Earth Tube Material:
When designing earth tubes, choosing the type of pipe is the first decision. There are a variety of materials to choose from, from baked clay tiles, to steel duct work, to common PVC or the most modern HDPE plastics with anti-microbial coatings… Perhaps I will eventually come back and put this in a table, but for now, I will just list some of the pros and cons to each.
Note that the thermal conduction properties of the material do affect the rate that heat conducts thru them, but it doesn’t seem to affect the overall performance of the earth tubes. Partially, this may be because the total resistance to thermal conduction includes both the R value and the thickness. Although concrete conducts heat better than plastic, concrete pipe is typically much thicker and 2 inches of concrete ends up with a thermal resistance similar to 1/4 inch of HDPE. It is also somewhat because a somewhat stable temperature gradient is setup that eventually lets the heat thru. But the real reason the material conductivity doesn’t matter very much is because it is the conductivity of the earth that is the bottleneck. Aluminum conducts heat very quickly, but can’t draw it from the earth any faster than a plastic pipe can.
More important aspects to consider include durability, cost, ease of installation, environmental concerns and the interior wall friction factor that has a direct effect on the frictional pressure losses of the system.
This steel earth tube helps make it affordable to heat and cool community facilities for an isolated, off-the-grid, tribe in the Yukon Territory of Canada.
Metal Ducts
Metal Ducts are commonly used in homes as part of HVAC systems, so there are a wide variety of connections/fittings available and it is not hard to put the system together yourself or to find someone to do it for you. The prices are also reasonable and it is somewhat intuitive to believe that the metal will conduct heat better (I don’t think it actually matters since the earth limits the conduction speed anyway).
However, buried metal ducts will corrode over time, particularly in moist or acidic soil, even galvanized ducts are not recommended for burial. Rectangular sheet metal ducts, commonly used for indoor HVAC systems, are particularly poor in an outdoor/underground environment where their shape does not help them resist earth loading. Their joints open up and bugs, water, earth and roots get into the pipes. While I could not find experts who recommend using regular HVAC ducting, I did find corrugated steel duct earth tubes being used in a variety of projects. Mostly these were “earth ships” in dry areas of the southern states where corrosion is less of a problem, but the attached image is of an installation used to ventilate a large First Nations (Na-Cho Nyak Dun) tribal center in the Yukon (Canada). No comments were made on the expected life of these ducts.
Clay or Cement
Clay or cement duct work has also been used. The idea is that if it is good enough for drainage tile or sewer systems, it is good enough for air. Their durability is not in question, however they are brittle and could be cracked with impact, most often during assembly when these heavy sections are lowered into the ground (typically with expensive equipment). The rough walls of these pipes provide a lot of resistance to airflow. The Friction factor for cement pipe is 200 times that of PVC. This friction has a direct effect on the frictional pressure losses. I suspect that the larger standard diameters can more than make up for the higher friction. The surface roughness can also make cleaning them impossible. The many joints are appreciated by bugs and mold.
It sounds like a bad idea to me, but proponents say that you can seal the joints against radon and insects while the permeability of the pipe allows moisture to escape (thwarting mold). Because many of these materials can absorb and release moisture, they can actually solve some of the humidity problems often associated with earth tubes.
This Earth tube is approximately 600 linear feet of 2ft diameter cement pipe with rubber gasket joints. It is laid in a 5000 sq ft area, buried 10 ft below the building.
PVC Earth-tubes often crack during installation and need to be mended (see top pipe).
PVC (Polyvinyl Chloride)
PVC (Polyvinyl Chloride) is a frequent choice. It is popular because you can go to any hardware store and buy as much or as little of it as you want. There are also a wide variety of fittings available. You can easily buy the tools and glue needed to assemble it or find someone to do that work for you. The downside is that PVC infamous for being one of the most hazardous consumer materials ever invented. Not only is it toxic in is fabrication, but many of those production chemicals are not actually bonded in the plastic and can leak out over time. No one wants dioxin or other carcinogens in their air supply. Structurally, PVC is brittle and gets more brittle over time (especially if it spends any time in the sunlight before it is installed). It is easily broken during installation (as testified to in the blogs of many who installed them). Flexible rubber joints have been used to repair breaks and some recommend them as a way to prevent breaks (flex instead of crack). Even after a successful installation, cycling temperatures cause thermal stress and micro-fractures. The joints can catch and hold water and make the pipes difficult to clean thoroughly.
