Cost efficacy of earthbags/strawbales/cordwood/ect

Hey guys,
I was wondering if y'all could help with something.
My husband and I are trying to build a house on a very very shoe string budget, and we thought natural building was the way to go both for ecological reasons and budget reasons. But when I sat down and actually did the math, down to the nitty-gritty minor details, I found that buying lumber and doing a conventional stick frame with a conventional pour cement slab foundation is actually cheaper than any of the alternative methods I looked in to. Cordwood, earthbags, strawbales and timber frame, all of it. We can't build with cob in our location so that's out unfortunately.
For example, I was able to source earthbags for super cheap, misprinted bags for somewhere around 20-25 cents per bag. Barbed wire is easier to find super cheap here, the foundation would be a gravel filled rubble trench, lime based plaster for the inside and out (since cob isn't allowed here) ect. But somehow it still came out that conventional stick framing, even with the necessary siding and insulation, would be around $5k cheaper. That 5k could mean the difference between being able to move in before next winter or not. I know that isn't a TON of money, but it is a lot for us.
Has anybody else experienced this? How could building with dirt and locally sourced materials be more expensive than the conventional and unsustainable practices?
We REALLY want to live in a naturally built house house but ironically it's looking like we won't be able to afford it

I have been dreaming of having a cob home for many years and it has just never been in my future.  

Before that, it was a log home. Too many options made us decide on going another route.

It is not cheap to build a home no matter which route you decide to go.  Just plan to make it as energy efficient as you can because in the long run that is where you will save money.

I am sorry I could not answer your question though maybe some others will have some suggestions.

A lot of times it comes down to labor. Are you willing to do it yourself? That seems to be the make or break. Additionally, look at where you live? What’s readily available? Wood prices are up 180% so it’s going to be more expensive no matter what you build unfortunately.

Ecological usually means more expensive. The way to cut down expenses is by doing the work yourself, but don't fool yourself, it is much cheaper to use conventional materials your local builder companies are used to work with. In my country these are concrete, bricks, terracota and cast (plaster), wood is in short supply.

Hi Abraham and Angela!
Yup labor was one thing we didn't want to pay for, we intend to do 100% of the work ourselves, from digging the rubble trench foundation (or pouring the cement) all the way up to framing the roof. We thought this would make things cheaper, we figured how could building with dirt be anything but incredibly cheap?
And yet, even with the outrageous prices of lumber, it's still cheaper to build a stick frame ourselves than to build an earthbag or strawbales house ourselves. I just don't get it. How can a house make of dirt or dry grass be more expensive than inflated lumber?
I dunno
But thank you for your comments! :D

Have you watched mylittlehomestead on you tube. its a family that have built houses and other buildings using earthbags.(actually its a continuous tube), but very informative.
I have also seen a couple that contacted an animal feed company and sourced boxes of miss printed feed bags to use.

regards Phil  

Hi Phil!
I LOVE mylittlehomestead! I found it when we first got interested in earthbags. You're right they're incredibly informative, and not just for earthbag building! Also for wells, water saving swales and mounds, and lots of other stuff. Love em'
We found a company that sells misprint bags, that's how we got the .20-.25 a bag price. The bags are very affordable and we were super excited with that price. If I remember correctly, the major money eaters are the lime for the lime plaster inside and out (can't use earth plaster unless stabilized with lime where we are), and the gravel for the rubble trench. Scouring the internet for the best prices (while keeping transportation and associated fuel use low) the lime needed for the stabilized plaster ended up being more expensive than cladding and insulation for a normal house. Same with the gravel for the rubble trench, as opposed to just having a cement truck come out and pour for us.
Again, in the end the difference was minimal, but for a family of 5 living in as small a house as possible, a finished shell (no appliances or cabinets or anything, just finished walls, floors, and roofs) the earthbag house ended up costing around 42k, and the stick frame house with the exact same floor plan ended up being closer to 36k. Even with the outrageous price of lumber! Straw bale was closer, at around 39k.
I know that a 3k difference, or even a 6k difference is negligible over the long run, and that maybe that money is worth the extra security, lower utilitiy bills, and green peace of mind, but with our income even a thousand dollar difference would mean a lot to us. I'm not even really sure what my question is for this thread, it's mostly just bewilderment haha

I truly do appreciate everyone's comments, and I'm not trying to shoot them down out of the gate. I am just so surprised that even doing the labor all ourselves, and doing things as cheaply and locally as possible, it's still cheaper to just go buy lumber at a big box store

These are good questions.  I had a similar question about rotisserie chickens in the grocery store.  How is it possible that it costs less than five dollars to incubate an egg, house and feed the chick, raise it to an edible size, process it, package it, and ship it in a refrigerated truck across the country?  That just doesn't make sense.

