Waterproofing Earthshelted Buildings

Rob Roy is Director of the Earthwood Building School, which has specialized in cordwood masonry and earth-sheltered housing instruction since 1980. Rob and his wife, Jaki, have built four innovative cordwood homes for themselves since 1975, including the Earthwood home where they have lived since 1982. Earthwood is a 2400 SF two-story round, load-bearing cordwood home, earth-bermed and earth-roofed. Details of construction are in Rob's Earth-Sheltered Houses (New Society, 2006) and Cordwood Building: A Comprehensive Guide to the State of the Art (New Society, 2016) two of sixteen books he is written in the alternative building field. Rob and Jaki have taught cordwood masonry at Earthwood Building School in West Chazy, NY, all over North America, as well as in Chile, New Zealand, Australia and Hawaii. They have helped scores of owner-builders with their cordwood and earth-sheltered projects, including homes, saunas and outbuildings. Earthwood has produced a 3.25-hour DVD on cordwood construction, which, with his books, can be accessed through the Earthwood website, or on the Cordwood or Earth-Sheltered Housing  pages here. Rob is considered to be one of the leaders in the fields of cordwood construction and earth sheltering. He does individual consultations at a flat rate of $75/hour, but answers questions here without charge. See also Earthwood Facebook.

Questions and Answers

Q: Do you have any strong recommendations on what waterproofing to use for concrete retaining walls for earth-sheltered buildings?A(John MacMillian)There are many outstanding waterproofing methods available today. They usually revolve around a thick rubber membrane being applied to the outer walls. With proper installation, they should provide decades of service! Truthfully, the real goal is to design your landscaping so that water never reaches your waterproofing! An architect once told me that no matter how good the waterproofing is - it can fail. There can be a mistake in application or some other damage to the product that can lead to failure at some point.

A:(Paul Shippee) My Dad and I personally applied the waterproofing to my home. The product we used came in 4' by 8' sheets that were applied in an overlapping method, then the edges were sealed with a special product. All went well with the application; however, I was unable to be present when they backfilled. Their instructions were simple, backfill and compress the soil in 18" layers. When they were finished everything looked great! When heavy rains came a few weeks later two things happen. First, parts of the home leaked, and second, the soil settled almost two feet!. While I have no direct proof, what I believe happened is that the contractors did not backfill and compress the soil properly. When the heavy rains came, the soil began to settle fast and dislodged some of the waterproofing. The problem was completely solved by adding fill to the area that settled and then making sure that the landscaping created a swale (shallow depression) to divert the rain water away from the walls. I was lucky in one respect, since I was building the home slowly, I had not put any finishing material on the wall that leaked. After it had dried, and passes several more rain tests, I added an interior waterproofing product as added protection. There have been no additional problems. The bottom line is do two things. Use the best waterproofing and then design the landscaping to prevent the waterproofing from ever getting wet!

Q: I'm thinking of creating an alternative living space in Austin, Texas and would like your opinion of my plan. Primarily, I'd like to excavate a large portion of my lot and lay a slab, sink two metal cargo containers (8'by 40'each), weld them together, and finish it out (including large room, a closet or two, and a small bathroom). I'd like to cut out holes on top for a a stairway and several ports for sunlight(facing south with slanted glass) and than pour another slab to top it off. Id like to lay a think layer on top (just enough for grass to grow) I'm aware that moisture is an issue and am thinking of wrapping the containers with a thick rubber membrane or a some other type of moisture barrier. In your opinion is this "do-able"?

A: (John MacMillian) Certainly anything is possible. It's an innovative approach that could work. I would suggest putting a thick layer of Styrofoam around the outside to protect the rubber membrane from any sharp edges the metal may have.

Q: I wish to build a living structure using locally harvestable minerals and what ever else that requires. I can work out most of that. What I am failing to find information about is building underground. My living space might be described as a cave (in the best possible way). I can imagine windows along the hillside, but I don't know about moisture issues and rainwater diversion. What do you think?

A (Paul Shippee) It's hard for me suggest something to help you seal out water infiltration into a cave. About all you can do is attempt to divert water flow away from your area. If you can seal off unused areas of the cave, and then seal the walls themselves, you might be able to run a de-humidifier to make the home more livable.

Q: What are the environmental implications of the damp proofing treatments?

A: (Paul Shippee) If I understand you correctly, damproofing methods contain toxic materials, such as chemicals, plastics, and petroleum derivatives. So, the environmental implications have to do with long term and short term effects of toxicity in the earth, in the water, in the air, and in your home. However, there are a variety of nontoxic damp proofing materials available...

