon in December, do not attach a sunspace there and hope.

  3. Draw the night position

    Show the door or damper closed. If you cannot close it, the plan leaves a hole in the house every night.

  4. Draw summer shade and water

    Overhang or vine for high sun. For a growing room, also draw where water goes when you overwater.

  5. Ask what the town will call it

    Footing, glazing safety, egress, and whether barrels are a structural load. Adjust the sketch to the answers.

  6. Only then price glazing

    Glazing is expensive. Buying it before the night strategy and the permit answers is how sunspaces become unusable glass porches.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.