A wood stove for our earth sheltered home?
The building inspector, mortgage company and common sense will probably dictate that I should have a “proper” automated heating system in the home. I would call that my “backup” since I am hoping that I have designed the system well enough to call passive solar my primary heating system. However, I also assumed I would have a back up to the back up in the form of an efficient wood burning stove. We have a lot of “free” wood on the property (4 acres of Oak and Cherry) and you never know when power will go out rendering the other “backup” useless. There is also this idea that it may take some time to “charge” the thermal storage soil around my home (some earth sheltered homes report a 3 year period before the home stabilized), and a wood stove would be a “free” way to do that. Of course, there is also just something nice about sitting around a wood fire…
Modern Woodstove
So here is my first choice, picked out several years ago… I liked that it was a full 360 degree stove. I had mentally situated it between the entry, dining and living rooms so that we could sit around it like at a camp fire. It looks simple, but has many of the advanced features you would expect from a more traditional stove (blower, outside air intake, re-burner, etc.) Its manufacturer, Focus Creation, has a lot of cool wood stove designs. I expected they would cost more than a more traditional wood stove, but this one turned out to be nearly $15k and the one on the next page of the catalog (similar, but telescoping) was $44k… It is an advanced stove, but you could get a 2012 Mercedes Benz SLK for a lower list price than that. “Ooo, but it telescopes!!!” Anyway, maybe if I already had that car, I wouldn’t mind shelling out for the unique stove…
We continued to shop around for more standard domestic wood stoves and found that they are generally inexplicably expensive… They are about the same weight as a motorcycle, but much much simpler mechanically and yet, more expensive. They are nothing compared to the technology or entertainment potential of a high end 3D TV, and yet cost much more… I wonder why that is?
Jotul F100 wood stove.
Anyway, market mysteries of supply and demand aside; I eventually ended up going with something smaller and relatively simple… The Jotul F100. I liked the arches on the door which would be similar to the vaults of the room. It is only supposed to keep 1200 feet warm, but that should be good enough for us. It does have decent efficiency, but not some of the advanced features that more pricey wood stoves had.
The main problem was the back of it… Actually, the back of pretty much all the domestic wood stoves I looked at… They all looked like junky old CRT televisions, many even had the big energy efficiency sticker like you would find on your clothes dryer. An that was before you added the even uglier blower assembly… The only solution is to put it up against a wall.
I spoke to the sales guy who was quick to correct my pronunciation… “oh, do you mean the ‘yot’l’ wood stove?” “Yes, sorry, I am not up on all the in-crowd Northern European wood stove name pronunciation”… Anyway, it “starts” at $1,168. But, at that price, you just get a paperweight. If you want the fan, that is 250$. If you want the “outside air kit” (to prevent it from sucking all the warm air out of your home), that is another $100. I don’t think the legs were even included in that base price. Then I asked about stove pipe… They sales guy said, “$800 to $2,600”. I asked him to break it down for me and he said that he could get me a deal on the first 8ft out of the stove for only $899. Well we were already past the low end of his estimate and I hadn’t even reached my ceiling yet. He said it was about 100$ a foot after that… I have since found double wall stainless steel pipe online for about 50$ a foot, so I will keep shopping around.
I also looked into the cost of a professional install… I love how they like to ask all sorts of questions and keep asking to come out and measure, but then really don’t have a very complicated formula for the price… “Well, I have never done anything like that [earth sheltered roof], but its usually either $500 or $1000.” Assuming that I look after getting the pipe thru the cement ceiling and out the outside of the dirt roof, he figured the rest of the work was on the low end, ~$500.
I don’t know if you have been adding that up, but I have to figure that my little Jotul wood stove will come in at close to $4,000 and that is before I put any gas in my chain saw… Hmpf, free wood heat indeed!