I also found it can be quite expensive (~8$/ft for 6″ Dia) compared to other options such as HDPE (~$3/ft for 6″ SDR17). Of course, there are various grades of PVC; for instance, PVC SDR 35 (thinner) Sewer pipe can be purchased for less than 3$ per foot, but it breaks relatively easily. The equivalent HDPE pipe (6″ DR 32.5 pipe) is much tougher and can also be purchased for less than 3$ per foot, but will require a couple more dollars per foot to fusion weld it together (if you hire someone else to install).
HDPE Pipe
HDPE (High Density PolyEthylene) was my favorite choice until I discovered Double Wall pipe (below). It is an inert plastic with none of the health concerns of PVC. It is also more flexible, smoother, stronger, and tougher than PVC or any other tube material I could find. This toughness is important during installation, burial and for the life of the tubes. HDPE handles the thermal cycling with ease. You can bury it and it will last as long as you need it, probably forever, but it is also recyclable. Sections of HDPE are fusion welded together in a way that results in joints that are as strong as the rest of the pipe and provide almost nowhere for water to collect. This type of pipe has the lowest friction factor available, which has a very direct impact on reducing frictional pressure losses.
One downside to HDPE is that you may need to hire a professional with the right tools to make those fusion welds. You can’t just pick up the pipe or the fusion tool at home depot and do it yourself (which was the main advantage of PVC). It comes in long pipe lengths that you will need to order in bulk and then unload when it is delivered. It also has a fairly high coefficient of thermal expansion, so if you plan to solar heat the air (as I do) flanges are recommended to prevent the HDPE from pulling itself thru the wall when it cools down.
I looked it up and noticed that the fusion welding temperature on the professional rigs was not very high (450°F), so I experimented with a piece of scrap HDPE pipe that I was given. I tried it three ways. First, I used my wife’s electric frying pan, which has a handy temperature dial. Second, I used my benzomatic torch directly. Third, to get a more even application of heat, I used the benzomatic to heat a thin piece of metal on one side and then touched the plastic to the other side…. In all three cases, I was able to soften the HDPE plastic and fusion weld it with ease. When I used the benzomatic directly, I was worried the HDPE would burn, but it didn’t. It just softened nicely. When I used the metal plate to transfer the heat, the plastic stuck a little (I over heated it past softening), but adding “parchment paper” solved that problem. The electric grill worked perfectly, but is probably overkill considering the other methods worked so well. I cut the samples up later and looked at the fusion cross sections… They looked good, although I could have gone with less softening. However, aligning the pipes was a little bit tricky. It would be good to make a simple jig for that purpose. I am pretty confident that I could do my own fusion welding for this low pressure application without hiring a pro.
Some people may prefer to have an expert fusion weld the HDPE pipes together… If you do that and want to keep your HDPE installation costs down, you will need to plan ahead more. Ordering all your HDPE for one delivery is a good idea (be ready with a fork lift to unload it), but you should also plan to have the fusion welder out for just one day. This will require organizing to make sure your trenches are dug at the right stage (after the house is cited and perhaps after foundations are poured). If you are planning for a geothermal ground loop, it would be at this same time also… You would then have all the HDPE pipe laid and fused at once. It will be important that both ends of the tubes are protected from critters from the start. These trenches will then need to be filled in (protected) before the next construction phases can begin.
Some builders create a temporary connection box to terminate the earth tubes in while other construction details are taken care of. The remaining distance to create the final connection to the house would then need to be done later and would require additional expense to mobilize the fusion equipment and operator.
It is possible to buy a 500 ft coil of 4″ HDPE pipe. At first I thought this may be a way to reduce the hassle of fusing sections together. However, an HDPE expert I was talking to told me that wrangling a 500 ft coil is very difficult and requires special straightening equipment that heats up the pipe as it is unwound, so maybe this isn’t really an option.