Well, in the case of my question the answer is "it doesn't cost less than $5" because rotisserie chickens are loss leaders for grocery stores.  But there are a lot of commonalities between the two.  Both have the plunder of the natural world down to an exact art, skating by at the thinnest of margins at incredible volume.  Raising a sapling to harvest size, clearcutting, processing the lumber, kiln drying, and shipping 2x4s is down to saving pennies at each stage to equal big dollars. Concrete is the same way. How can they save 9 cents here to make 11 cents down the line?  So there is infrastructure in place. Skids that fit certain dimensions within an inch or two of truck width and store shelves a specific size to make it all flow easily.  And as customers we benefit from that cost cutting. There is no such scaffolding in place for earthbag building. Everything is boutique.

Even so, I have questions about your numbers.  I am not doubting your calculations.  It sounds like you have done a thorough analysis. I hope to learn more from you about how you came up with your total to help me plan my own earthbag venture.  These are just questions and curiosity your post brought up.

I don't know what dimensions you're planning for but $36K seems low to me for a stick built house.  There are hidden costs all over the place.  Again, not seeing your numbers, are things like structural screws, tar paper, hurricane clips, seals, hot dipped galvanized nails, etc in the mix? Galvanized braces and door sweeps? Tyvek home wrap and sealing tape? I don't know, just asking, because having dabbled in home construction and roofing I know my foreman was always cutting costs to the bone and still venting money.  Now if you have access to secondhand windows and a supply of free barn wood that starts to make more sense.

By the same token, $42K seems high for an earthbag home. Again, not doubting your research, but mylittlehomestead values are more like $5K per room, which includes basement, main floor, and loft.  So if you built a couple completely separate bedroom domes, a living room dome, a kitchen dome, and maybe a mixed use dome, that's in the $25-30K area.  My guess is you would not want five separate domes, so the materials cost should go down further (fewer bags, fewer exterior doors, smaller roof.)

The biggest question for me is you said "with the exact same floor plan."  I don't consider earthbag or strawbale home to have the same floorplan as a frame-built house.  Strawbale walls are 5-6 times as thick as frame walls.  Earthbag walls are round while stick built are square.  So that makes me wonder if you mean "roughly equivalent 2 bedroom, 5 room house with roughly the same square footage" or if you are trying to replicate an exact floorplan? Some people argue that round homes have more efficient uses of materials.  Some argue that round walls waste a bunch of liveable space.  But everyone agrees the construction and layout are vastly different.

I suppose I'm asking for more detail so we can help you refine the tradeoffs in time, labor, cost, and especially layout.  Also I appreciate you raising these questions because it's making me think harder about my own venture and what costs I may be overlooking.

Hi Rob!
First off let me thank you for the very detailed information.
Secondly, both the house numbers I came up with are also including all permit fees, perc testing, cost to install septic, ect. ect. This equals about $14,500 all told so that can be subtracted from the total cost of both for the actual shell. After your post I went back and checked my numbers and realized I had done this. So, the corrected cost for the finished shell would be ~21.5k or ~27.5k for stick or earthbag respectively.
The earthbag house, unfortunately, cannot be a dome. It has to be a square/rectangle/"normal" shape as our area is slowly gentrifying from what used to be totally rural to something called "Residential Rural" octagon houses are allowed but I haven't looked into that. Maybe I should. Anyway, the floor plan for the earth bag house must also then include buttresses. Some of the interior walls are used as these buttresses, and some are external like those around windows or doors. When I say "the exact same floor plan" for the stick house, I mean minus the buttresses, with the internal wall at a normal depth, and with the external walls having the same exterior foot print (so if the earthbag house has an exterior of 32x32 with 15" thick walls, the stick house has an exterior of 32x32 with 6" thick walls, if that makes sense). Perhaps the extra cost to create these buttresses also adds cost? Since it's "walls" that do not have to exist otherwise?
Anyway, here is my cost breakdown at the best prices I can find relatively locally:

