Underground Construction






Features & Methods:

PAHS – Passive Annual Heat Storage

(AGS) Annualized Geothermal Solar

Underground Heat Exchange System (UHES)

Geothermal Intro

Roofing – Pond Liner Methods (Alternative)

From what I’ve gathered here, pond liner is now recommended for the roofing, the roofing material should also be extended a good distance away from the sides, french drains are recommended around the perimeter, and it is no longer recommended to set the posts into the ground.

I found a material that is half the price of pond liner, but much much better. This material is 1/8 inch thick UV stabilized HDPE . It comes in 4 foot wide rolls 50, and 100 feet long and is half the price of pond liner if you get the black 4 foot wide roll of what ever batch they are currently producing. The only thing that will stop most people from using it is welding the sheets together. However, it took me 30 minuets to get this down using a harbor freight Plastic Welder and a 1000 foot roll of HDPE ‘welding rod’ . This stuff is impenetrable for all practical purposes!

1. Carbon Dioxide (the stuff we breathe out) is heavier than air. Any time we breathe in a closed space it settles to the bottom until stirred a bit. If you live in an airtight home that is underground or not without proper ventilation from and to your lowest point, CO2 can become lethal. I’ve seen families living in their “green, low energy” home that was so airtight they had to move out because the hamsters in their child’s bedroom in the basement kept dying from too much CO2 in the air. If you insist on doing it without a good air exchange setup I would urge you to invest in hamsters or canaries.

2. Radon, anytime you’ll have a house entierly underground you might want to have a radon test done before you build or it could be $10,000 down the tubes and a large medical bill for your family down the road.

If I was going to build. What ever the material. At least 2 ft of sand or sand gravel mix all around. Go deep with same below the footings and clear drainage channels filled with sand. Then build accordingly. Like a root it must be an anchor. Land shifts. It must ride it a little. Do not forget to pack solid any dirt on your bottom. Pack as hard and deep as you can.

The ground can be your best friend. But it has its own mind too. Work with it. I would douse your property. See where the trails are and see how the veg is growing. There are often moist spots we do not see. Avoid them to build in/on. I have 2 seapy spots on my prop. Think Hobbit House. A layer of silca sand and DTE will help with infestations. Also ventilation. Treated wood can and will give off toxins. You need vents and air exchange. By law you must have egress. And it is a safety issue. You do not want to be trapped no matter what.

I read you posting about plastic moisture barriers  and your wish not to use plastic requesting those with some experience to answer. While I can personally understand your not wishing to use plastics or buy from big box stores – plastic is 1 of the best low end cost, modern materials available to you so, I personally think,  you might should not discount it. It does last long , when protected and has it’s place in some of today’s better build earth sheltered structures when used correctly.

For our designs & specifically our Georgia Adobe Planetary Home, ( currently our top of the line modern design )  we specify EPDM Pond Liners at the furthest extension of our heat sync, with the addition of 6 inches of 2 pound closed cell polyurethane foam insulation,  to be spray applied onto all earth to structure exterior sides of our buildings, since this provides a superior strength and sealant ability. 6 Mill Plastic sheeting is also used in the floors during the foundations start , and layering of drainage and mud floor installations.

Eliminating mold growth and reducing the likelihood that mold spores will develop,  is the designed beneficial goal of a moisture tight structure. Increasing your air sealing while decreasing the unwanted ventilation leaks within the structure, also it prevents any un-wanted moisture from its accumulation upon your external materials, inside the living space ;  but barriers can also trap that moisture being naturally released in the course of everyday activities such as exhalation, bathing, cooking, plant growth mediums, Greywater systems , etc. Therefore, without proper ventilation in addition, improving the air sealing of structure  has the very real potential to exacerbate any existing moisture problems of the earth sheltering or even create new ones from occupation.

Other materials include Roofing Tar Mastic materials, a low cost choice and the less expensive 1.5 pound polyurethane foam has about the highest R-value of readily available insulation used today in most structures and we have specified the same before both in sheet form and in spray form , all depending upon the specific conditions. Foam sheets while the easiest applied by the do it yourselfer is often more expensive over all and unless it is sprayed or somehow is design/manufactured to be applied without any gaps it could also allow moisture leakage problems too so,  be careful with your use of any of the above.

