A lot of specialized experience and understanding are required to properly build an earth sheltered home.
Hindsight
Many contractors have experience, usually learned from mistakes made. Specifically, they learn from mistakes that negatively impact their portion of the build. They often don’t understand how the design or their adjustments and shortcuts will effect other portions of the build or the final performance. Many engineers have understanding of what effects the final performance, but don’t have the experience to understand how their instructions will be interpreted during real construction.
Contractors who do larger portions of the build are more likely to discover negative impacts of their earlier work and, therefore, build more experience. A General Contractor (GC) sticks around for the whole project and hopefully accumulates more experience (via hindsight) that he can apply to the next job. If you are GCing your own home, you need to figure out how to avoid mistakes rather than just learning from them.
Building Inspector
The building inspector is often a retired builder or someone with similar experience. They should be on the side of the home owner and help to keep the builders in line with acceptable practices.
However, for practical reasons, their inspections are only at certain key points in the process. The inspectors have a lot of other homes to inspect and can’t spend too much time at each one or keep up on understanding all the design intent. Even with a good building inspector, there is still a lot of room for serious errors that the inspections won’t catch. The GC and home owner still need to be vigilant.
For example… The building inspector checks out the footings before the concrete has been poured. He can see that the rebar and the forms look OK and may approve it, but he is unlikely to check all the rebar or all the forms. Even if they do check the prep carefully, they may not see what is under it and they are not around when the concrete is actually placed and the builders are stomping the rebar into the ground.
Anecdotes
Here are some anecdotal examples while I sort this out in my head… I am sure I will see more as I continue my build.
Rebar:
The engineer understands that concrete is strong in compression(20 – 40 MPa or 3000 – 6000 psi), but weak in tension (2 to 5 MPa or 300 to 700 psi). Reinforcement is used to increase the tensile strength and resist cracking. The Engineer understands that when the concrete footing is under building loads, its top portion is in compression (unlikely to fail or crack), but the bottom is in tension and needs reinforcement. at some point between these two, the compression and tension balance out to zero. Therefore, the engineer will often specify that the rebar should be placed in the bottom of the footing.
However, the engineer also understands that the reinforcing is pretty useless if it is not fully encased in concrete so that its tensile strength can be shared. So the engineer may specify something like placing the rebar 2 inches from the bottom of the footing.
Later, the guy installing the footing has none of this understanding. If you are lucky, he knows that he can use rebar chairs that will put the rebar right in the middle and he will probably do that if you let him. When laying out the rebar, they may not overlap corners like they should or worry about rebar being placed in certain critical directions where the tensile load is likely to be greater.
In other situations, the installer may have no concerns at all about pulling the rebar or welded wire reinforcement (W.W.R) up into the concrete to place it at the correct depth. They will just stomp it down and focus much more on the surface finish (that you see when you pay them) and not spend any time worrying about what you can’t see.
Getting this wrong is not something that they notice during the install (no failure as far as they can see), so you can’t trust them to get it right by experience. The guys on my project made all of these mistakes and probably more. I talked to them about these things as they came up and realized that while they have many years of experience installing footings, they had no real understanding of why they were putting in rebar or why it mattered where it went. As a GC on your own home, you will need to keep a careful eye on your installers and inject some understanding (or just tell they what to do) as needed.
Forming
Here is a case where experience is the most important thing, probably because failure happens during the install. An engineer can calculate lateral loads in mega pascals, he can understand how those loads translate into forces that will try to tear the form-work apart. Meanwhile, the experienced builder has felt the weight of the forms in his hands and the compactness of the dirt under his feet. He has seen these things fail and really wants to prevent that from ever happening again.
In my case, one of them said “put as many stakes in as you think you need to keep the forms in place, and then add two more.”
I still had some bow outs on my build because they overestimated how well the sand could hold in stakes, but generally speaking, their experience was an asset.
For my garage, the plans were a little unclear on the depth of a grove that was to be formed in the top of the concrete to catch the base of the wall. I understood what it was for, but thought it was based on the depth of a 2×10 board (1.5″ x 9.5″). The first guy to work on it had never done anything like this and did the work on a day when I wasn’t around, so he put the wrong sized board in the wrong place, etc. I contacted the boss the next day and he send out someone else to fix it. The new guy stiffened the board and placed it 3.5″ into the form. While he was working, I noticed it was “too deep” and went to ask him about it. He just looked at me and politely said, “I have done about 20 garage slabs for quonset huts and they always do it like this.” Recognizing experience (and confidence), I backed down and went to check the plans. He was right (and I let him know).
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.
Most of the electrical system for an earth sheltered home is pretty conventional, but there are a few differences.
Ground
Good electrical ground is able to dump many amps of electricity into the earth with as little resistance as possible. It is important for occupant safety, and surge protection (including lightning).