EARTHBAG
Gravel for trenches: $1700 for 40 tons use crushed limestone. 40 tons was the number I arrived at after calculating the rubble trench at depth, minus the area taken up by the filled bags and drain pipe, around the perimeter of the home plus each buttress, for a total cubic yardage required. Again, this for a 32x32 house with buttresses at each corner, plus every ~12 feet or every window/door, whichever looked more balanced. This ended up being around 24 buttresses (I say around because some are the full 4-5 foot length for stability, and some are only a bag long for window and door support)
Bags: $2125 for 8500 bags
Wall  Framing (2x6 studs): $1000 (for interior walls that could not be earthbag, like plumbing walls)
Stair framing: $1000 (we have to have normal, code compliant stairs. No fun suspended or spiral stairs)
Windows:$4000
Rafters and ceiling joists: $5,000
insulation: $900 (not very much is needed due to the earthbags, but some still is
plywood for floors: $2310
Barbed wire- 40 rolls of 1320' for $1600 (calculated the amount needed for walls, exterior and interior, and buttresses. equalled a wierd number so I had to go up one full roll)
Corrugated metal roof panels, to be cut into foot high shingles: $2,200
Underlayment specifically for metal roofing: $1,100
Wall panels for framed interior walls: $980
Lime for base coat and top coat of earthbag walls: $1620 (this is with the lime being on a 50% off sale at a local feed store)
masonry sand: $580
Straw: $240
Dry ball clay: $807

Total for earthbag shell: (apparently I did my math wrong previously, after re-adding everything) $27,162

STICK
Studs for any 2x6 walls: $2,387
Studs for any 2x4 walls: $1,128
Top and bottom plates for 2x6 walls: $814
Top and bottom plates for 2x4 walls: $374
rafters and ceiling joists: $5000
floor joists: $1584
exterior sheathing: $2541
"outsulation": $1540
insulation: $1104
house wrap: $240
wall panels: $900
concrete for slab: $2400
rebar: $1242
Stair framing: $1000
Windows:$4000
plywood for floors: $2310
Corrugated metal roof panels, to be cut into foot high shingles: $2,200
Underlayment specifically for metal roofing: $1,100

total for stick house: $21,254

I admit I do not know as much about traditional framing as I do about earthbagging, and even then I don't know much about earthbagging so that tells you where I am haha.
After your comment I realized I never included the cost of the fasteners, like nails and screws and metal panels for the rafters, ect. Would this make up for such a large difference? Hopefully this clears some stuff up, if not I could try drawing the plans out on the computer and posting them, let me know! But yeah, these are the numbers I was able to come up with on my own

Thank you again!

I think you may be clouding your mind when you speak of timber as being outrageously expensive.
Thats the price, if you focus on whether is a fair price or not you you may go mad.
Really you pay the price or find something different.

In Australia we do a lot of Post and Beam, whereby the roof structure is raised, I use steel poles now, but in the past large timber was available.
And the infill panels between the poles are filled in with earthbags in your case.

What are you doing for a floor in the earthbag house?
You have included floor joists but no plywood.

Is $900 enough for internal wall panels?
Why cut the roofing into short shingles, full length sheets would be more waterproof I think.

So we're talking a 30x30ish building with no basement but it has an upstairs?

Do you have to have all the plumbing, electrical and finish work done before you can move in?  That could put a hitch in your giddyup if you aren't planning on it.  

Why are you cutting the roof panels into shingles?  

Where in the world (approximately) do you live?  How big a foundation do you need - rubble vs concrete?

