Monday, November 25, 2013

GREENER OR BROWNER?


Back in the not-so-Jolly Old England of the Middle Ages, where many of America’s  building traditions originated, no one had ever heard of structural engineering. Instead, carpenters used common knowledge gleaned from trial and error and handed down over the centuries. With no way to analyze the strength of their buildings, they just built them as stoutly as they could, using massive timbers hewn from lots and lots of trees. 

For example, records show that one six-room, two story house built in Cambridgeshire around 1600 required seventy-two small oak trees to be cut down for the framing lumber alone. Seven more mature oak trees (which yielded wider planks) were sawn into floorboards. The total wood used was equivalent to about 68 acres of oak forest. A larger house could easily consume over 300 trees--more than 280 acres of woodland as it then existed.


Given the rate at which these houses gobbled timber, the English were already managing their forests for harvesting by 1200. Even so, over the next few centuries, the island famed for Sherwood Forest became the sparsely wooded place it is today.

Fast forward to America during the postwar Baby Boom era. Our houses are constructed with a light framework of slender wooden studs, doing away with the need for heavy timber. It’s a relatively efficient system--an average house of 1950 uses about 9000  board feet of framing lumber, or about 9 board feet per square foot (a board foot is a hypothetical quantity of lumber twelve inches by twelve inches by one inch thick). At a crude average of perhaps 200 board feet per tree, this means we cut down something on the order of 45 trees--mostly softwoods--to build a house. No matter how we, er, slice it, it’s quite a bit less lumber than your average English house of the Middle Ages required. 

But fast forward once again to the present, and you find a strange irony: Today, even though we manage to use even less lumber per square foot than in 1950--only about 8 board feet--the amount of wood we use has nearly doubled, to about 17,000 board feet per house. That’s back up to about 85 trees--even more than were used during the Middle Ages. 

What gives?

For one thing, modern building codes require wood-framed structures to withstand much higher wind and earthquake forces than before, and that takes more lumber and plywood. For another, houses also have more rooms, and hence more interior walls. 

But the main reason we’re back to gobbling wood is simply this: Today’s average house is much bigger--over double the size of a typical house of 1950. In short, we’ve wiped out all the gains we’ve made in using wood more efficiently, simply by using a whole lot more of it. 

Now, one bright spot in cutting down so many of England’s oaks is that some wonderful houses of the Middle Ages are still with us. How many McMansions will still be standing in 2500 A.D.?

Monday, November 11, 2013

A LOT OF HOT AIR


Nowadays, when you’re feeling chilly, you just nudge your thermostat up a few degrees. Not too long ago, you’d have been in for a lot more effort: Until the 1880s, most American houses were still heated by an open fire. 

In those days, any room you wanted to keep tolerably warm had to have its own fireplace and chimney. This is one reason houses had such boxy, compact floor plans--the idea was to have as few of those expensive fireplaces as possible. Often, they were placed back to back so they could share a chimney. All this finally changed in the late nineteenth century, when the innovation of central heating made it possible to warm every room in the house with a single source of heat.

Of course, Americans were hardly the first to have central heating. As early as 100AD, the Romans used the hypocaust system, which conducted warm air from a fire into hollow spaces beneath a tiled floor. Ancient Korea may have used a similar system, called ondal, even earlier. By the 12th century, Muslim engineers had improved the hypocaust by using pipes--our modern heating ducts--which did away with the need for hollow floors.

The English had an early version of central steam heating as early as the 1830s, though it was of course limited to the fabulously wealthy. It took another fifty years for a proper central heating system to make it across the ocean and into ordinary Yankee homes.
Some central heating used steam or hot water piped to radiators, but most heated the air directly. Early systems burned wood or coal, which meant you were still liable to freeze unless you kept the furnace stoked. Later on oil and natural gas prevailed as fuel, since they could be fed automatically. 

All of these early “gravity” heating systems relied on the fact that hot air tends to rise (or more accurately, that gravity makes the denser cold air sink). Of course, since warm air just wafted its way into each room through big ducts, the furnace had to be located below the living space--one reason older houses had basements even on the West Coast.

Once individual rooms no longer needed a bulky and expensive fireplace for heat, houses could be laid out much more freely. The characteristic rambling floor plans of late Victorian houses such as Queen Annes--among the first adopters of central heating--were a direct outgrowth of their liberation from the fireplace.

After World War II, central heating systems began using a fan to actively push warm air through the ductwork. These so-called forced air units could use smaller ducts than the old gravity furnaces, and could be located anywhere in the house, even in the attic.

Thanks to our increasingly urgent quest for energy efficiency, today’s central heating systems make even those postwar units look antiquated. Gone forever is that consummate energy-waster, the standing pilot light, and many forced air units now boast efficiencies in the high nineties--about double that of old gravity furnaces. Electronic burner controls and programmable thermostats make it easy to forget that your furnace is even working. But don’t: next time you turn up the heat, think about how far we’ve come in just a hundred and twenty years.