Code can be met a few different ways, but most electricians use one (or more) 8 ft grounding rods buried in the earth. Since it is all about conducting electricity into the earth, it all works better if there is moisture in the earth. Most electricians will try to bury one or two rods (separated by at least 6 ft) somewhere near a corner of the house where a downspout is likely to dump water.
If you are putting in an insulating umbrella that will keep the earth under your home dry, you will need to move your grounding rods out much further away from the home than an electrician normally would.
Since many earth sheltered homes have concrete footings full of rebar, it may seem logical to use a “concrete encased electrode” in the foundation (aka Ufer ground). Normally, a concrete encased electrode is a very effective grounding system and the NEC allows it to be used as your only grounding. However, this method is not as compatible with the earth sheltered umbrella and other water proofing measures that will keep the soil around your foundation very dry. Again, dry soil will not conduct electricity as effectively.
There have been cases when lightning strikes actually cause the moisture in the concrete to rapidly expand, crack and occasionally explode
Another popular option is attaching the grounding system to a galvanized well casing (an uber grounding rod). This can work well and should certainly be done if you have a metal well casing. PVC casings cost about 1/3rd of a galvanized casing and never corrode. Obviously it is not worth paying three times the price just so you can use your well as a grounding rod, but some areas still require the metal casings for all or at least the first 20 ft of the well. Find out the rule and costs for your area and plan accordingly.
Grounding is very important to the electrical system of a home and is probably one of the areas where you want to exceed code. I will probably start with the “concrete encased electrode” and also put in a few extra grounding rods and tie onto my well casing (if it ends up with a galvanized casing).
Conduit
With my construction method (steel covered in shotcrete), I don’t have wood frame walls (or even wood furring strips) that I can run wiring through in a traditional manner. This is probably the reason that electrical contractors all gave me such ridiculously high bids. My walls start with a framework of steel studs and rebar. I will need to run my electrical conduit in through that. There are several types of conduit that the NEC (National Electric Code) will accept for concrete encasement. Some are more expensive or difficult to use than others.
The above chart only tells part of the story. The first two rows, PVC Conduit and Electrical Metal Tube, come in straight 10 ft pieces. They can be bent, but that is much more work than the flexible tubes. There are also a lot more fittings when you have to put at least one every 10 ft, and the cost of fittings really starts to add up. EMT and PVC also take a lot more work to make each connection.
The Liquid Tight Flexible Non- Metallic tubing is really designed for situations, usually industrial or agricultural, where the conduit frequently gets liquid on it. This is really overkil for a residential wiring application. It does come in long flexible lengths, but its fittings are expensive and a take much longer to apply than ENT. The real killer is the price, although I did see some very reasonably priced at the reuse place.
That leaves us with the Smurf tube as the cheapest and easiest overall solution for most of the conduit.
My recommendation is to use a combination of PVC and ENT. The PVC is the cheapest per foot, comes in a wide range of sizes and can be buried directly. You can also use it in straight walls or even bend it a little with some heat. The ENT, with its long flexible coils, is great for longer runs through curved walls.
And the Electrical Metal Tube will look great in my steel Quonset hut garage.
ENT, Smurf Tube
The NEC stubbornly refuses to call it Smurf tube and instead insists on calling it ENT (Electrical Non-metallic Tubing). They cover it in article 362 of the NEC. Here are some of the highlights;
ENT is allowed to be encased in concrete, but it is not considered strong enough to be buried or used in places where the ambient temperature is greater than 122°f (50°c). Because of the temperature limits, it is not allowed to carry more than 600 volts. There are other weird rules about not allowing it to be exposed if the residence is more than 3 floors tall. Also, even though it is flexible, you still need to pay attention to the conduit rules about the number of turns (the degrees can not exceed 360°.
The flexible connections just snap together (no need for glue or any other tools), but I think I would tape the joints before adding the shotcrete.
Boxes
Your choice of Boxes is related to your choice of conduit. If you are encasing them in concrete, you should probably go with plastic (rather than metal which has gaps and can rust).
You want the kind that connects the conduit right into the box (round holes) rather than the kind where you just push the wires through little square trap doors. If there are any little gaps, tape them to keep the concrete out.
The boxes generally assume that you are attaching them to wood. Some come with nails built right in. You will need to figure out a way to properly attach the boxes to the rebar. When I figure it out, I will come back with pics. It needs to be tightly fixed to make the inspector happy, but you will also want it to stay where you put it when the heavy shotcrete is slamming into it.
Books
If you need to save a bit of money by doing the electrical yourself, or if you just want to keep a better eye on what your electrician is doing, you may want to do some reading.
I summarized my favorite Electrical Books here…
