Since I was a little girl, my dad has been indoctrinating me with a love and respect for the environment. He’s a true outdoors-man and conservationist and has been recycling, composting, shutting off the lights and turning down the heat since before it was easy or cool, despite the (occasional) aggravation of his family. Now that he and my mom are getting close to their retirement years, he has decided to take his environmental consciousness to the next level. About a year ago he began researching and making plans to build an energy efficient straw bale house in the East Mountain area outside of Albuquerque, New Mexico. The ultimate do-it-yourself-er (and all around smart ninja), he has designed a home and written his own software program to lay out preliminary building plans and model a dynamic energy balance for the house. With the model done and property bought, he is now moving on to the next stages of development. He has handed his conceptual designs over to an architect to get final design and construction drawings and he is working with a builder in Albuquerque who has experience in straw bale construction. This weekend he will be visiting the site in New Mexico. I asked him to say a few words (I got more than a few) for green.mnp about his concept, design and software:
Solar House Software
I’m a software developer. This house project has many aspects where software can help – creating building designs and plans, analyzing energy systems, and keeping track of the many kinds of information and resources involved in a construction project. So I’ve developed my own “House” software, which is geared to the design and building of an energy efficient solar house. It’s a combination of a people and resources organizer, an architectural drafting program, a building project estimator, and a solar heating and cooling energy analysis program.
The principles of passive solar heating are very simple: heavily insulate the building envelope, include a lot of thermal mass inside the insulated space, and put most windows on the south side, with overhangs or landscaping for summer shading. Depending on where you are located, these simple elements can greatly reduce or even eliminate your fuel use and heating costs. A big step in reducing your carbon footprint.
Ever since I started focusing on this project I’ve been horrified to notice how most of our current housing stock totally ignores passive solar design principles. Drive around almost anywhere and look at the windows on the houses. You will see virtually no orientation to the sun. This should change drastically, at least for new construction, if we are to start doing our part on global warming.
One of the books I found most helpful on energy design was The Passive Solar House, by James Kachadorian. He is an engineer, and he includes all the information and tables you need to perform a rough energy analysis of passive solar heating. He advocates a system of storing heat energy in the thermal mass of a concrete slab resting on a bed of cinder blocks. The cinder blocks are arranged to form channels through which air circulates, storing heat energy in the slab or pulling from it, depending on the time of day and the season. The ability to store and retrieve thermal energy is important in a passive solar design, to even out the temperature swings during the 24 hours of the day. So I included a cinder block based slab in my initial designs.
Starting with some of Kachadorian’s energy calculations I began writing energy analysis into the program. To cover some additional aspects I had to give myself a crash course on heat transfer engineering. The program now uses the accepted principles of modeling the “heat energy balances” of the outside surfaces, inside surfaces, and room air temperature. And it allows me to model different strategies and systems of heating and cooling.
An energy analysis is just not a simple thing. You are looking at lots of variables and how the all interact. I groped around for a while with how to display and evaluate the findings. What I arrived at was a graph showing room temperature, and the temperatures of the walls, thermal mass, etc. over the 24 hours of a day. By viewing this graph under various scenarios I can evaluate a range of design options and solar strategies for both winter and summer needs.
Focus first on the green line – room temperature. It shows the large temperature swing (about 8 degrees) of the basic passive solar, without running air through the cinder block channels in the slab. The temperature swing would be much greater without all the passive thermal mass in the slab and interior cinder block walls.
The green room temperature line shows that running the room air through the slab can moderate the highs and lows some, but only reduces the temperature swing from about 8 degrees to about 6 degrees.
East Mountain Energy Strategy
I’ve come to feel that our chosen location in the East Mountain area of greater Albuquerque has one of the easiest possible climates for a solar home. While the winter temperatures can get down in the teens at night, there is plenty of sunshine for solar heating. With good insulation and some thermal mass there is no problem bringing in enough passive solar heat. The main heating problem is avoiding too much temperature swing – overheating in the afternoons.
And, being at a higher altitude than Albuquerque itself, the summer temperatures never get above the nineties – ok, so far! But more importantly, they always fall to the low sixties or below at night. That means that cool night air can be brought in and absorbed by the thermal mass, which then will keep the house comfortable through the heat of the day.
This all sounds fine in principle, but the real test is quantitative. Can the winter and summer temperature swings inside the house be kept within an acceptable range. That’s where the 24 hour graph tells the tale for each scenario – each combination of outside temperature range and heating or cooling strategy.
From cinder block slab to hot water tank.
What the 24 hour graphs showed was that the cinder block slab system did only a moderately good job of evening out winter temperature swings. I could have a range of around 8 degrees – say 71 in the early morning and 79 in mid-afternoon. That might be fine or it might be a little much – hard to say until you live in it!
But I also began to realize that there was not enough extra heat storage to do much about cloudy days. I would need to burn some kind of fuel for the cloudy or stormy times. This is not the end of the world, and I enjoy using a wood stove. But I also liked the idea of getting as close to 100 percent solar as possible. So I looked into using a set of solar collectors on the roof and a large water tank for thermal storage. Water is a much more efficient and flexible medium for storing heat energy than cement. Running hot water through tubing in the slab to provide radiant heat is a well accepted technique. And I liked that using such a system would also provide domestic hot water. The big benefit was that I could store up enough heat for several days of cloudy weather, depending on the size of the water tank. A bonus was that I knew Miki would love to have a spa, and the spa would make a good “heat sink” for the excess hot water generated in the summer.
So I tweaked the software to model this system as an alternative to the cinder block slab. This “active” solar system adds to the initial cost, though that cost is partly offset by eliminating the slab cinderblocks. Combined with the basic passive solar design, it provides much better temperature control (around a 3 degree swing), all our hot water, heat for two or three cloudy days, and heat for the spa. As a result, I feel that the hot water system is best for us. It will be more expensive to install, but cheaper to run and more environmentally friendly.
Here’s the water tank system with the pump (red dots) providing radiant heat via the slab in the late night and early morning hours, and the outside vent (blue dots) removing excess heat in the afternoon. The green line shows the three degree temperature swing.
This is where things stand now. I need to verify or calibrate my energy analysis against a reliable commercial or government program. I’m working with an architect to turn my conceptual plan into a finished design and construction drawings. I also have a builder in Albuquerque who is experienced with straw bale construction. We start with the garage this fall, and then the main construction next year. I’m planning more installments of this article to document the building process. I would welcome any comments or suggestions.
-Jack Partridge, Bahama, NC
Be expecting more posts on the progress of my dad’s straw bale house over the next weeks, months…year. I am especially excited to get to construction time, when we will all go out to New Mexico to help stack up the straw bale frame. Building the house will be a whole family & friends affair.
I know this is a long post already, but here is a great video from strawbale.com that dispels some of the myths about straw bale construction, as well as showcases some beautiful straw bale homes: