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September 29, 2003

Alternate Options Revisited

Back a while I was writing about alternate alternative housing materials, including my then-favourite construction type, the straw bale home. Having written that article, I realized that I was still eager to see what else was out there, and what, if anything, might surpass a straw bale home's lower cost and higher energy efficiency. That's when I found Monolithic domes.

Monolithic domes are interesting beasts. They are thin-shell steel-reinforced concrete structures that happen to make really efficient homes. One funny aspect of Monolithic domes is that they're built in reverse order to conventional homes. First, you inflate a dome made of a specialized tarp material that is the final shape of the outside of your home. From there, you spray 3" of foam on the inside of this bubble. Once that's complete, you hang steel reinforcing bars (rebar, for short) from the foam, and then spray on 3" of concrete over the rebar. Instead of building the inside and working out, you inflate the outside and work your way in, from the inside. Freaky.

Aside from being ack basswards to build, Monolithic domes are also extremely energy efficient. The reason for this is all that concrete and steel, which gives the home an enormous thermal mass. Concrete absorbs a lot of heat energy per unit mass. What this means is that a Monolithic home will tend to stay at a given temperature with very little change. A good way to visualize this is with a candle, a pebble, and a boulder. If you lit the candle and held the pebble above the flame for even just 10 seconds, that pebble is going to get stinkin' hot very quickly. It has a small thermal mass. The boulder, on the other hand, will barely notice the candle. Any heat put into it will be spread throughout its entire mass, so the same amount of energy that heated the pebble up to flesh searing temperature barely even registers in the boulder.

Now, the sunlight is the candle, and your wood-frame house is the pebble. Bake it under the August sun for a few hours, and you're begging for air conditioning. Do the same to the Monolithic dome, and it just quietly absorbs the extra heat, raising its temperature ever so slightly, and will release the heat at night when the air gets cooler. What's more, a Monolithic dome's thermal mass is only 3" thick, and has a huge surface area. This means that unlike a big pool of water or a pile of rocks, temperature changes can be immediately moderated by the high thermal mass. It's right there, surrounding you, always moderating the temperature.

If you followed me thus far, it's probably sounding like a great kind of home to live in, doesn't it? I'll tell you, I've been smitten with Monolithic domes for a couple of months now, and have tons of sketches to prove it! However, once I understood the principle behind a Monolithic dome's ability to maintain a comfortable living space with a high thermal mass, my sneaky old brain decided to do some figuring on its own. Never satisfied with something that's good, it has to go and find something better. And that it did.

My brain wondered if there weren't materials that had an even higher thermal capacity, and that might even be stronger than concrete. One material immediately jumped to mind: steel. Steel is extremely strong - in fact, the concrete in the Monolithic dome relies on the steel rebar to be as strong as structure as it is. "Well," my sneaky brain asked suggestively, "why build with concrete, when you can build with steel, get the same or better thermal mass, have a stronger home, and be able to weld on your walls?"

My brain was right. Steel has just over twice the thermal capacity that concrete does, meaning that I could build a steel dome 1.5" thick instead of a 3" thick concrete dome. I'd get an equal or larger thermal mass, get even better thermal properties than concrete, and have an obscenely stronger dome to boot. The only stumbling block was how to turn 1.5" steel plate into a curved dome, but my nefarious mind even found a way around that, too.

Steel plate in inch-plus thicknesses is not as cheap as I might hope, and even though making a dome out of steel plate is possible, the plate would need to be bent, and welded together. For the size dome I was considering, I calculated something horrendous like 5000 feet of welding. That is 5000 feet of 1.5" deep weld, not exactly a speedy process. At this point I realized that between the steel cost, the labour involved in all that welding, and having to handle all that steel somehow, there was no clear benefit over a Monolithic dome anymore. Monolithic domes are already efficient, and able to withstand 300mph winds, runaway dump trucks, and even fire. Steel would simply be gross overkill.

Back to Monolithic domes, right? Well, not when my brain is involved. Now that the notion of "high thermal mass" had been processed, I had to go looking again to see what other home designs there were that operated on the same principle. Sure enough, I found something. It had a high thermal mass, it was additionally a passive solar design, and it was made out of everyday common building materials. What's even more, the shape of the house is the familiar rectilinear shape with flat walls and ninety-degree corners. This means not only that your fridge will fit against an outside wall (just try that in your dome with compound curved outer walls), not only will you not have to relearn how to lay out your living area (quick design a pie-shaped living room), but any unskilled Joe like myself can perform a significant portion of the manual labour involved in building the house. Nirvana!

The structure of the house is dead simple. A rectangular concrete pad is poured, with the long side facing south. On top of this pad, cinder blocks are dry-stacked to make walls. (What is "dry stacking"? If you've played with Lego, you've dry stacked before.) Once that's complete, sheets of ready-made rigid foam insulation are installed on the outside of the wall for insulation, and the inside is finished with a layer of surface bond cement, showing no mortar lines to remind you it's good old cinder block making your walls.

So, what happens with the south wall, you ask? The entire wall is glass, set at a slight angle off vertical. If you're home efficiency-savvy, you'll know that glass is an abysmal insulator, allowing all your heat to escape at night. And during the day, so much window area will bake you out of house and home, right? Nope - that's where the thermal mass comes in. During the day, all that free solar energy coming in through your windows gets stored in your cinder block walls, your concrete pad, and the earth underneath the pad. At night, all that heat slowly radiates back into the cooler living area of your home, keeping you comfortable. Properly executed, you should not need any mechanical heating or cooling equipment. No furnace, no air conditioner, no fans... just imagine how quiet your home would be!

Can you guess what comes next? You bet... my brain absorbs this information, and starts looking for more. I will say that a high thermal mass passive solar home is at the top of my list right now - the prospect of never having to pay for heating or cooling again (or hearing a fan come on, or paying for ductwork, or trying to arrange my furniture around vents and returns) is very attractive. Having been in a dome state of mind for so long, however, I'm going to have to turn around and start thinking rectilinearly again.

A very few things are for certain: 1) I want to be a significant contributor to the labour in building my next home. 2) My next home will be as efficient as humanly possible, and use solar energy in as many ways as possible. 3) My brain won't rest until I'm actually living in my new home, and even then, it'll only take a short sabbatical. Stupid brain.


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