Over the Christmas break I went on what I am now referring to as my “Zero Energy Design Binge.” I started thinking about the through wall air conditioner we are considering to handle the cooling demands for the home and thought it would be fantastic if there was a way to economically eliminate the cooling demand altogether via passive design. This thought quickly put me into research mode and when I couldn’t find anything in stock at my local Borders or B&N, I found an 800 page e-book on the subject from www.ZeroEnergyDesign.com.
While I normally don’t buy e-books if they are not immediately downloadable, I made an exception and the CD-ROM arrived just in time for Christmas break. The book consumed many hours over the break. While there is a ton of great info in the book, it became very evident very quickly why it was an e-book and not actually available in bookstores… Skipping over the 200 pages of political ranting helped cut down the time to read a bit.
The biggest thing in the book that caught my attention was the concept of a “Thermal Buffer Zone (TBZ)” or double shell envelope that could significantly reduce the cooling and heating demands if designed and built properly. Basically the idea is to build a double envelope on the North and South sides of the house as well as above and below the house. This would create TBZ around the interior shell of the house where air that is not conditioned but is milder than the outdoor air could freely circulate. See the image below for clarification.
The concept allows each envelope to be built economically without the need for extreme amounts of insulation and top of the line energy efficient windows. This is because having two smaller differences in atmosphere (outside – TBZ & TBZ – Interior shell) are more efficient than on large difference in atmosphere between the outside and inside of traditionally built houses.
The diagram above shows how the home would operate in the summer with cool air originating under the home, gaining heat and humidity as it rises and being exhausted out the top of the house. In the summer the TBZ could get up to 80 degrees or so when it is the hottest outside but it will still be cooler and less humid than the outside air.
In the winter the design functions differently. The South face of the house is actually encouraged to use as much single pane glass as possible in order to let in the most heat from the sun in the winter. The South side is on the right of the diagram above. The air on the South is warmed by the sun during the day, rises to the top and slowly falls to the North side of the building to warm the side of the house that is not getting direct sunlight. This air then cools and continues down under the house and flows naturally back to the South side where the process starts over again. Temperatures of 80 degrees or more in the TBZ have been recorded in houses of this design during the winter when snow is on the ground.
One of the things that got me excited about this concept was the possibility of incorporating a glass garage door on the South side of the house on the ground floor. This had been investigated early on by ISA but thrown out due to thermal losses that would occur. With the TBZ design we could possibly incorporate it again and have a large atrium type space in the back that could be opened wide to the back yard when weather permitted.
To cut my story down a bit, we ultimately agreed that the TBZ design is interesting but should be shelved for a future project due to cost constraints on this project. Besides simply materials costs, there will most likely be added labor costs for throwing too many new ideas at our builder and subs at once. The design we have now is still very unique and extremely energy efficient compared to the average home and probably even the average LEED home.
Here is a list of some of the things I took away from the ZED book that may be applicable to our project. None are really groundbreaking but it helps to have a review again close to the end of the design process on passive design:
- Minimize or eliminate all windows on the West side of the house. This is not an issue for the 100K house but it is for the corner house that has a 40′ exposed wall facing West.
- Minimize the amount of windows on the North side of the house. We have a lot of glass there now and may be able to reduce while still letting in a lot of light.
- Radiant barriers are extremely important, especially on the roof, for keeping the house cool in the summer. Traditional insulation is not enough.
- Designing windows that open on the bottom of the North side of the house and on the top side of the South side will aid in passive cooling via breezes in the summer (especially at night). The windows at the bottom should be half the size of the windows at the top.
- Exterior shading is extremely important for the South side of the house and low-cost, permeable shading material may be able to shade the entire back yard in the summer if designed correctly.
- Interior, thermal shades can be effective in both the summer and winter months for maintaining the atmosphere in the conditioned space.
In a future project with a larger budget I think the TBZ could be implemented without sacrificing the design of the house or impacting the budget too much. It would also be nice to explore low-cost home automation in more detail. If you were able to design the home to automatically control windows, vents, exterior shading and interior shading the cost of mechanical HVAC could be reduced dramatically if not altogether.
As it stands with our current design, we still feel that it will be energy efficient enough to add a reasonably sized PV solar array on the roof to supply most or all of the power to the home. If incentives return to PA in the near future this could be a $5K-$10K decision for the homeowner which is manageable. Then the homeowner can consider things like selling their car in exchange for a nice bike and a solar array that will get their home to zero energy for the rest of their life.