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Part V: Passive Solar Homes and SunspacesChapter 8. Designing a Passive Solar House

Passive Solar Principles in Plain Language

Collect, store, and keep: the three jobs a passive solar house has to do on a winter day.

Difficulty
Beginner
Cost
Free
published

A passive solar house does three jobs on a winter day. It collects sun through glass, mostly facing the winter sun. It stores that heat in mass so the afternoon does not become a spike and the evening does not become a chill. It keeps the heat with insulation and air sealing so the night does not give it all back.

If any job is missing, the house misbehaves in a predictable way. Glass without mass overheats by day and crashes after sunset. Glass without a tight, insulated shell collects heat and leaks it. Mass and insulation without sun are just a comfortable ordinary house, which is a fine outcome and not a solar one. People get into trouble by adding glass first because glass is the visible part.

The site survey tells you whether the winter sun actually reaches the south wall. True south tells you where that wall should aim. Heat loss tells you how big the "keep" job already is. Start there.

Difficulty

Level
Beginner
Time
An evening of reading, then a walk around your own south wall
Skills
Holding three jobs in your head at once

Cost estimate

Free

No parts. The next pages turn this into glass ratios, mass, thick walls, and retrofits, which are where money shows up.

Materials

ItemQtyNotes
A floor plan and a south arrow1Mark which rooms the winter sun can actually enter.
A note on the envelope1Attic insulation, obvious leaks, and the worst windows. The heat-loss sketch is enough.
A summer complaint, if you have one1Rooms that already overheat are not candidates for more glass until they have shade.

Collect

Winter sun in most of the United States comes from the south and sits low. Glass on the south wall sees it. Glass on the north wall mostly sees heat leaving. East and west glass sees a low sun that is hard to shade and is a summer liability. The useful collection is south glass, sized with some restraint, shaded in summer by an overhang. The overhang page does that geometry.

There are three classic arrangements. Direct gain lets the sun fall into the room. Indirect gain puts mass between the sun and the room, as in a masonry wall behind glass. Isolated gain puts the sun in a sunspace that can be closed off from the house when the sun is gone. Direct gain is the one people mean when they say passive solar. It is also the one that overheats if the glass fraction and the mass are out of balance.

A tight house needs less glass to stay warm, and it overheats sooner if you give it the glass area an old leaky house seemed to want. Insulating first changes how much glass is safe.

Collect, store, and keepThree jobs: south glass collects, mass stores, and insulation plus air sealing keep the heat overnight.CollectSouth glassStoreMass in the sunKeepInsulate and sealGlass without mass overheats by day. Glass without a tight envelope gives the heat back at night.
Collect with south glass, store in mass the sun can reach, and keep the heat with insulation and a sealed shell.

Store and keep

Mass works when the sun strikes it or when it is broadly coupled to the room that got the sun. A concrete floor in the sun, a masonry wall in the sun, or a measured volume of water. A rug, a bookcase, and a back bedroom do not count. The mass page turns that into gallons and cubic feet.

Keeping the heat is ordinary building physics. Air leaks and thin insulation dump the afternoon's gain before bedtime. At night, the glass that collected heat at noon lets it out. Good double glazing is still around R-3. A wall is several times that. Insulated shades, on the quilt page, belong in a serious direct-gain design.

Summer is the test people skip. An overhang, trees where the shade page says they belong, and windows you can open at night are the cooling half of the same design.

  1. Name the three jobs for your house

    Where would the sun enter, what would it strike, and what is the envelope like tonight. Write the weak job in a full sentence.

  2. Confirm winter sun and true south

    Use the site survey. Glass on a shaded south wall is just a window.

  3. Do not add glass to a leak

    Air sealing and attic insulation come before a new opening. A tight house wants a smaller glass fraction, not a larger one.

  4. Pick direct, indirect, or isolated gain

    Direct gain needs mass in the room. A sunspace needs a door you can close at night. A masonry wall behind glass is structural work.

  5. Write the summer sentence too

    Overhang, shade, or less glass. If you cannot say how July stays comfortable, you are not ready to enlarge the south wall.

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.