Another downside of HDPE is the availability of the pipe. As I noted, you can’t just walk into Home Depot and pick up a few pieces. You will need to find a proper supplier, a supplier that is used to dealing with much bigger customers (think cities or oil companies). The supplier may keep some HDPE pipe in stock, but there is a good chance the stuff you want won’t be… Larger-diameter thinner-wall pipe for low-pressure flows isn’t something a lot of people are ordering. Basically, the factory has a large extrusion pump that pushes the plastic thru a die to make the pipe. It pushes pipe out continuously and they slice off the lengths they need. When you order, you are asking the factory to stop the machine and switch dies for your order. If the factory is moderately busy, they are going to need a minimum size order to even consider doing that. It may be something like 500 or 1000 ft of pipe. You also need to wait your turn. Other customers are ahead of you and priority customers with larger orders may cut in line, so order early.
My local HDPE pipe distributor was very friendly and helpful, even though my job was small potatoes. He tried to push me towards the thicker pipe they had in stock (for higher pressure water or oil pipeline applications). He explained the factory processes and warned me that a customer order may take some time to fill. However, the thinner pipe also takes a lot less plastic and the price is about half as much. It will be easier to move around and easier to fusion weld, so maybe the hassle is worth it. I will come back and let you know how it actually works out for me.
You can buy very expensive HDPE with an anti-microbial inner coating designed specifically for earth-tubes and marketed towards people concerned about microbial growth. However, i suspect that the other properties of HDPE, particularly its very smooth walls and joints and inert chemical makeup, combined with proper installation, already prevents most of the problems and the expensive coating is not needed.
Google HDPE or try plasticpipe.org for more information.
Corrugated Drain Pipe
Corrugated Single Wall Drain Pipe for Earth Tubes has many good properties, but it can also hold water… So be warned!
Corrugated Drain Pipe is another polyethylene product, so, like the HDPE pipe, it is tough, long lasting, inert, etc. However, It is much thinner than HDPE, so it is corrugated to keep it from collapsing. This pipe is definitely the most flexible and lowest cost of all the piping options, which is why it has been so enormously popular for “budget” earth tube applications. It is also very commonly used in perimeter drain systems used by both conventional and earth sheltered homes.
As with the other types of pipe, the 6 inch corrugated drain pipe costs more than two 4 inch pipes (probably more due to lower production than increased cost of manufacture). You can buy “solid” corrugated drain pipe, which means it doesn’t have any holes. This is usually a better choice than the perforated or slotted pipe usually used for drainage systems. There is also “leech” pipe which has even larger holes and is commonly used in septic fields. On average, 4 inch drain pipe costs less than 40 cents a foot (2012 pricing), while 6 inch can easily get up to $1.20 per foot. You can buy large rolls, 100ft or even 200ft long. This sort of pipe is easy to install yourself, for additional savings.
Of course, there are drawbacks… In fact, I suspect that much of the bad press surrounding earth tubes comes from the use of this sort of pipe. Because the pipe is corrugated, regardless of how well it is laid, water will not fully drain out to the end. Water can sit in the corrugations. This can be worsened if it is not laid straight, which is not always easy with coiled pipe.
Using perforated or slotted pipe can help by letting that water out of each corrugation, but those holes are notorious for letting bugs and radon (and possibly more moisture or water) in. Also, the factory slotted pipe has the slots on inside ridges, so there is no draining the outside ridges (I assume this is to prevent the slotted pipe from snagging while it is uncoiled). This pipe can come with a fabric sock that will help keep plant roots and many of the larger bugs out.
Earth tube experts warn that it is better to buy solid corrugated pipe and cut your own slots. Notch each of the outward corrugations, but only on the bottom side of the pipe, so they will drain (just notch, don’t split the length of the pipe or it will collapse). Lay the pipe very carefully to make sure the notch is on the bottom. The hope is that any water droplets will have a very short distance to run before they can exit the pipe.
My wife is particularly concerned about this sort of pipe and absolutely will not let me even consider it as fresh air inlets for our home… This is a concern shared by many (and protested by others). We will be using this sort of corrugated pipe for drainage around the perimeter of our foundation. My plans for “By-Passive Solar” include earth tubes that would not go into the house, but would instead circulate solar heated air under my umbrella. The perimeter drains are already in a good place to do that second duty, I would simply need to lay them out a little differently so that I had a complete circuit and attachments to the solar air heater… I might even hook them up so they can enter the house (if I want). Design is still on going.
