Passive Solar Heating

SECMOL buildings are heated without emitting CO2 or burning anything fuel such as firewood or gas, nor electric heaters. Passive solar design absorbs heat from the sun and then stores it as long as possible. It does not use circulating water pipes, air blowers, or moving parts. With only passive solar heating, the SECMOL buildings have let us run residential programmes every winter for the last 15 years — even when the minimum outside temperatures falls to -25°C.

The temperature in the main building at SECMOL Campus has been:

Normal evening temperature in coldest part of winter: +14°C
Minimum in a normal winter: +10°C
Minimum observed in 19 years: +7°C

In the main building at Phey Campus, the stairwells have houseplants flourishing even in January.

The main features that keep the buildings warm are:

• South facing windows, as the sun moves low in the southern sky in winter.
• Greenhouses are attached the south side for winter.
• Greenhouses are removed in springtime to prevent overheating.
• Skylights are covered with glass or clear plastic to keep warm air indoors.
• Thick earthen walls and floors to store collected heat (thermal mass).
• Insulation in the roof, outer walls, and in some places under the floor.
• Natural lighting so electricity is not needed for light in the daytime.

In winter, we roll huge plastic sheets down to make a big greenhouse which works as a solar collector for each building. In summer, the plastic is rolled up to prevent overheating. This UV-stabilised plastic is commonly used for agricultural greenhouses in Ladakh.

Thermal mass in walls and floors

Most of our buildings are at least three feet (1 m) below ground on the north side. The earth’s temperature at that depth is relatively warm in winter and cool in summer. It also helps us get the building material — earth — on site. The earth we dig out becomes the walls of the building.

All of our buildings are made of earth, so the building material comes right from the site and is not transported hundreds of miles. When construction is finished, there is no debris to be thrown away: no addition, no subtraction. Earth buildings stay warm in winter and cool in summer, and also moderate the humidity of the building.

The walls of the big building at Phey campus and the Leh office are rammed earth. This means the earthen walls are cast in place, in a simple wooden frame. Sand and clay are mixed in the right amounts to get a very strong constitution. It is then packed in the frames and rammed with pounders.

Some buildings here are made of straw clay bricks for insulation. Our earliest buildings were made of the common local style earth bricks.

The thick earth walls are not just structural (to take load) but also have an essential function as the heat bank (thermal mass). They absorb the excess solar heat during the day and release it to the rooms at night. The same property also keeps rammed earth buildings cool in summer.

Rammed earth is an ancient technique used in monasteries, castles and forts around Ladakh. These structures have survived, unprotected and exposed to the elements, for hundreds of years.

While we resurrected this method in Ladakh, we were pleasantly surprised to learn that there is a resurgence of this technique in Europe, North America, and Australia.

Insulation in ceilings, outer walls and floor

Insulation protects the heat of the day, but need not be expensive modern materials. The wood waste generated during the construction is stuffed in the ceiling to stop heat loss through the roof.

Insulation below the floor also helps. Layers of various sizes of rocks create insulating air-pockets between the rocks. A top layer of gravel and cement acts as a heat bank.

Sometimes the top layer of the floor is slates from nearby mountains. This reduces the use of cement, and the slates also become a thermal mass or heat bank as they are now cut off thermally from the cold ground.

This kind of floor absorbs the excess heat during the day and releases it during the night. Otherwise floors can be an area of big heat loss.

The outer walls are insulated by a jacket wall outside the main structural wall. The six inch gap between the two walls is filled with low cost insulation: saw dust, wood shavings or sometimes paper and plastic garbage like bottles and bags. Sometimes we have also used cow dung as an insulating plaster. Mixed with the right amount of earth and clay, it makes a strong and thermally effective plaster.

If comparing ‘thermal conductivity’ (insulation property) of common Ladakhi building materials, you find that mud is the best:

1 ft mud wall = 2 ft concrete = 4 ft stone = 1.5 inch of saw dust. = 1 inch of Thermocol, rockwool etc.

Natural lighting: the sun

Windows and skylights ensure that no place in the building needs electric lights in the day.