Part V: Passive Solar Homes and SunspacesChapter 9. Sunspaces, Greenhouses, and Solar Rooms
Low-Mass Sunspaces for Daytime Heat
A light sunspace that delivers afternoon warmth and is closed off at night so it does not rob the house.
- Difficulty
- Intermediate
- Cost
- Varies
A low-mass sunspace is a heating room, not a greenhouse. The floor and the walls are light, so the air warms quickly when the sun is out. You move that air into the house during the day. At night you close a solid, insulated door and you let the sunspace go cold. If you wanted the room to stay warm for plants, you wanted water barrels and a different detail. The choice between those two jobs is the planning guide.
Light does not mean flimsy. The porch is still an addition: footing, a connection that does not rot the house wall, and glazing that will not fall on someone. Light means you are not counting on concrete or barrels to carry the house through the night. The house's own insulation does that, after the door is shut.
Difficulty
- Level
- Intermediate
- Time
- A design pass, then a construction project with a permit
- Skills
- Closing a door at dusk, and not storing tender plants in a room you intend to freeze
Cost estimate
Varies
A small attached porch is a foundation, framing, glazing, and a real door. Materials often start in the low thousands and climb with tempered glazing, a footing below frost, and someone else's labor. A plastic sheet on an existing porch is the cheap experiment and the short lifespan.
| Piece | Where the money goes |
|---|---|
| Footing and a sound connection to the house | Required even when the floor is light |
| Glazing and a solid night door | The heating performance |
| A small fan and a damper | Only if a door is not enough |
Materials
| Item | Qty | Notes |
|---|---|---|
| South wall that the winter sun actually hits | 1 | From the site survey. An east or west porch is a summer problem with a weak January payoff. |
| Light floor and light walls | The porch | Wood, not a slab you poured for storage. Do not add barrels and then strip the room at sunset. |
| Insulated door or panel | 1 | Weatherstripped. A hollow door with a gap at the sill is a hole you meant to close. |
| Glazing with a summer shade | The south face | Overhang or an exterior screen. See the shade guide and the overhang sketch. |
| Optional fan and backdraft damper | 1 | Daytime only. The damper or the door stops night flow. Read the damper guide. |
Why low mass heats the afternoon
Sun on a heavy floor disappears into the floor. Sun in a light room shows up in the air, which is what a fan or an open door can move. The porch stores very little, so you deliver the heat while it exists.
The porch will overheat if the house cannot take the air, or if the fan is off and the door is shut on a bright day. Vent outdoors, or open the path indoors, before the temperature is a problem for the glazing or for anyone working out there. A low-mass room overheats sooner than a barrel room.
At night the same lightness is why the porch collapses toward the outdoors. Glazing is a poor wall. An open door at 10 in the evening undoes the afternoon. The design depends on a habit: the last person who leaves closes the door, and a fan thermostat is not allowed to run just because the porch is merely warm. It should run only when the porch is usefully hotter than the house.
An 8 by 12 foot porch with an 8 foot average height holds about 770 cubic feet of air. Six changes of that air in an hour is 770 times 6, divided by 60, or about 80 cubic feet per minute. That is a small fan, not a whole-house machine. A bigger porch scales the same way: volume times changes per hour, divided by 60. Too much fan, with no low return path, is the subject of the next heat-moving page.
What does not belong in this room
Tender plants do not belong here if you are serious about closing the room and walking away. A night near the outdoor temperature will kill them. If the plants win, you have changed the design to a greenhouse and you owe them mass and a shut door that keeps their heat with them.
A concrete slab "for later" fights the quick warmup and loads the footing. If you want that mass, design for it on purpose, on the mass page, and do not also plan to empty the room every sunset.
Summer sun will cook a light porch faster than a heavy one. An overhang sized with the noon sketch, plus a way to vent, is part of the heating design. West glazing is a mistake you will feel in July. Shade it or do not build it.
Write the job on paper
Daytime heat for the house. The porch may freeze. If that sentence is unacceptable, use the greenhouse page.
Confirm winter sun and a night door
Site survey, true south as close as the wall allows, and a weatherstripped door you will actually close.
Keep the interior light
No barrels, no surprise slab. Glaze the south face, shade the summer, and vent a bright afternoon.
Move air only while the porch is hotter
Door or a small fan with a damper. About six air changes an hour is a starting size, not a code.
Permit the addition
Footing, glazing safety, and what the town will call the room. A fan does not make it a bedroom.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/Sunspace/sunspaces.htm
- https://www.builditsolar.com/Projects/Sunspace/LowThermalMassSunspace.htm
- https://www.builditsolar.com/Projects/Sunspace/LTMSSGuide/LTMSSGuide.htm
- https://www.builditsolar.com/Projects/Sunspace/SolarRooms/SolarRooms.htm
- https://www.builditsolar.com/Experimental/GlazingTransmittance/GlazingTransmittance.htm
- https://www.builditsolar.com/Projects/SpaceHeating/Glazing.htm
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.