The corrugations also add wall friction ( very high surface roughness which leads directly to high frictional pressure losses) to this sort of pipe. If you are taking it more than a hundred feet, I recommend paying extra for the 6 inch pipe (even larger sizes would be better, but they are prohibitive expensive). If you use a duct fan, make sure it is the high pressure centrifugal type and not the low pressure axial “booster fan” type. It may not be practical for other reasons, but some suggest pushing the air (pressurizing the pipe) rather than pulling the air (reducing the pressure in the pipe). This positive pressure should help keep some things out (including Radon) rather than drawing them in.
Warning: Corrugated drain pipe seems great! It is tough, flexible, cheap, easy to install, etc. but it can also hold water (potential mold problem) so it needs to be laid very carefully.
Some experienced earth tube experts (such as Larry Larson) recommend these corrugated tubes (but at the larger 8 inch diameter) because they feel the corrugations help mix the air, which improves thermal transfer. He also says you must lay them in a serpentine pattern to help with the mixing. I assure you (see the sections on Pressure Drop and Reynolds Number calculations) that the flow will be turbulent in even the smoothest pipe. The corrugations and serpentine path will dramatically affect pressure loss (Larson mentions that you can’t even feel the air moving). Larson’s site goes into detail on other steps you need to take to keep mold an other potential hazards at bay.
Corrugated Double Wall Drain Pipe
I am not the only one to notice the serious problem with draining corrugated pipe… Fortunately, some of the others were in a much better position to solve the problem. They invented “Double wall” pipe. This is pipe that has a corrugated outer surface for strength and flexibility surrounding a smooth inner wall that drains cleanly.
Since this uses much less plastic than the solid HDPE pipe, it costs quite a bit less. It also weighs much less and is more flexible, so it is easier to get into position. The best part is the press fit soil tight (water tight is also available) connections that make assembly a snap. Most brands also feature a design where the snap together mechanism works within the outside diameter.
I hunted around and found some local distributors for ADS Pipe in my area, N-12 is the product name. They both quoted me the exact same prices, so I guess price is determined by the head office. With the solid wall pipe, I needed an unusually thin wall so I needed to give weeks worth of notice to get my special order filled, but with the ADS N-12 drainage pipe, diameters from 4″ to 60″ are standard and I could get delivery in 3 days. They also had a wide range of fittings such as T pipes, etc.
is a fiberglass duct type that I recently learned about. I have not had time to research it thoroughly, but it is used mainly in under-slab HVAC for commercial and industrial buildings. It is available in all the diameters and with all the fittings that you would need. I heard it was expensive, and it looks like it needs very professional installation but not sure how that cost compares to the alternatives. I will research it more when I have time.
The Equations!
Pressure Drop
Professional engineers and HVAC designers calculate the pressure drop (head loss) for a given duct system. You can use similar equations for earth tube design. If the pressure losses in the system exceed the pressure driving the flow (passive or active), the flow will stop and the duct will become useless. When air flows through a duct, there is pressure drop due to friction losses as well as dynamic losses which are caused by change in direction or velocity (usually at the fittings). For a commercial HVAC duct system, 1 Pa/m loss is typical.
Pressure drop can kill a passive system or require an active system to use a lot more electricity (and make more noise) than would other wise be necessary.
Frictional Pressure Losses
I will move the actual calculation to this page (link coming).
However, even without entering numbers, the equations tell most of what you need to know about the relationship between the parameters. Understanding these relationships leads to design insight.
- The DArcy equation predicts the frictional pressure losses in ducted air systems like Earth Tubes
The friction losses are due to viscous interactions between the air and the pipe walls and can be expressed with the D’Arcy-Weisbach Equation. In this equation;
–∆Pf = frictional pressure drop
–λf = friction factor (based on material, Re and Dm)
–L = length of duct
–v = mean duct velocity
–g = gravitational constant
–Dm = hydraulic mean diameter (cross sectional area / perimeter)
Some things are immediately apparent from the equation; for instance, the pressure drop is proportional to the friction factor. In other words, the rougher the pipe wall, the higher the frictional pressure loss (which seems pretty obvious). This should affect your material choice; PVC is 200 times smoother than concrete, HDPE is even smoother. Increased length is also a factor; while we want length to provide more contact for heat exchange, too much and the flow could stop. Velocity is very important because this is squared. In other words, if you double the velocity, the pressure drop is affected by a power of 4. If we reduce our velocity from 700 to 175 ft/min, we reduce the velocity by a factor of 4, and our pressure losses by a factor of 16. Passive systems tend to move the air relatively slowly, but adding a fan to increase the velocity may actually be counter productive (choose a fan with high pressure rather than high velocity ratings).