That's economy of scale for you. The more a product is in demand, the more it is subject to mechanization. As long as the raw material is in supply, everything around it will be cheaper the more it is demanded. Your local builders use lumber, apparently, so it is logical that the majority of the houses in your location are made of that material. If all your neighbouring houses are made of lumber, it just makes sense that all the stuff that a lumber house requires is in high demand, so they are produced in big numbers, thus cheaply. Moreover, if you use techniques that are not known to you or your local building company, you'll probably do it wrong several times, and this is a concern (more money and time spent) unless you want to learn and build more houses yourself.

In an ideal world, only sustainable materials should be affordable, truth is that the cheapest material is not the most sustainable (blame cheap oil). The next best option is to use less friendly materials, but invest in energy/resource savings techniques (better insulation, solar gain, attached greenhouse, rocket mass heaters, grey water system). Some of these saving techniques will pay for themselves, you can consider them economic investments; if nothing else, go for them. There are not many things where a worker can invest his/her money and expect a reasonable return, cutting bill expenses is one of them.

I've seen a few videos where houses were build with scavenged local materials, and that's great, however I wonder how many house builders can afford to scavenge for second hand building materials. Others used this sandbags with local sand, because they had good local sand they didn't have to pay for, but what if you have to purchase the sand? I must say I am not fond of the idea of building with sandbags, I don't see it as a truly sustainable technique since it requires a plastic fiber. Of all these alternative building materials, the one that I think that is true and tested for sustainability is cob (not saying it's cheaper or easier, it's not). Here is an old technique now adapted for T-bricks, using a wood frame to support newly made blocks until they dry:
http://jeanandaaron.blogspot.com/2016/08/t-brick-shed-progression-video.html

I looked into building a straw bale storage shed last year.  I was disappointed when I crunched the numbers.

Hello AnnaLea!  Thanks for posting more info.  It doesn't suggest a clear winner one way or another but it really helps see where you're coming from.

As to your question, no, I do not think fasteners are going to run into the $1K range or above.  I'd expect a few hundred dollars at most.

Some further comments I have based on your numbers:

In both cases you list $1000 for stairs.  I'm sure that if you went to a contractor and asked they'd quote you that.  But I feel like you might be able to get code compliant stairs for cheaper.  I have personally watched as a carpenter laid out two large planks of wood, did some math, and within the hour stairs were up.  Now that is a carpenter who has been doing that job a long time. But there are stair calculators online and at its heart you're talking about cutting two planks with a saw with cross braces and treads.

You've also listed $4K for windows in both scenarios. For stick built, that's probably pretty accurate.  For earthbag I think you could save a few thousand dollars there.  You can tell where I stand by my signature but I understand you're in an area which has restrictions.  So look for secondhand windows, mismatched, it doesn't matter.  You can save well over 50% by going to window vendors and asking for their seconds.  The multi-ton walls and generally organic aesthetic hide a lot.  

The gravel and earthbag and lime and such all seem on point.  Why do you have straw in there?  Just curious.

Why do you have floor joists for the stick house and no floor joists for the earthbag house?  Why do you have a concrete slab for one and not the other?  I'd say go with the gravel trench foundation and do earthen floors in ether case and save a thousand or two as long as you don't mind tamping and burnishing clay.  It's beautiful by the way. :)

I think you'll need more money for insulation on the earthbag house.  Earthbags are not insulative at all.  They retain heat well and slowly release it, which is a nice feature, but three days of cold/heat and your home will be the ambient temp. So I'd insulate the hell out of an earthbag home unless you are in a hot or temperate climate without huge temp swings.

I agree with Mike I'm curious why you are cutting the roof panels.  My general approach to roofs is reduce seams as much as possible.  Also, every junkyard I have ever been to has corrugated metal roofing.  One coat of primer then some dark neutral color to hide when the rust inevitably reappears and no one will know they weren't new for at least 5 years. Maybe 10.

I don't think this definitively tips it either way.  And thanks for the numbers! They're helping me plan as well. Good luck!

Which region, terrain, ecosystem?

My stick frame, denim insulated, all wood interior house burnt down in a 70mph woodland firestorm even with metal exterior and roof.  (CA hilly dense woodland).  It was also really hot to live in.  

Now I wont build anything with wood more involved than than a temp structures platform or straw bale exterior house's frame with wood.