But Mr. Pronunciation did fill me in on some other rules that I was not very familiar with. The pipe must extend at least 3 feet out of the roof, but must be at least 2 ft taller than anything within 10 feet. Hmm… I have a 22ft radius house with a 10 ft radius “storm room” on the second floor. Since much of the other layout is already in place, this means I have three options for where to place the stove…
1) I keep it where it was, about 2/3rds of the way out in the living room… But then I have an ugly backed wood stove with a very tall (18 ft?) shiny metal pipe sticking out of my earth covered roof, probably with guy wires to keep it steady…
2) Move it to where the piano is currently and let the chimney climb right up the side of the storm room… I kind of liked this idea and imagined a traditional stone chimney as well as tapping into the pipe with a second stove in the storm room (some day when I find a cheaper one on craigs list). But the cost would definitely be higher and the stove would then be in a major transit path next to the kitchen. My wife was concerned about the logistics of sitting around a hot stove in the middle of a traffic pattern.
3) We move it out and put it agains the outside wall, pretty much 11ft from the tower and hope that any rising smoke doesn’t just impinge on the storm room… This is a serious problem because the prevailing winds will most likely drive it that way. On the bright side, the little pipe could appear to be coming out of the entry cottage (if we do it right). This also knocks out a window on that internal wall, or maybe reduces it to a high transom.
We didn’t really have much option for where to place the wood stove…
Anyway, this third option is what I sent the architect…
At this point, it is in the budget, and I am expecting to put in the pipe to make a hole when we shotcrete the ceiling, but I also plan to save purchasing the stove for last… If we have any budget left.
Costing this out has really undone my theory of using the fire place for “free” supplemental heating while we charge the earth that first year. I could buy a lot of convenient conventional heat with my geothermal furnace for the cost of a wood stove. However, I would still like to get one eventually for its ambiance and grid independence.
We will see how it goes.
So, at some point, I decided that I wanted to build myself an earth sheltered home. This is not like saying I would like to build a colonial or a bungalow or something that has some know design DNA, or that uses known systems and processes… This is a wide open endeavor with a wide range of possibilities and even more unknowns. I have always been an Architecture buff, so it would have to be nice looking and functional. The engineer in me wanted to build an “engineered house”, what ever that is… I wanted it to be light, but authentic, and a bunch of other vague ideas that would have left any architect scratching his head. Of course, this wouldn’t just be my vage want list, I had a wife and kids who were also giving vague input. My kids wanted tunnels that connected things impossibly (they were not worried about the engineering or cost). My wife’s wants were harder to nail down, but she knows when she doesn’t want something ;^). But everyone was pretty supportive of the overall idea, so I started trying to add specifics to the vision…
Of course, I started by searching the web for other examples (check out my links page). The range was quite wide, from free-form (curvy) shotcrete vaults without a straight line in sight to more “practical” homes with concrete block walls and simple flat roofs. Some basically just buried Quonset huts (maybe a nice cottage idea some day). I have a big folder full of pictures in my computer. I wish I could share them all here, but getting permissions would be too much pain. Instead, here is a google image search, and here is another. The variety is amazing.
Many of the photos came with detailed blogs about their construction. Many were home builders who kept their budget low and did all the work themselves. I did like that idea, but I am full time employed and too busy to build my own home by hand. I am willing to spend a little more (not mortgage phobic) and wanted something more architecturally interesting. You could also see that many had a lot more money than I did (Bill Gates lives in an earth sheltered home), so I won’t be on that end of the spectrum either. There was also a strong contrast between those that were built without any compromise on the scientific principles (no north windows, brown cement floors, insulating shutters, tromb walls, etc.) and those that were built as art, without any consideration for the scientific principles. I wanted to stay married, plus I am a little artsy myself, so I plan on compromising where I need to. For instance, I plan to have a number of windows on the north side to capture the view even though I know they will leak precious heat. I also plan to have rugs on my floor although they inhibit solar gain into my storage mass…
I shared these ideas and pictures with my wife in an “agile” way, trying to get as much input as possible. For instance, i found it interesting that my wife really liked the concrete block or “ICF” idea because she liked the simplicity of the Lego-like construction method. I also discovered that she thinks about thinks like curtains very early in the design process… ;^) So, we initially co-imagined a rather rectilinear cement home (but with fibonacci proportions) with an undefined roof type (research pending) covered in some unspecified thickness of earth…
We cut these standard room sizes out and arranged them on the dining room table
For the actual layout of the home, I got some plan books and looked at single story homes that were about 2400 square feet. I took the room sizes from those and made a bunch of scale room tiles. Sherri and I printed these out and shuffled them around endlessly as we talked about design concepts.