On the bottom of the expression, we see the hydraulic diameter (proportional to pipe radius), which means increasing the pipe radius reduces the frictional pressure drop.
Dynamic Pressure Losses
The dynamic pressure losses can usually be found in tables. For instance, for a given velocity, the pressure drop around a 90 degree mitered turn may be 50%. The same flow around a smooth bend may only lose 15%. The problem is that most of the tables are for higher velocities used in home or industrial HVAC systems. However, looking at these charts, you quickly get the idea that fewer fittings is a good idea. You can also see that lower velocities have lower losses for a given fitting.
Gentle curves are much better than tight turns. A gentle turn has a radius at least 6 times the pipe diameter. However, even gentle curves or serpentine layouts cause the flow to change direction and, therefore, induce dynamic losses that will reduce your flow rate or increase your energy costs.
The charts also show that diverging or converging sections should be as gradual as possible. Most HVAC texts suggest that divergence should not exceed 12° and convergence should not exceed 30°.
Reynolds Number and Turbulence
A second equation, important for understanding earth tubes, is “Reynolds number”. When scientists were studying flow, they knew that it was sometimes laminar and then as the velocity would increase, it would transition to turbulent. They could graph it but it took until 1883 before Osborn Reynolds showed that the change depended on ρVd/μ, which was named “Reynolds Number” in his honor (and usually shown as “Re”).
This is important because the turbulence of the flow affects both the pressure drop and heat transfer rate of the system. By calculating the Reynolds number for a given design, you can predict if the flow will be turbulent or laminar. In a viscous flow, friction is able to stop the air molecules adjacent to the wall. If the flow is laminar, layers form with each layer away from the wall moving a little faster than the layer below it. This forms a “boundary layer profile”. The flow is called “laminar” because these layers are stable. Streamlines stay nice and straight and never cross. While the flow at the walls is stopped, the majority of the flow moves easily and smoothly thru the duct/pipe. The problem is that heat transfer between the walls and the majority of the flow is greatly reduced. In turbulent flow, the fluid is always mixing and the system is better able to transfer heat from the walls to the majority of the flow or vice versa.
Many Earth tube designers incorrectly assume that their flow will be laminar. They tell you that you need to “add turbulence” to increase heat transfer by choosing tubes with rough or corrugated surfaces, laying the tubes in serpentine patterns, etc. These “enhancements” increase pressure drop dramatically. A few quick Re calculations show that they are not necessary for most earth tubes.
Reynolds number is proportional to the density, velocity and diameter and inversely proportional to the dynamic viscosity. The density and viscosity are properties of the fluid (such as air) and are both inversely proportional to the temperature. Thicker fluids (like syrup) have higher viscosity and tend to form laminar flows (Low Re), air is not very “viscous” and goes turbulent easily. The velocity and diameter are aspects of the duct design, increasing either parameter will increase your Reynolds number and turbulence.
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- The Reynolds Number can be used to predict transition to turbulent flow (Re > 2300 for round ducts)
ρ = fluid density
- V = mean flow velocity
- d = hydrolic diameter (inside tube diameter) (keep in mind that this may be different from the “nominal diameter”)
- μ = dynamic viscosity of the fluid
Lower Reynolds number flows are laminar. Higher Reynolds number flows are turbulent. For a round duct/pipe, this transition happens around Re~2300. We can easily calculate a table of Reynolds numbers for various nominal duct sizes (actual diameters would vary based on duct material). In this chart (above), I have colored Reynolds numbers >2300 red. These are turbulent flows.