Here's what folks in the Warm West tend to recommend:
Straw bale exterior walls for insulation,
covered with earthen plaster for durability,
add Earthen interior walls and/or floor for thermal mass, as labor permits later.

I understand the above as having the lowest lifetime cost of ownership and being the most habitable.  Its among the most passively cooled designs that're not earth sheltered.

I recommend considering stick frame houses - tents.

Have you considered building a tiny cabin first so you can move there while you build your main house?  Then you experiment and learn prior to building your family home.  You'd also eliminate the cost of your current housing while you building.

Any form of earth building or even strawbale housing is very labour intensive.
I have worked with many people building their own, and a 3 year program is pretty standaard.

As for stick homes and firestorms almost anything will burn in that environment.
Many people build in bushfire prone areas with completely unsatisfactory designs and materials.
In Australia, I have seen one home survive a full firestorm blast.
I t was build from Hebel Bricks which are autoclaved from memory.

Windows and roof lines, spouting are the weak points with 'ember attack'.

Going back to the OP.
Perhaps more details about the resources you have, the size and the area would help?

Hi guys! Sorry it took a while for me to reply. The past day and change has been pretty busy!
Anywhosal, I realize I forgot to add the cost of floor joists to the earthbag house, though I did remember the plywood. That was an oversight on my part, but it just adds another thing to the cost...sigh.
The internal wall panels aren't plywood or osb or drywall, they would be of a thing very creatively called "brown board" I found at the local Habitat ReStore that is smooth and paintable, and you use putty to fill the creases in between, either drywall or wood. Much cheaper, but not as strong. So anything more than a couple pounds has to be hung on stud, but that isn't really an issue for us

To answer the burning question about the corrugated panels being cut into long strips to use as shingle (so if a piece of corrugated metal is 4x8, it would be cut into 8 four foot long strips), it was the cheapest solution I could find to the expensive metal roof question. My husband has decided he NEEDS a metal roof (and I don't disagree, they are very reliable and long-lasting) but we can't afford standing seam. I don't want to use exposed fastener style roof panels because I dont like the idea of drilling a hole through my roof sheathing and relying on a tiny rubber washer to keep the water out. I have seen those things crack and just fall off, and then you have a moist roof problem. So my idea was to fashion long shingle out of incredibly cheap metal (corrugated) and hand them in the same fashion, so the fasteners would be hidden under the next layer, and the edges possibly sealed with some form of roofing caulk just to really seal everything in. It may end up being more trouble than its worth though, one of those things that sound nice as an idea but may or may not actually work out in practice.

In either case, earthbag or stick, there is no true "basement" but there would be a partial crawlspace under one side as the entire property sits on a slope. Speaking of the property, it's in the mountain region of southwest-ish Virginia (although my family in the Rockies refuses to consider the Appalachians anything other than foothills with delusions of grandeur), between roanoke and smith mountain lake, for anyone who may or may not be familiar with the area. Any foundation in our county has to go below the frost line (30'') from the lowest point, and be level all the way across. (so, the part of the house that sits the furthest downhill, down 30", and then have to dig that much further into the hill side) There is of course code for slab on grade and for pier footings, but with all the red tape it's easier for us to get approved with the partial crawlspace (and not have to hire an architect to approve the plans, at yet another cost) I think with the slope on the sight we chose, this works out to 30" deep on the downhill side, and around 5 feet on the uphill side. It's one of the most level spots on the property. This applies regardless of if we use a rubble trench or poured concrete or concrete blocks :) The land itself is mostly between 10 and 15% slope, with some bits as high as 25% (but very few). The mountains mean more snow than virginia usually gets, I think our area is somewhere around 30" a year. There is a spring on the property that flows strong all year through, even when the neighbors' wells dry up in heavy draught. 80% of the land is sloped to the south, and we get something like 40" of rain a year. January low is 0* and July high is 90* so, serious temperature fluctuations, sometimes even on the same day (anyone who lives in VA knows mother nature here is bipolar).

