Science

Urnes Stave Church 1130 CE

SOLID WOOD

Why were the timber homes of Scandinavia so energy efficient, even in the 12th century?

In 1981, the National Institute of Standards and Technology (NIST) constructed six test buildings in Gaithersburg, Maryland, and tested them for energy efficiency. Much to their surprise, Building 5, with walls made of solid wood, was the most energy-efficient. This was attributed to "thermal inertia," a phenomenon where the solid wood walls stored energy during the day and released it during the night.

This form of timber construction is well known in Scandinavian countries, where it is the prevalent method of building because of its insulating properties and longevity. When interviewed during the 1994 Winter Olympics, a Lillehammer couple casually remarked that their solid wood home had been built in 1406!

THERMAL INERTIA

Just below the earth's surface, within reach of the average basement, is an infinite reservoir of heat that never drops below 50° F. The night-day cycle is more than ample to raise that temperature into the comfort zone with a simple shift in time. Thermal inertia makes the use of daytime heat at night and night-time cool by day possible, and the engineered lag-in-time is a property of the thickness and specific heat of the solid wood walls.

HEAT PUMP HOUSE

Inspired by the ancient Persian engineering feat of the thermal chimney, our inventor designed a home that serves as its own heat pump. In the Enertia Building System, the house itself is a heat pump, using the natural energy of rising solar-heated air to extract and enhance the pool of geothermal energy just beneath the building's floor. The design is simple and foolproof, with no CFCs and no electric bill (see "Heat Pump House," Popular Science, June 1992, p.42).

THE ENVELOPE

How does a house, office, or ground-based structure become a natural energy machine? The secret is an air path, or envelope, just inside the structure's solid wood skin. On a sunny winter day, it is a heat path, a continuously recharging convection loop. It is also a radiant heat source and provides extra insulation for a cold winter night. Sunspace plants can also oxygenate the miniature biosphere, providing a buffer zone to outside noise and wind.

The Enertia house works because the walls have the ability to gain, hold, and release heat. They do double duty as structure and storage. Their thermal mass and thermal lag lead to "floating," where stored daytime energy cancels out night-time needs. Floating can last for days, keeping the house comfortable during periods of little or no sun. Massive houses also experience seasonal "float" and can coast a month or longer when lightweight houses need artificial heating or cooling. Enertia® houses float right through heat waves and arctic blasts that would endanger occupants of other buildings.

("Floating," Abstract: A Field Study of the Effect of Wall Mass on the Heating and Cooling Loads of Residential Buildings, Doug Burch et al. National Institute of Standards and Technology, 1982)

FLOATING

HEAT CAPACITY: THE 'C' FACTOR

Since the envelope is recharged only during the day, the 'C' Factor, or heat capacity of the wall, becomes essential. Besides being a natural insulator, solid wood has a large heat capacity due to its mass and the latent capacity of the cellulose and resin. In contrast, conventional light framing has little mass to absorb heat, and fiberglass and other low-density insulations are designed for high "R Factor" alone.

DELTA T

All architects and engineers work with the Heat Loss Equation. Heat loss equals the temperature difference (Delta T) divided by resistance (the familiar R factor). All modern attempts at energy efficiency have focused on the "R" factor - i.e., more and more insulation into a thicker wall. However, reducing Delta T has the same effect. While insulation is necessary to increase the "R" factor, Delta T can be reduced naturally. The envelope presents an entirely different Delta T to the living space than it would see if the house were single-walled. Because of this, the living space never "sees" conditions colder than the geothermal temperature under the house! As the sunshine raises the envelope temperature in the closed wintertime loop, Delta T goes to zero, and the "R-value" is meaningless. At this point, the Enertia house is in equilibrium, and Heat Loss = 0.

RADIANT HEATING

Radiant walls and floors are inherent in the Enertia Building System, the ultimate in heating and cooling. When your feet are warm, you feel warm all over. It is an efficient and even heat. Radiant heat is invigorating because the air you breathe is cooler. On the coldest winter day, the envelope keeps the north wall warm, and you can lean up against it to feel the heat.

RADIANT COOLING

The same thermal mass that acts as a heat source in winter becomes a heat sink in summer, enabling an energy shift in time. The envelope is opened to the outside for summer cooling - basement windows for intake and rooftop windows for exhaust. Natural ventilation carries off internal heat captured by the massive walls during the day. The permeable walls allow humidity to migrate towards the outer surface, where it is evaporated by the sun. Because of the envelope, the house is wrapped by the cool in-ground climate. (The House that Needs No Fuel, Architectural Designs, August 1988, p.6)

In the summer, Enertia homes rely on the geothermal factor, earth coupling. The earth remains below the first few feet at a constant temperature between 50-55 °F in most areas. The thermal chimney effect is what causes the distribution of energy in the Enertia home. For a "passive solar" house in a cold climate, the sunroom wall must face directly south, but because of the equalizing loop, the Enertia home has more tolerance for sitting. It can be sited within 10 to 30 degrees off south, and the loop will redistribute the energy. This loop prevents the sunspace from overheating, another common fault of passive solar designs. The thermal chimney effect also causes the Enertia home to self-ventilate in summer. Heated air rises out of the roof vents and causes cooler air to be drawn in through the north basement windows.

CLIMATE CHANGE PROVIDES IT'S OWN SOLUTION

BIO-MIMICRY

THE GREENHOUSE EFFECT

We use a miniature version of it to heat our houses, all of which have an artificial Earth-like atmosphere, complete with internal trade winds and buffered by geothermal energy.