Using the velocity and the nominal diameters (again, the actual internal diameters would vary based on duct material), we would get this table showing cubic feet per minute. Again, the “turbulent” flows are colored red. If you can stay below these flow rates, you may have laminar flow (flow could still be made turbulent by seams/joints, dirt, upstream turbulence from the fan, etc.) which would flow with less resistance, but much less heat transfer (much less slope for pressure drop over velocity).
- Flow rate in cubic feet per minute was calculated in this table, and colored red to indicate turbulence based on the previous table. It shows that for any significant flow rate, you should expect turbulent flow, even in a straight smooth air duct.
A related question is how quickly the turbulence will form. Assuming the flow enters the duct as laminar flow (unlikely), how far will it go before it becomes fully turbulent?
- A flow profile will form within a short distance of the inlet, and in almost all Earth Tubes, this flow will be turbulent.
Friction between the flow and the walls (friction exists even in a relatively smooth pipe) will bring the molecules immediately adjacent to the wall to a stop. This slows the flow next to it, and the flow next to that, etc. The result is a growing boundary layer profile that shows the gradient between the stopped flow at the wall and the free stream velocity. This boundary layer grows as the flow moves down the pipe until it meets in the middle and a stable flow profile develops. If the viscosity is high enough that the Re<2300, the flow can remain laminar. However, if the flow is not viscous enough, the friction at the wall can actually cause some flow reversal (wall roughness can cause this to happen even sooner). This flow reversal starts transition to a fully turbulent flow. A boundary profile can still develop in a turbulent flow, but it is really the “mean” turbulent velocity profile; the average of many small fluctuations in velocity and direction. Since this average is relatively constant, the resulting wall shear is constant and the pressure drop becomes linear with X.
- The distance to a stable flow profile is between 18 and 20 times the diameter up to Re = 10000
The distance before this stable profile develops is a function of the Reynolds number and Diameter and can also be calculated. Often this distance is expressed over the diameter. For all the Reynolds numbers on the above chart, this works out to between 18 and 20 times the diameter, which for these pipes is between 6 and 20 ft. Any ridges, fans, screens or other upstream obstacles will only cause this to happen sooner.
Just in-case my point got lost in the engineer speak… here it is plainly. Turbulence is good for heat conduction, but trying to intentionally induce additional turbulence is unnecessary and bad for pressure loss. In designing your system, you can assume the flow in your earth tubes will be turbulent no matter how smooth the walls are. There is no need to add features to increase turbulence, they will only increase your back pressure and reduce your flow.
Virtual Build
Last post, we talked about finding problems with the architects drawings. Mostly, the issue was just that the drawing was not quite to scale. The dimensions shown were not actually the dimensions used. I had followed the dimensions as shown when building my virtual model, so by the time I got around to fitting the kitchen door, it didn’t work. Some things I could fix, such as the radius of certain rooms that had been mislabeled by a number of inches. Other things, such as the width of the arch shown on the kitchen wall elevation, were just plain wrong and I had to figure out how to deal with the misfit.
Note: In an earlier post and on a page, I talked about how I went with the architect who offered only 2D drawings because it saved me a lot of money on this difficult-to-3D-render home. At the time I understood that 3D is better for most rectilinear homes because it does help to find these problems, changes are properly propagated, etc. But for this complicated unconventional home, I reasoned that I would be paying a lot for the 3D cad skills when I really only needed the architects design skills and the final builders would only need 2D drawings. I never did get a lot of architectural input on my 2D design, and now that I am doing the 3D model myself, I am finding all the problems that a 3D architect would have found. Choose wisely, even if I am not really sure what the wise choice would have been yet.
I am using SpaceClaim to build my virtual model. It doesn’t have a lot of the fancy textures or architectural features like widets for making sloped roofs or easily adding doors to walls. I also have Autodesk Revit, which does have those features. I used it for my earlier models (pre architect) and that tool was great for layout, etc, but I found it much harder for the complex geometry of my roof. SpaceClaim can handle the complex geometry. Spaceclaim is also great and modifying a geometry to fix a problem. It is a “direct modeler”, so you can grab any surface and “Pull” it to adjust it. Everything gets taken care of along the way. This made fixing the model pretty easy.