- How do we divide the public and private areas of the house?
- How do we keep the TV noise out of other areas?
- Where can Simon’s office go? (I work from home 80% of the time)
- Why do people put laundry rooms by the garage instead of next to the bedroom?
- Do we need somewhere for our guests?
- How can we keep it affordable and make it interesting?
Also, we didn’t like the idea of an entry that appeared to be leading to a bunker. We wanted the functional areas of house to be on one floor, but I liked the idea of a “storm room” like a gazebo/conservatory on the roof that would give me 360 degree views. I thought a zero flush urinal in the boys bathroom would be a good idea. I really wanted a courtyard, but that idea morphed into a more practical green house or sunroom.
We some how expected that we would be entering from the North side (the earth bermed side) and wanted to avoid the bunker look, so we came up with the idea of the entry cottage. It would look like a little house to disguise the entrance to the underground portion of the house. I liked the idea of a modest little cottage concealing the larger home inside it.
We worked out the look of these rooms with extensive notes (in ppt) for each room.
Layout from Nov 2007, featuring the entry cottage and a decent sized courtyard. It also had a lot of small sunrooms… Funny now to look back at this.
By Dec 2007, I had realized that the courtyard wouldn’t work well in our norther climate, and moved it to the south side
By July 2008, we shrunk the courtyard, tilted some rooms (solar angles) and added the start of the rotunda idea. The walls were still straight because I had not yet figured out a roofing plan. I was considering individual square shotcrete domes for each room.
I used my ppt “skills” to record/share our ideas and layouts for each room in a rudimentary, but useful way. We could edit these easily as we tweaked our design.
I really spent the most time thinking about the structure of the house. I was I going to safely hold up all that dirt without spending too much money or having a house that looked like a bunker? I ended up with this idea of a central tower (the rotunda), with site-cast concrete ribs radiating out from it. There would be shotcrete arched vaults between the ribs passing the load to the ribs, which passed the loads to the ground. It was this central “structural umbrella” concept that changed the design from angular concrete block to the more circular layout built with shotcrete. It was somewhat of a Eureka moment for me as I saw it all come together in my head.
From there, I spend few minutes here or there each day for a month researching shapes for the ribs. I ended up looking into the mathematics of natural shapes such as shells or eggs. These had been studied by ancient mathematicians and Renaissance men and there was a lot of information on the web and in books. Since the ribs would need to be cast on site and lifted into place, I was particularly interested in designs that could be drawn in the field with just a piece of wire (or a stick) and a pencil. I tried these shapes (such as the golden egg, a mathematically sacred shape based on the golden ratio) built into arched ribs at many different angles and simulated loading the ribs with an estimated 68000 lbs of load until I eventually settled on a particular orientation of a 2300 year old euclidean 5 point egg design that had the right proportions.
Golden Egg design
“Golden egg” on its side as part of a rib. However, the heights at the ends were not right, so I would need to tilt it up…
Golden egg, 19 degrees tilt, but still not the right proportions…
Golden Egg, 38 degrees, still not right…
I tried a lot of other eggs, the Euclidean 4 point, the Moss Egg, etc. But this Euclidean 5 point egg, tilted at 41 degrees, had the right clearances, proportions and strength…
I created computer simulations for most of the promising candidates and loaded them with the estimated 68000 lbs that I expected they would each need to carry…
Eventually, by the end of 2008, I got into Autodesk Revit where I could CAD up the model in 3D. Now it was getting interesting. We had spent a lot of time discussing each little detail and there was a reason for everything. All the rooms had purpose, even the angles of the windows were designed to line up views thru the house (and even functioned as a solar clock).
An early version in Autodesk Revit, windows are missing, furniture is still sitting outside, but you can get the idea.
