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

Retrofitting an Existing Home for Passive Gain

What you can change in a house that is already built: glass, overhangs, mass, and insulation.

Difficulty: Intermediate
Cost: Varies

published

An existing house already has a south wall, a set of mistakes, and a structure that is carrying the roof. You do not get to pour the slab in the sun and frame the glass around it. You get to seal what leaks, insulate what is thin, shade the glass that already overheats, and only then decide whether a new opening is worth a header.

The order matters more than the catalog of ideas. New south glass in a leaky, unshaded house enlarges the swing. The room is hotter at 3 in the afternoon and colder at 6 in the morning, and the bill barely moves. Do the air sealing and the attic insulation first. Then shade east and west glass you already have, using the shade guide. Then read the glass fraction before you cut a wall.

Difficulty

Level: Intermediate
Time: A season of small jobs, or a permitted structural project if you enlarge an opening
Skills: Leaving bearing walls alone until someone sizes a header

Cost estimate

Varies

Air sealing and attic insulation are the inexpensive end. A new south opening with a header, an overhang, and better glass is a construction project. Uncovering a slab sits between them. Price the structural work locally.

Move	Where it sits
Seal and insulate the attic	Usually the first money
Shade and night-cover the glass you have	Next, and often enough
A new south opening or a sunspace	Only after the envelope, and only with a header

Materials

Item	Qty	Notes
The site survey	1	Winter sun on the real south wall, not on a wall you wish faced south.
A list of bearing walls	From someone who can tell	Enlarging a window in a bearing wall needs a header sized for the load above it.
The mass you already have	A look under the carpet	A slab under a rug is the cheapest solar storage in an old house.

What is usually possible

Seal the ceiling, the rim, and the obvious holes. Insulate the attic to a real R-value. Those two steps cut the loss the sun would otherwise have to replace. They also make the house more likely to overheat if you add a lot of glass later, which is an argument for a modest glass fraction, not an argument for skipping the insulation.

Shade the windows that cook the house in summer. West glass is the usual villain. An overhang on a south window helps only the south window, and only around midday. Use the overhang sketch if you add one, and do not remove a summer shade tree until that overhang exists.

Night covers on the glass you keep are a retrofit that does not touch the structure. Quilts and shades make existing south windows behave more like a passive design and less like a hole.

Add south glass only when the site survey shows winter sun, the fraction is still inside the band after you add it, and the wall can take the opening. A wider window in a bearing wall needs a header. That is a permit and a carpenter, sometimes an engineer. It is not a reciprocating saw and a Saturday. Do not remove a bearing wall to "open the house to the sun."

A sunspace is sometimes easier than cutting the south wall, because the new glass is outboard and the house wall stays. It only helps if you can close it off at night and if you have a way to move daytime heat indoors without depressurizing combustion appliances. Read that page before you enclose a porch.

Seal, insulate, shade what you have, then glass, then the mass that is already under your feet.

Mass without a new foundation

Look for mass the sun already hits. A slab under carpet is the usual find. Tile or a hard finish on the sunlit strip puts that slab back in service. A new masonry wall in a retrofit needs a footing. Most houses should not get one just to satisfy a rule of thumb. Use the mass sketch, compare it with the glass you propose, and shrink the glass if the mass is not there.

Mass in a room the sun never enters will not store solar gain. It can smooth the furnace's output a little. Do not pour a floor in a north bedroom and count it as the south window's battery.

Thick-wall retrofits, such as an exterior truss full of insulation, belong to the wall comparison. They are a siding-off project with a drainage plane, not a passive-gain extra. They do reduce the glass you need.

Seal and insulate before you redraw the south wall
Attic, rim, and the leaks you can find. Confirm winter sun with the site survey.
Shade and cover the glass that is already there
West and east shade first. South overhangs from the noon sketch. Night insulation on the windows you will keep.
Uncover mass you already own
Carpet off a sunlit slab, or a hard finish on that strip. Do not count a back room.
Add glass only inside the fraction, with a real header
If the wall is bearing, the header is designed and permitted. If the fraction would land too high, keep the wall.
Consider a sunspace instead of a cut wall
Closeable at night, with a heat path that does not backdraft a fireplace or a water heater.

Difficulty

Cost estimate

Materials

What is usually possible

Mass without a new foundation

Seal and insulate before you redraw the south wall

Shade and cover the glass that is already there

Uncover mass you already own

Add glass only inside the fraction, with a real header

Consider a sunspace instead of a cut wall

Further reading