I needed to reduce the radius by 4 inches. Even on complex objects like this concrete over my quad-deck, it was easy to pull things into place…
Other tools, like Rhino3D, are popular with architects and could handle the geometry and has much better rending functionality (and my architect’s junior guy did model some aspects of my design in Rhino3D), but it doesn’t have the tools for easy modification and I didn’t have a licensed copy. I am told that Sketchup Pro, very popular with architects, could handle this. However, I tried the very popular free version, aka “Sketchup-Make” and it could not handle the ends of the vaults in the circular portion of the house (although it would be great for something more rectolinear).
Once the radius problems were fixed (and I added all the changes to my errata sheet), the door still didn’t fit because of the out of scale arch used in the architects elevation. I had to think about my options. I was not willing to re-scale the rib to match the architects mistake in one drawing. I decided that the rib at that location was “architectural”, not structural. This meant that I didn’t want to move it outward or upward and lose its architectural look/fit with the rest of the space. It also meant that I could cut the spandrel without needing to re-engineer the arch because that arch was not really bearing the load of the roof like the other arches were.
This left me with two main options.
1) I could cut the arch to fit the door. This would give me a full rectangle door to work with. I could get my “architectural” look back somewhat by coloring the door some how to continue the arch. Maybe I would add a veneer of granite, or stained glass or just stain or paint, shaped so that when the door was closed the concrete arch lines were continuous across it. Well, my wife did not like that idea. She is concerned about the structural aspects and she is probably concerned that it will be more work (the house is enough work as it is and she hates it when I add to my potential work load).
2) I could accept the fact that the arch crosses a big corner of the door. To test this idea, I have actually put duct tape across the corner of my office door in a way that matches the profile of the arch crossing the kitchen door. It has been there a while now and I have not minded it at all. Even if I brush my shoulders against the door frame, my head still does not hit the duct tape corner. If I go with this plan, I have multiple ways of proceeding…
2a) I could cut out a section (or just prevent concrete from forming in that middle space) so that I could fit a rectangular sliding door frame tucked into the arch. If I take out 4 inches of concrete from the middle of the spandrel, there will still be 4 inches on either side. Plenty of structure for an arch that is filled in on the underside with concrete anyway. The door would slide into the wall between the kitchen and basement stairs. I like this idea, but it will take some careful planning if I am to form the concrete rib with the void in exactly the right place… (planning the wall void is easier because I am building it right in place over a framework.)
2b) I could simply hang a sliding barn door (but a modern looking one with nice or hidden hardware) on the outside of this kitchen doorway. The door would be hanging in the mudroom and could slide the opposite way along the mudroom wall, so I wouldn’t need any voids in the wall between the kitchen and basement stairs. This is Sherri’s preference, at least partially because she thinks it will be easier to implement. I think the architect may also have suggested it at one point (because he didn’t know how I would get the mechanism inside the concrete wall). I don’t like the “fit” of it as much, but I will try to keep an open mind and think about it some more.
In the mean time, here is how things are looking in the kitchen (I modeled in some cabinets to make sure it all fit)… The three open blocks above the cupboards will be 8″ glass blocks and are there to let light from the main living space into the basement stair well.
And here is a wider view of the north side of the house (the original plan was to virtual-build just the section over the basement). Of course, this is just the initial concrete structure (plus door bucks). No earth cover, windows, etc. You can see that the mezzanine windows have been moved closer together to allow the dirt to cover the roof better. Inside, I added other details, including the spiral stairs, etc. Maybe I will include some of those pics in the gallery at the bottom.
Fusion welding HDPE Plastic Pipe
I got a section (about 6 ft) of 8 inch HDPE pipe from a contractor a while ago. It was old and cruddy and maybe had a bit of oil on it, but I took it so I could experiment with it. The expensive part about building earth tubes with HDPE plastic is that you have to hire someone to fusion weld them together… Or at least, you can’t buy a fusion welder from Home Depot. The fusion welder equipment is very expensive and only intended for professionals. I thought that maybe I could make my own fusion welder. The professional equipment specs I found on-line called for Teflon plates that could reach 450ºF (230°C), along with some jigs to help align the heater plate between two ends of pipe and then move the heater plate out of the way and press the pipes into alignment.