I hope some of this helps you !

Joe Woodall, Eco-Architect , Managing Partner
Georgia Adobe Rammed Earth & Renewable Energy
231 Harris Lord Cemetery Road
Commerce, Georgia 30530 USA

Found a possible earth- and wallet-friendly replacement for new EPDM pond liner: Used billboard tarps.

True, they’re 20 mil so they’re less than half than 45 mil pond liner. But they were destined for a landfill, and at the prices I’ve seen you could double it up and still come out ahead.

Given that Mike’s original spec was much thinner (6 mil?), even a single layer should work just fine. A massive 48×96 tarp, enough to umbrella at least 1500 square feet with lots left over for thermal mass, is just $565 plus freight shipping. Bargain!

Though the freight shipping might make it higher. I understand you can meet the truck at their station and save a bunch.

If you layer pieces you can get 14 x 48 pieces for $50. They’ll ship it standard shipping, so that seems like a better deal.
About used billboard tarps:

The vinyl billboard tarps are reinforced with fibers.  It looks to me like the PVC when cold is not quite flexible enough and I noticed a few spots at the folds with a bit of wear, where I would wonder about over time, if I may not have problems with them.

The feel of the PVC tarp is more like the feel of a piece of PVC water pipe than like soft flexible vinyl.  If buried flexing much is not a problem and they may work well but while not having holes, I did note weathering slightly and potential for possible problems.

As Andrew noted above, possibly a going over with Gorilla tape may be in order.  I would more consider these for the top layer of plastic above the 4 inches of fill over a primary EPDM or heavy 20 mil poly water barrier.  Note that Mike’s original design used 6 mil poly but he now recommends the EPDM for a trouble free water barrier.

In my opinion these may be usable but for foolproof, the EPDM looks like the answer.

Used Billboard Tarps instead of pond liners? – https://billboardtarps.com/bulkcontainer/

Roof Structural Integrity?

If the posts are buried in the ground per Mike’s original suggestion there is a chance of the posts rotting off.  I did three posts charred and wrapped in plastic as Mike suggested.  All three rotted off and had termites.

Fortunately it took about 4 hours each to dig the holes with a jack hammer as my ground is hard,  so I decided that was too much work and went to an alternative method.  The alternative method requires temporary bracing of the posts before back-fill.

You can either pour a concrete pier  with a steel rebar – 5/8 minimum is what I went and drill a hole in the post or if your ground is extremely hard like mine (claystone and rock) you can drive the rebar into the ground using your jack hammer then put a plastic square vapor barrier out of 6 mil poly with a couple inches of rather dry concrete mix under the bottom of the post for solid contact to the ground.  You could also drill into the ground with a rotohammer for the pin.

I now recommend a French drain around the entire perimeter draining to daylight on the low side of the hill.

Mike mentioned similar problems with his ridge house but attributed it to the posts being exposed.  My posts were not exposed much but still had the rot problem –the water from the post will drain down into the plastic bags if the wood is wet then it becomes like a continuous recycled rainforest inside the plastic bags making a perfect growth area for mold and termites. If it could be kept dry with no holes in the plastic it might work with extremely dry posts, but in my opinion that will never happen – the plastic gets holes – the wood checks and cracks – no way to stop water entry.

I had a queen in one of the posts – maybe all of them.  I had to jack up the three posts and make steel foundations to bridge the holes, support the posts and bolt to the post braces about a foot above ground.

You can replace the posts with temporary jacks under the girders but I would rather just keep the post out of the soil and avoid the problem.