As to why poured concrete instead of rubble trench, or vice versa, I don't think I have a real reason other than "Well an earthbag house is crunchy and needs a crunchy foundation. Stick house is normal and needs a normal foundation" So it honestly hadn't even crossed my mind! Something to keep in mind for sure

Rob, I followed the link in your signature and I honestly love the idea. Especially using the tube to store stuff like spices, or as a little terrarium. I think it's a great solution, and if we end up earthbagging I definitely plan on giving it a try. The straw is for the lime plaster. It would be chopped up fine and added to the lime and sand mix. If I remember correctly, it helps discourage cracking. You could also fluff from cattail punks (no cost) but in our area, since we're so close to the lake, the wetland plants are protected, so chopped straw is the ticket :)

I may be misinformed about earthbags, I know they operate on the property of thermal mass as opposed to insulation, but I figured we wouldn't need insulation if the thermal was thick enough. I thought that if we were heating or cooling internally, the earthbags would retain that temperature and then radiate the heat or cool, reducing costs. Is this not true? Would it just go straight through and be lost to the outdoors? I may be misunderstanding how thermal mass works.

Efren, when it was just my husband and I, all we wanted was to test ideas out on the perfect tiny house. Life happens though, and we now have a five year old and a two year old. I know some people could do it, I know some people HAVE done it (or are doing it) and maybe we could do it if it was just the five year old....but the two year old....I don't know how to describe him other than we've had parent friends refuse to watch him because he a terror. A very happy, incredibly cute, very personable, terror. He was 6 months old when he figured out the remote made the tv screen change. He was 9 months old when he figured out he could roll into the table hard enough to knock stuff onto the ground. When he was born, he didn't cry, he laughed. At a year old, he had figured out how to plot. Like, creating elaborate diversions to distract me while he scooted over to the pantry, grabbed the maple syrup, and drank it. At a year and a half he figured out how to take the safety plugs out of the outlet, and how to work the baby locks on the cabinets (we have to replace them with different, more complicated ones with a diversion button.) Maybe this is normal for a child. Nowadays he uses his teeth to climb up furniture, and uses his head as a battering ram on our poor drywall. Maybe this is normal for a child. Maybe not. Our five year old was a beautifully well behaved angel so we were NOT expecting this one. But I do not think I could attempt a tiny house with this tornado of a toddler. Again, maybe others could, but not this lady. (Also sorry for the long anecdote about my toddler. I know its off topic but he's just so wierd, he does everything with a happy giggle and is never cross or in a bad mood. Just meddling)

John, we are prepared for hard manual labor. Both of have worked manual labor jobs all our lives, though we may still be unprepared for the level of work this project will require. We had planned on working ten hour days three days a week, every week until done, with friends present occassionally to help, and had hoped to be in the house before christmas. We have a friend who has volunteered to do the wiring with us so we don't screw anything up, and another who has volunteered to do the same with plumbing. We may end up needing to hire out HVAC though. Do you think the Christmas goal is too unreasonable? Are we totally deluding ourselves if we broke ground next month? Either for stick framing or for earthbag

Thank you all again so much for the comments and advice, and new ideas! I really, trully, do appreciate the time you're taking to have this conversation with me!

What is this please?

hire out HVAC though

If you have no hangups about concrete, it is usually the best type of foundation if build to the conditions.
I stand to be corrected if freezing etc is an issue.

Back to the roof, I think shingles fixed the way you speak of will blow off in a gale.
Exposed fasteners to the bets of my knowledge do not cause issues.
Particularly if an underlaying insulating sheet is installed.
But I will investigate.
It seems some fasteners can work out, here is a comment I found

The four most common causes of exposed fasteners backing out are:
- Incorrect initial installation. Overdriven screws damage the fibres in either the wood or sheet substrate reducing holding power.
- Ice dams can, as you say, put pressure on the heads.
- Too few fasteners combined with light gauge roofing can allow wind to create repeated movement in the panels slowly working them loose.
- Moisture in the furring or plywood reduces its holding capacity.

The best solution, but for a variety of reasons not the most palatable, is to replace the roof with a concealed fastener snap-lock roof.
Barring that, and given that the substrate is in good shape, I'd replace all the gasketed fasteners with ones of a much larger diameter.

Another comment also mentioned

I've also heard that the expansion and contraction of through fastened steel due to temperature changes can work the screws loose. This is exacerbated when the substrate is thin plywood or osb. Solid wood strapping or purlins are the preferred substrate.

