Passive Solar Building Design

Passive solar design uses the sun’s energy for heating and lighting without any mechanical systems, pumps, or electricity. It’s been used for thousands of years—Roman bathhouses, Puebloan cliff dwellings, and Persian wind catchers all employed these principles. In a post-collapse world, passive solar design is how you stay warm without burning through your firewood supply.

The core concept is simple: let sunlight in during winter, keep it out during summer, and store the heat in heavy materials that release it slowly overnight.

Solar Orientation & Geometry

Finding True South

True south (in the Northern Hemisphere) is not magnetic south. The difference—called magnetic declination—varies by location and can be 5-20° or more. For building orientation, you need true south.

Shadow stick method:

Place a straight stick vertically in level ground
Mark the tip of its shadow in the morning
Draw a circle on the ground using the stick as center and the shadow length as radius
Wait for the afternoon shadow to touch the circle again. Mark this point
A line between the two marks runs east-west. Perpendicular to this is true north-south

Noon shadow method: At solar noon (when the shadow is shortest—not necessarily 12:00 clock time), the shadow points true north (in the Northern Hemisphere).

Orient your building’s long axis east-west, with the largest window area facing within 15° of true south. This single decision captures the most winter sun while minimizing summer heat gain.

Seasonal Sun Angles

The sun’s angle above the horizon changes dramatically with seasons:

Winter solstice (Dec 21 in NH): Sun is at its lowest. At 40°N latitude, noon sun is only ~27° above the horizon
Summer solstice (Jun 21 in NH): Sun is at its highest. At 40°N latitude, noon sun is ~73° above the horizon
Equinoxes: Noon sun angle equals 90° minus your latitude

Quick formula for your latitude:

Winter noon sun angle = 90° - latitude - 23.5°
Summer noon sun angle = 90° - latitude + 23.5°

This difference is what makes passive solar work. A properly designed overhang blocks the high summer sun while admitting the low winter sun through the same window.

Window Sizing

South-facing glass should equal approximately 7-12% of the building’s total floor area for moderate climates, up to 15% for cold climates. Too little glass means insufficient heating; too much means overheating on sunny winter days and excessive heat loss on winter nights.

Critical: Minimize east and west windows (they admit low-angle summer sun that’s impossible to shade with overhangs). North windows should be small—they never receive direct sun and only lose heat.

Salvaged window glass is a priority resource. Store any glass you find carefully. If no glass is available, use oiled animal skin, oiled cloth, or thin-scraped animal intestine—these transmit some light and provide some insulation, though far less effectively than glass.

Overhang Design

A properly sized overhang is the passive solar thermostat. Too short and summer sun overheats the building. Too long and winter sun can’t reach the floor.

Sizing formula: For a window of height H at your latitude L:

Overhang projection = H × (overhang factor)
Overhang factor at 36°N ≈ 0.55
Overhang factor at 40°N ≈ 0.50
Overhang factor at 44°N ≈ 0.45
Overhang factor at 48°N ≈ 0.40

The overhang should begin at the top of the window or slightly above. A gap between the wall and the overhang (about 1/3 the projection) allows hot air to escape upward in summer rather than pooling against the wall.

Thermal Mass

Thermal mass is the battery of a passive solar building. Dense, heavy materials absorb heat during the day and release it slowly at night, smoothing temperature swings.

Mass Materials

Material	Heat Capacity	Availability	Notes
Water	Excellent (highest per volume)	Containers needed	Best mass-to-weight ratio
Stone/rock	Very good	Abundant	Heavy, slow to absorb
Adobe/rammed earth	Very good	Make from soil	Excellent for walls/floors
Brick	Very good	Salvage or make	Consistent performance
Concrete/urbanite	Good	Salvage rubble	Abundant in ruins
Dense wood	Poor	Abundant	Insulates more than it masses

Water is the champion thermal mass material—it stores roughly 2x the heat of stone per unit volume. Salvaged barrels, drums, or tanks filled with water and placed where sun strikes them directly are remarkably effective. Paint containers dark (ideally black) to increase absorption.