I built using the methods in Mike Oehler’s “$50 and Up Underground House” book.  Mike uses what he calls the PSP method-post, shoring and polyethylene-Basically you put in posts then a shoring of boards with poly and tar paper behind them and wrapped about 1 foot around the bottom.  You continue stacking the boards behind the posts and pulling up the plastic as you go up.  I used tarpaper to protect the plastic from rocks in the backfill.  You hand tamp the backfill equally on opposite sides.  Drainage is one of the keys to keeping water out.  Roof plastic extends past the sides a few feet to keep runoff away.  Mike is now recommending EPDM sheeting for the roof as he has had a few leaks over 30+ years.  Another friend of mine recommended torch down roofing.  The roof has a minimum of two layers of 6 mil poly with tarpaper underneath on mine.  Tarpaper over the boards with a lap then poly then 4 inches soil then another layer of poly. We have also used good heavy duty reinforced poly tarps in some areas.  The dirt cover prevents UV deterioration nearly indefinitely.  Over the top layer of poly I put compost and soil mix for the garden.  Gardening requires that you don’t go deep enough with a shovel to harm the poly.  With good composted horse manure you don’t need to shovel much so we use non-tillage methods and minor surface tillage mostly.  The top soil layer should be about 14″ giving a total of about 18″.  Another key to keeping dry is the uphill patio.  It is excavated on the uphill side to stop missed runoff before it gets in and gives you light and a back way out also.  A greenhouse over it also helps.  If  you are in a wetter area than I am French drains along the walls through under the floor to the low side of the cabin would take away any missed runoff. The French drains can be combined in plumbing trenches.  I occasionally get a little seepage from gophers digging around one area where I don’t have an uphill patio.  It usually only gets damp- not major wet.


Calculating Earthen Load

Calculating earth load is actually pretty easy, its all about math and knowing a few common multipliers.

I own a gravel pit so I know a cubic yard of gravel weighs about 3000 pounds. Loam is a bit more and sand is a bit less, but 3000 pounds per a cubic yard is close enough for these calculations, especially since on a WOFATI the soil is dry. Anyway the other multiplier to know is, there are 27 cubic feet in a cubic yard. Knowing those two multipliers, any soil load is able to be calculated on any house.

Lets say you have a shed roof (one pitch) Wofati, that is 24′ square and you want to cover it with 18 inches of soil.

24 x 24 =576 square feet, with 18 inches of soil, that would be 864 cubic feet of soil on its roof. To divide that into cubic yards, we just divide by 27 cubic feet, which equals 32 cubic yards. Knowing a cubic yard of soil is 3000 pounds, we do some multiplication and that is 96,000 pounds………..but do not be scared yet, that weight is distributed over a lot of square feet. So lets figure that out. 96,000 pounds, divided by the square footage of the roof (not the interior of the home) is 166 pounds per square foot. But with a WOFATI we are not done yet, because it is underground, we have to calculate the snow load too. That is easily found online for your location. Mine is 72 pounds per square foot, so combining the snowload with the earth load, our WOFATI structure has to support 238 pounds per square foot. That is actually quite a lot, because standard commercial structures are built to hold 112 pounds per square foot. In other words, this WOFATI needs a little beefier construction than a conventional house, something we knew from the start though.

So how much is 238 pounds in real world terms?

Well whatever you build has to be able to hold a rather heavy person standing with feet close together in one spot with no deflection. That can be done in a variety of ways. Putting posts, beams and other structural members closer together (say 8 feet on center instead of 12 feet), using deeper structural supports (like using 2 x 8 rafters instead of 2x 6 rafters), or thicker planking and logs over the structural members (like using 3 inch thick planking instead of 2 inch planking). It really is easy. 238 is the worst case load on this particular, imaginary, WOFATI built in Maine, so if I can build its support structure to hold my chubby Uncle from causing deflection in the structure, I have built a very safe house.

If I built my WOFATI using 8 x 8 square beams, 12 feet on center, with 3 inch planking laid upon the roof, experience in construction, some calculations of beam loads in the past, etc determines quickly that my chubby Uncle is not going to be able walk on this roof without deflection.

(This is not meant to offend those with weight issues, it is just a quick, easy way to try and put load calculations of a structure into real world terms that people can picture so they can build a safe WOFATI, and that is assuming a person considers a 238 pound person overweight. That is about average for a working guy in Maine. Myself, my Doctor considers me overweight at 195 pounds, only 43 pounds less then this example, so I am in the same “chubby” category. Honestly I do not think disclaimers like this are required, but I love Permie People and wanted to be sure I did not offend and explained my reasoning).




Personal Notes to Explore:

Organic Carpet Flooring over earth Over pipes/in ground heating system/the system where heat can be directed into the subearth ?