Laws buildings obey ... even when their builders do not

Posted Wednesday, April 16, 2014 in Sustainable Maine

Laws buildings obey ... even when their builders do not

by Paul Kando

 A comfortable, well functioning house, new or old, begins with a basic understanding of how the physical world works; in this case how heat, air and moisture interact in a heated building.  A house is a physical system, subject to the laws of thermodynamics. If we ignore these laws in designing and operating any physical system, that system is bound to fail to perform as expected. Take, for instance, our unlimited growth dependent economy. It staggers between booms and busts, serves the needs of only a few, fails to provide decent livelihoods, fosters growing corruption, and substitutes money and debt for the real wealth, which always seems to be  scarce –  in contrast to nature, which manages splendidly without money, and within its own limits.  Houses built to be “affordable” but ignoring or disregarding nature’s laws will leak air like a sieve, cost too much to heat, have peeling paint, grow mold,  and may even rot over time.

Man-made systems that ignore natural laws can, of course, hobble along for a time. A person who is broke can live high on the hog until the credit cards are maxed out and revoked. Some drink several fifths of whiskey daily for years before their liver fails, or smoke tobacco for decades before lung cancer develops. But only a fool would take this for proof that the laws of nature I briefly summarize below can be ignored with impunity.

The 0th (zeroth) law of thermodynamics holds that objects in equilibrium with one another have the same temperature. –  The more energy a material object absorbs, the more its molecules vibrate and temperature is but a measure of this vibration. The more intense the vibration the higher the temperature. When the vibrations break the crystalline bonds of a solid, ice melts, when  the inter-molecular attractions in liquid water are overcome, it evaporates. – In spring and fall a house and the outdoors are in thermal equilibrium: no need for heating, cooling or heavy sweaters.

 The 1st law of thermodynamics states that energy can neither be created nor destroyed.  Material objects absorb or release energy in a reversible process, but no energy is ever lost . – Your heating system warms the air. The warm  moisture evaporates as you shower and the warming air absorbs the vapor. When the warm, moist air leaks into the attic, the moisture condenses on the cold underside of your roof. All the heat absorbed along the way is now also released, melting the snow on the rooftop. 

According to the 2nd law of thermodynamics, the capacity of energy to do work decreases Entropy increases as things tend toward equilibrium: high to low; hot to cold.  A perpetual motion machine is thus physically impossible. -- The boiler in the basement burns oil at hundreds of degrees to warm water to 140ºF, which heats the living room to 68ºF –  and a 15ºF outdoors. The energy is still out there, but it is impossible to heat a house with it again. – When the economy slumps, “experts” and pundits call for restoring growth. But unlimited growth in a closed system (our planet) would be perpetual motion – creating more out of less. Economic theories not withstanding, this is physically impossible. Fortunately, the sun constantly replenishes Earth’s energy supply.  Until it becomes unaffordable, you may buy oil to replace what you have already burned. But substituting money for real wealth (oil) can never, in the real world, replenish (real wealth) oil reserves.

Heat, air and moisture define indoor comfort as they interact in a house in accord with the above laws. When water absorbs heat it evaporates. Heated air expands, becomes lighter and rises, allowing cooler air to take its place. This is convection, one of three ways heat travels (the other two are conduction and radiation).  Warming air absorbs more and more water vapor. The warm, moist, expanding air pressurizes the house. The house leaks like a punctured balloon, so  the pressurized air easily escapes. But as the air cools, the vapor it can no longer hold precipitates on the coldest nearby surface, e.g. the outermost inner surface of an exterior wall. The liquid condensate can cause rot and serve as fertile ground for mold spores to grow. Of course, the cooling air and water also release all the energy they had absorbed along the way –  close to a thousand BTUs of heat for each pint of water. That’s  heat you’ve burned expensive fuel to produce!   

 

A properly functioning house is well insulated to retard heat flow. It is airtight to control air and airborne moisture flow, and adequately ventilated to remove excess moisture (without wasting  heat), and to provide fresh air. Such a house controls the movement of heat, air and moisture along the same plane, otherwise unplanned convective currents and damage can result. The best design-build system to accomplish this cost-effectively is Passivhaus, which, when supplied with renewable supplemental energy, turns into affordable  zero energy or plus-energy buildings. 

 

Next week:  Passivhaus foundations.

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