Mass Placement Rules

Place mass where sun hits it directly. Thermal mass in a dark corner does almost nothing. Mass floors, mass walls opposite south windows, and water containers in direct sun paths are effective
Thickness matters—but only to a point. For daily heating cycles, mass elements should be 10-15 cm thick for dense materials. Thicker mass responds too slowly—it’s still absorbing when you need it releasing
Surface area beats depth. A 10cm-thick mass wall with 20 m² of surface stores and releases more useful heat than a 40cm-thick wall with 5 m² of surface
Dark colors absorb more. Mass surfaces in direct sun should be dark-colored. A dark slate floor absorbs ~95% of incoming solar energy; a light-colored floor reflects most of it back out

The Heat Cycle

On a sunny winter day:

Morning: Sun enters through south windows, strikes thermal mass
Midday: Mass surface warms, begins radiating heat into the room
Afternoon: Mass is fully charged, room temperature peaks
Evening: Sun sets, mass continues radiating stored heat
Night: Mass slowly cools, keeping room above outside temperature
Dawn: Mass has released most stored heat, room is coolest

With adequate mass, indoor temperature swing can be limited to 5-8°C even when outdoor temperature swings 20°C or more.

Insulation

Insulation and thermal mass serve opposite functions. Mass stores heat; insulation prevents heat transfer. A building needs both, in the right places.

Rule: Mass goes inside the insulation envelope, exposed to the interior. Insulation goes outside the mass, facing the weather.

A common mistake is putting insulation between the sun and the mass—this prevents the mass from charging. Another mistake is putting mass outside the insulation—this heats the outdoors, not the indoors.

Natural Insulation Materials

Straw — R-value ~1.5 per inch (in wall thickness). Excellent in bale form. See straw-bale-construction
Wool — R-value ~3.5 per inch. Excellent but scarce. Works even when damp
Cattail fluff / milkweed — R-value ~3.0 per inch. Tedious to harvest in quantity
Dry leaves — R-value ~1.0 per inch. Settles and compacts. Better than nothing
Wood shavings / sawdust — R-value ~2.5 per inch. Fire risk. Treat with lime
Moss — R-value ~1.5 per inch. Naturally fire-resistant
Earth (dry) — R-value ~0.2 per inch. Poor insulator. Use only in underground-earth-sheltered designs where earth mass is the strategy

Ventilation & Summer Cooling

A building designed only for winter heating becomes an oven in summer. Passive cooling strategies must be integrated from the design phase.

Cross-Ventilation

Place operable openings (windows, shutters, vents) on opposite walls. Wind entering one side and exiting the other creates airflow. The outlet opening should be slightly larger than the inlet—this accelerates airflow through the Venturi effect.

Position inlets low and outlets high when wind is calm. Hot air rises naturally (stack effect) and exits through high openings, pulling cooler air in through low openings.

Thermal Chimneys

A thermal chimney is a dark-colored, sun-exposed vertical shaft that heats air inside it. The hot air rises and exits at the top, creating a draft that pulls air through the building. No wind required.

Construction: A south or west-facing dark masonry or metal column with an opening at the base (connected to the building interior) and an opening at the top (vented outside). Even a dark-painted salvaged metal pipe works.

Earth Tubes

Below about 2 meters depth, ground temperature remains nearly constant year-round (roughly equal to the annual average air temperature). A buried tube running from an outside intake to inside the building pre-cools summer air and pre-warms winter air.

Practical requirements:

Tube length: 15-30 meters minimum
Depth: 1.5-2.5 meters
Diameter: 15-30 cm
Slight downward slope toward the building for condensation drainage
Screen the outside intake against rodents and insects

Salvaged PVC pipe, clay pipe, or even stone-lined channels work. The longer the tube, the closer the air temperature approaches ground temperature.

Integration with Building Methods

Passive solar principles apply to any construction method, but some combinations are particularly effective:

straw-bale-construction + thermal mass floor: Straw bales provide superb insulation (R-30+), while a dark-colored stone or adobe floor provides thermal mass. South-facing windows complete the system
earthbag-building walls: Earthbag walls provide both moderate mass and moderate insulation. Add external insulation in cold climates
underground-earth-sheltered + south glazing: Earth-sheltered buildings use the ground itself as thermal mass and insulation. A south-facing glazed wall admits winter sun into the earth-mass interior

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Passive Solar Building Design