Saturday, September 30, 2006

So you want to heat your house with a's how.

As a young teen, I became obsessed with solar power and energy efficiency in general. My mom was the librarian at school, and one of the consequences was that I learned how to find information pretty efficiently. So over the years, I have seen or read about practically every trick in the book to heat a house inexpensively and efficiently. Here's a partial list of topics that I researched over the years.

Geodesic domes, These are visually fascinating, "modern", with all the pizzaz of Buckminster Fuller. They also generally leak and finishing all those funny angles is challenging to say the least. Geometrically speaking, domes present 1/3 less surface area per cubic foot of living space enclosed when compared with more conventional square/rectangular/cubic construction. So, all things being equal, they can be energy efficient if you are willing to put up with a few leaks. The average building inspector, along with his whole department, will likely give you trouble. For a brief time I worshipped in a church like this in Caldwell Texas that had the most amazing acoustics for singing.

Monolithic domes, Made out of cement or Gunnite or Shotcrete and reinforced with steel, cool or weird depending on how I feel that day. They can attain very high levels of thermal efficiency because they minimize surface area and they use a kind of foam insulation that may attain r-6 to r-7 per inch, which is twice what you get with fiberglass. As far as real world performance, we are probably talking quadruple the real world insulating power of fiberglass batts in a conventional building. They are also tornado proof. makes for interesting reading.

Tire houses, This is where you stack a tire, ram full of dirt, repeat 2,000 times. Lots of code issues and not that great for insulation. Poor control of moisture. Vast enormous quantities of labor required.

Adobe houses, Omit the tires, just use the dirt. Lousy insulation, but tremendous thermal mass. They work great in sunny, cool, dry or sunny, hot, dry climates, and just about nowhere else. Vast enormous quantity of labor required again.

Bale houses, or strawbale construction, This is where you stack bales of straw rather like Lincoln Logs or giant bricks. Construction itself goes pretty fast and is forgiving of beginners if the design is good. The have pretty good insulation values, which, coming from me is very high praise indeed. Conceptually, I want to take this to its logical extreme and build a house out of big round or big square bales with wall thickness of four feet or more. In many places, straw is considered a waste product that is difficult to dispose of, so this kills two birds with one stone. Some code problems in some places. I am still very tempted...

Log home construction, Romantic, pretty, but real world r-values are not that great and they require considerable special skills to construct. Finding good logs of the needed size is not a trivial task in most places and it's an inefficient way to use the lumber. You could take it apart and build two really efficient houses with that much wood.

Underground houses, These offer almost mythical energy savings, in the sense that real data that proves their claims is as rare as pixies and fairies. Devotees often exhibit religious zeal for this idea. Building a house underground does lower the Delta-T (the difference between inside and outside temperature) but they are still problematic on several levels. You can get equivalent or better energy efficiency in ways that are cheaper and easier to attain. There is a book called "The Fifty Dollar and Up Underground House Book" that is a cult classic. I was almost a hippy back then.

Berm houses, These are sort of underground and are neither fish nor fowl. Ultimately, there are easier and less expensive ways to achieve energy efficiency.

Cobb houses, The English are crazy about cob, it's the English version of adobe more or less, with somewhat better insulating capacity. In both relative and absolute terms, r-values are not that great.

Stackwall, This has had a big revival in the last 20 years or so. You basically buy a load of firewood logs, cut them into pieces all the same length and lay the logs up like bricks to make a wall 18-24 inches thick. r-Values OK but not great. A million cracks to caulk after the logs shrink. They are both pretty and distinctive. Here's a blog by a guy who did a double wall stackwall house that came out very good on the thermal envelope. He's the exception though and it involved massive labor inputs.

Double Envelope homes, They didn't really save a lot of energy, and if they did, it wasn't because they were double envelope. They were a short lived fad.

Vacuum panel insulation, These are small panels that are almost absolutely airtight and can be installed as insulation They are very much like a thermos bottle. A perfect vacuum doesn't lose any heat by conduction or convection, only radiant losses, which can be controlled. Unfortunately, they are expensive, delicate, and have questionable lifespan. They are not ready for prime time yet.

Active solar heating, tends to be fabulously expensive to provide 100% of the heat for you rhouse unless you cut your heat needs dramatically by superinsulation techniques. Spectacularly unsuccessful in cloudy climates like mine.

Drum wall, Make your south wall essentially all glass. Build a heavy frame to support 55 gal drums of water behind the wall. Build an insulating cover for that south wall that you can open and close so that it doubles as a reflector during the day. These work fairly well if you have a sunny climate and you insulate well. They look funny and it is not a trivial task to build an insulated door for the south wall of your house. Completely unsuitable for remodels.

Trombe Wall solar heating, This is another big bank of south facing glass. But now you build a cement wall about one foot behind it to soak up the heat during the day and release that heat at night when you need it more. It sounds great, but without nighttime insulation over the glass it is terribly inefficient because that hot cement wall radiates and convects that heat very efficiently right back out the window. It's also, ummmm, ugly. Take a big huge window and build a black wall right next to it so you can't see out or in through the "window". It is still widely promoted by the way. If you can insulate the cold glass from the hot thermal mass at night and on cloudy days, then they work fairly well, but then it's not really a Trombe wall either.

Passive solar heating, This can range from extremely successful to wildly impractical and expensive, mostly depending on how much they implement superinsulation techniques.

Ground source heat pumps, They are expensive, complex and have a history of durability problems. Durability has gotten better lately they say. They run on electricity, which often comes from coal or nuclear, so those are not huge enviro winners. 2/3 of all the heat from the original fuel gets wasted at the power plant before it ever arrives at your house, so system wide efficiency suddenly doesn't look so great. If you absolutely have to heat with electricity, this is how you should do it. You need a big lot and the backhoe will dig huge trenches everywhere. Superinsulation techniques would allow you to cut the size of the unit by half or more.

Air to air heat pumps, These are only efficient in milder climates. At temps below freezing, they become very inefficient because they have to use electric resistance heat to prevent frost build up. They are also expensive and complex to maintain, and run on electricity.

Ground water heat pumps, Also expensive, complex and requires at least two wells on the property, often against zoning regs to pump water back into the aquifer.

Outdoor wood furnaces, These have hideously poor efficiency, often less than 25%. The manufacturers often imply good efficiency, but you will universally find that they are short on indepent test data. They virtually all smoke (aka pollution) when operated as suggested in the ads. This is deforestation and pollution right in your own back yard.

Superinsulation. This is the best, period. I'm not biased or anything. Really.

So that's a thumbnail sketch of what is available, what works and what doesn't. I will support my wild claims for superinsulation in subsequent columns.

As far as actual progress on the actual house, nothing. Still fixing up the city house. Got about half of my cogenerator transfer switch unwired from the regular breaker box and will finish that tonight.

Finest regards,



Rachel Hoekman said...

Hi Troy

I'm sold - insulation or bust!


Small Group said...

So what about that classic standard, the aluminum foil house? You too can be the baked potato!


troy said...

The aluminum foil house is only recommended for those who need to repel the mind control rays of the aliens camping out on the far side of the moon where we can't see them.

finest regards,


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