I started by taking an old toaster apart. I was going to run the elements between two Pyrex glass plates that I found in the cupboard. I figured they were garage sale plates and not part of a set (there were just 3 of them). Boy was I wrong… Those were part of a special 3 plate cake holder thingy that my wife loves. Good thing I checked first.
I decided that her fold-able electric grill would be better because it already has two nice Teflon surfaces and dials for adjusting the temperature. I would just need to break it in half so it would fold outward instead of inward, and probably disable what ever safety switches its designers had included to prevent me from using it that way. Of course, I would also need to buy her a new grill (I already got her a new toaster), but that would be a lot cheaper than hiring a guy with a professional fusion machine, so win-win.
I put rings sliced from my pipe on the grill and the edge softened right away. I then lifted them off and pressed them together… Instant fusion weld… Actually, I guess I heated them too much (too soft) and pressed them together too hard, because I got a bit of a bead inside.
Later, I sliced up my samples, including a cross cut so I could see that the fusion weld was as strong as the rest of the pipe. For scale, the pipe shown in this image is 1/4 inch thick (twice as thick as the pipe I plan to use eventually), so the bead is about 1/12 of an inch. (sorry the pic isn’t very good, my camera doesn’t do macro well, but you can see the bump where the soft plastic at the join pushed into the pipe). Well, that was easy. I am sure I can handle that.
Of course, I wanted to see what else I could use to fuse the plastic… I have a small benzomatic torch. I thought maybe it would burn the plastic, but, even with the direct flame to the plastic, it only burned for a second (some surface residue) and then it just softened the HDPE nicely. The problem was the heat was not even enough… So for an additional experiment, I used the benzomatic to heat a piece of metal and put the plastic against the other side… That distributed the heat well to soften the plastic evenly and wouldn’t require any electricity. For one attempt after the metal was probably too hot, the HDPE plastic did stick to the metal a little, but a piece of my wife’s parchment paper fixed that problem (just like fusing perler beads). I later hooked up my benzomatic hotknife attachment and found I could cut the HDPE pretty well with that.
In general, I found that the HDPE plastic softened easily, once soft, it was a bit tacky to the touch, but would instantly fuse with other HDPE plastic. I found that the joints seemed as solid as the rest of the pipe. I also found that the heavy plastic also kept its heat well (high Specific Heat Capacity), so I had quite a bit of time to get the two pieces together.
The only hard part was aligning the two pipes perfectly. I imagine that would be even more difficult with 20ft long sections of pipe, but I am sure I could build a simple jig to make that alignment much easier.
Quotes (estimates)
I started back up the process of getting quotes last week. I probably called a dozen companies. Only one has got back to me with a quote (so far). A couple others just had follow up questions. And in one case, I am still waiting for a call from the “lady in the office who knows the email and such.” I need the email address to send in the plans.
The one quote that did come in this week was for the footings. It was about 1/4 the price of the last footings quote I got and this guy seemed much more interested in the project and much more pleasant to work with.
Previously, excavators had all told me that they would get down to the depth at the top of the footings and would let the whoever did the footings excavate from there. The other foundation people I spoke to agreed with this and included several thousand dollars of additional excavation in their quotes. However, this latest foundation guy said that it was very difficult to dig a precisely curved trench with their equipment and my sandy site probably wouldn’t be well suited to trench footings anyway. It would be much easier for the excavator to level out the area to the bottom of the trench depth (an extra foot) and then the foundation guy could lay out the curved forms (just thin plywood staked in place) in an open flat space in much less time and much more precisely.
This foundations guy is actually a full service concrete company that also has Shotcrete equipment. It looks like his experience is mostly limited to smaller jobs like turning “michgian basements” into real basements. I still prefer my other shotcrete guy, if I can ever manage to arrange a meeting with him. The foundations guy also said he would do flatwork and gave me reasonable rates for that.
Much Simpler…
I know I have mentioned this tiny house design site before, but I saw another post that I want to share… They have a few small underground homes and even more green roof homes and I recently stumbled on to another one (posted mid 2012) here. Man that look so easy to build compared to mine ;^)
I am not the only one to notice the serious problem with draining corrugated pipe… Fortunately, some of the others were in a much better position to solve the problem. They invented “Double wall” pipe. This is pipe that has a corrugated outer surface for strength and flexibility surrounding a smooth inner wall that drains cleanly.