At some point, I hit that little button that calculates the square footage. Ouch, 3917 sqft, plus another 1035 sqft for the garage. We could not afford that much house… How had this happened? We had started from the rooms of a 2400 sqft house, but then probably added a few feet with each alteration. We ended up making some drastic cuts to get it back under control.
Our original footprint was much too large, the house had to go on a diet. The purple outline is the original floor area, the white walls are the shrunk down version… At least for a few days… A lot has changed since then.
Then we went out to shop for some land… I will save that story for another day, but the punch line is that the lot we bought had primary access from the south and best views were to the north, so we had to scrap much of what we liked about these these early designs… More on that later.
So, why not just go with a more traditional, above ground, passive solar house?
PassivHaus construction calls for a tight thermal envelope
There is quite a range of what is considered a passive house. On the “uber” end of the spectrum is the “Passivhaus” (or “Passive house” in English), which is really a performance standard for high-efficiency housing commonly applied in Europe where high energy prices (necessity) have focused a lot more interest and invention towards this sort of housing… There are tens of thousands of these built in northern Europe and some here in the USA also (a greater challenge due to our tougher climate). To be certified as a “Passivehaus”, the home needs to meet a few strict requirements including low annual heating demand, less than 15kWh/m2 (4746btu/ft2) per year, and be as airtight as possible, the building must not leak more than 0.6 times the house volume per hour, as tested by a blower door.
Basically these are just standards of energy efficiency, and “Passivhaus” owners typically claim a 90% reduction in their heating bills. I guess it is a good thing to have standards and metrics, but the more I thought about it, the more hollow it sounded. While I am interested in reduced energy bills, I am also interested in increasing my robustness against interruptions to the grid. I am even more interested in climate comfort, temperature stability and a healthy environment. Don’t get me wrong, when I am done, I will check to see if I met the “passivhaus” standards, they are just not my focus.
Passivehaus construction is typically boring because all the money is used up on the super insulated and sealed walls
A more interesting example, but still a simple square to reduce the cost.
When I studied examples of Passivhaus construction, particularly the American examples, I did not think they were taking the correct approach. For instance, they spent a fortune on R60 insulation all the way around… The insulation its self is only part of the cost, they first had to balloon frame the walls with trusses instead of 2x4s, and then sheath both sides for stiffness, then they created an airspace and a second wall (double envelope house). Most also used a “cold roof”, which is a double roof with an airspace between (and costs about as much as two roofs). Most use triple glazed casement windows which needed to be imported because they just don’t make those here. In some cases, they put 14 inches of rigid insulation under the slab and 6 inches around the foundation. A lot of expense is also tied up in making sure that the envelope is well sealed against infiltration, yet permeable enough to allow trapped moisture to escape. In terms of performance, they typically get large temperature swings on a daily cycle, the passive solar heating is also not uniform within the space. In one example, I read about plastic toys melting in the living room. Because they are trying to keep the construction costs manageable and the volume to surface ratio as high as possible, most passivhaus examples I have seen are very simple boxes ( upper left) although many can still make those boxes interesting like this one (right).
I am all for fuel efficiency, but some “fuel sippers” crack me up.
It reminds me of those automotive “fuel sippers”. These are people who spend thousands more for a Prius or other hybrid, then drive it very slowly (coasting when ever possible) and risking their lives as big rig trucks over take them, in order to save a few dollars in gasoline. I had one such colleague tell me that she is saving so much money on gas that she “drive[s] all the time now”, she didn’t understand why I laughed.
Leakage is the main obstacle to keeping a home comfortable in a challenging environment. Sealing a home above ground is a difficult challenge. Think of it this way, the strict PassivHaus inspectors are impresses when you only leak 59% of your homes air every hour! The average home has much higher infiltration rates. While it costs a lot to seal a regular stick frame home above ground, below ground construction is naturally air tight (instead we worry about bringing in enough fresh air). Also, an earth sheltered house has insulation and thermal storage. Thermal storage works as a sort of dynamic insulation. Our particular earth shelter plans also call for some cellular concrete (R~1/inch) and a rigid insulation umbrella. It should give an average roof R value of 47, but at a relatively low cost (more on that later).