Xmas lock up move in, could be possible, but you need to plan all materials, fixtures and fittings.
A lot oif time is lost finding 500gms of nails or an extra length of material because you are short.
As a Civil Engineer we were taught Critical Path Analysis which is essentially a chart linking the path to completion with all the inputs.
IE Roof cannot go on until roff structure is complete and roof materials and fastenings are delivered.

HVAC = Heating Ventilation and Air Conditioning

I'm not a fan of the cut down metal shingles.  If you're worried about the exposed fasteners, put tar paper or another barrier between the roofing and the sheathing to allow a possible leak to drain away correctly.  And maybe plan to replace all the roofing screws (or more accurately, the washers) in 10-15 years.

AnnaLea Kodiak wrote:

I may be misinformed about earthbags, I know they operate on the property of thermal mass as opposed to insulation, but I figured we wouldn't need insulation if the thermal was thick enough. I thought that if we were heating or cooling internally, the earthbags would retain that temperature and then radiate the heat or cool, reducing costs. Is this not true? Would it just go straight through and be lost to the outdoors? I may be misunderstanding how thermal mass works.

I don't know all the ins and outs because I'm still planning and learning myself.  This is my understanding: Earthbags are no more insulative than, say, a thin sheet of metal would be.  The difference is the thin sheet of metal exchanges that heat immediately whereas an earthbag retains heat and discharges it slowly.  So slowly that if you live in a temperate climate, the sun has come up and recharged the heat before the wall has a chance to bleed off.  But in general yes, the answer is the heat would go straight through and be lost to the outdoors, just in slow motion.  So the question becomes how much temperature swing do you have, and how much do you want?  

If you have winter and it gets cold sometimes, and the home is not insulated, you need to keep the heat going much, much more.  Both because the outside cold will creep through the walls, and because the heat you generate will be lost through the walls.  Heat seeks equilibrium. With insulation, the cold does not creep in, the walls hold your generated heat longer, and you use dramatically much less fuel to heat.

If you have summers that get hotter than 72 degrees, the sun will bake the walls and that heat will soon make its way through to start heating your interior.  It might not snail its way through until 1AM or so, who knows, but at some point your interior will reach the temp it was when the sun was beating down, so you need to cool (if you have AC.)  If you insulate, that sun heat is much less likely to penetrate to the walls, so you'll get "shade" temperature all day.  

I'm currently investigating something I vaguely heard which is that if you dig down to the geothermal layer, you'll have a constant 55 degrees-ish base in your home, and if you have vents at the base of the walls and a vent at the top, convection will draw the cooler air in at the wall base and hot air out the top, pulling your geothermally cool air up as well, stabilizing the temperature of the whole interior.  And in winter, you can close the bottom vents and run a fan down a duct from the top to the bottom, sending the warm air down to re-heat the bottom floor.   This just something I'm trying to piece together from snippets I've heard and may not be accurate.  But I know it works in teepees.

Rob, its not so much a Geothermal layer, its just deep enough to have a constant soil temperature.
Its dependant on where you are, IE Freeze line etc.
Its about 4 feet down where I live.
There is plenty written about it, sometimes small pipes, 1 inch are laid in a trench and backfilled, water being passed through the line to transfer the coolness.
Others use larger air filled tubes.
Look for say, ground heat transfer .
Here is a technical paper on the matter heat transfer with the ground

And in winter, you can close the bottom vents and run a fan down a duct from the top to the bottom,

Actually, there's a fanless option. The thing is that you want your ventilation to draw air from the floor in winter and from the ceiling in summer. The air is cooler near the floor, so it makes sense to remove the cooler air in winter and the hotter air in summer. So, how? Install a vertical pipe, like a chimney, with two openings: one near the floor, the other near the ceiling, and the top at least one meter over the roof, so there's some drag. Have both openings with regulable doors, so you can open and close them, and even select how much open you do want them. Just remember to open the upper vent and close the lower one in summer, and reverse it in winter. Ideally this ventilation should be in your bathroom or very close to it to take out bad odors.

There's nothing natural here, but the bottle house is something