Q: I am in the process of designing a basic home that will be earth bermed and a super insulated roof. Planning on using cement block walls that are reinforced with rebar and poured solid with pilasters every 10' to 12' as needed. What is a good method to use to insulate and address moisture migrating to the inside of the masonry walls. I am planning on using an ice/water shield membrane (with primer) on the outside of the blocks...at this time it seems the best thing is to cover this with 2" of rigid insulation and then use expanded metal lath with parged cement where exposed/above grade...do you agree and could you offer an alternative way to address this situation when building with blocks.

A: (Paul Shippee) Two main problems with earth-bermed walls are: structural problems, and mold problems.

1. Build up of static water pressure in the earth against the exterior walls will cause an extra structural load against the walls due to hydrostatic pressure - which can be much greater than soil pressure alone. Some remedies typically used are to provide a way to relieve this static pressure by draining any water that could build up in the earth... by, for example, placing a vertical layer of permeable stone gravel against the ice and water shield that you are using to seal the exterior of the wall against moisture penetration. This also ventilates the exterior wall area to help keep it dry.

2. The other problem is moisture migrating from the inside of the house, through the cement block walls, and condensing inside the exterior waterproofing material. The fix for this is to seal the interior of the block walls with a good non-permeable paint and caulk all cracks extremely well. You want to prevent mold from growing inside the block walls.

Q: I am building a flat living earth roof. What material do you recommend to use for waterproofing wood structure from dirt? ie 4 ply hot mop build up or single ply rubber pond liner or ?

A:(Paul Shippee) I would go with the 4-ply hot mop. I used that on the SunEarth House in Longmont, Colorado (under one foot of dirt) in 1978 and have not heard of any problems. The hardest part seems to be the edges, that is, to keep excess water away from the walls. But this house was earth-covered, so the walls were banked up over the roof as well. I don't have any experience with the pond liner material.

Q: We moved into a 20 year old underground home 2 years ago. The concrete roof is covered with a rubber membrane and then has about 1 foot of soil on top of it. We have a problem with yucca plant roots digging through the rubber membrane and then causing leaks through the seams of the roof. We try to make sure we are killing the yucca as soon as we see them, but sometimes even very small ones will have very lengthy roots that will dig through the membrane. Is there any thing we can put over the membrane or any membrane that is much stronger that would prevent this problem?

A: (Paul Shippee) Gee, that IS a problem. I have not heard of or expected a plant to be that aggressive, but they do bore through plastic sewer pipe come to think of it. What is the membrane made of? Probably EPDM. It seems any fix might involve removing all that one foot of dirt. What a job that would be! Seems like you might have to remove the dirt and leave it off, then insulate the roof better.

Q: I have a cave under my house built from stones, and I have a problem of moisture and humidity affecting the kitchen wood and and the moisture is transformed in water on the wall and some times on the floor. Until now I have not furnished this cave because of this problem. Can you help me find a solution to this problem?

A: (Kelly) I can see how this could be a big problem for you. It sounds like the moisture is condensing within the space because of the temperatures and humidity there. Once you heat the space to a comfortable temperature, it may decrease some. I guess the only solution that I can suggest otherwise would be to use some sort of dehumidifier in there to deal with it.

Q: I have been installing concrete tilt-up buildings on my farm and have mastered it to the point of trying to build an underground home using tilt-up construction. The only thing that I am unable to find information on is a good sealing method. I used silicone in the joints, typical for tiltwall construction and have no problems. If I use silicone in the joints and put a good rubber membrane over it it will probably work, but I thought I would get your input on this.

A: (Paul Shippee) Yes, I think that will work. But I would check to see that the membrane is compatible with whatever chemical composition is in your soil. Usually asphalt products are used for underground concrete protection.

Q: I live in a bermed house that is located in Oklahoma and I am having problems with moisture, mold and mildew. The walls are 12 inches thick reinforced concrete with three feet of dirt on top of the roof. The house is very old, over 20 years I would guess. It does have a pipe on the roof and along the wall for drainage but I'm sure over time that has filled in with silt. If the drainage system is clogged up does this pose a safety risk. I think there is a layer of tar on the roof and there is no insulation. When it rains there are four water leaks two in the ceiling and two on the back wall. Could this be a sign that the structure is failing or is it something that can be fixed by removing the dirt and sealing the cracks with more tar?

A: (Paul Shippee) Yes, remove the dirt and before sealing the cracks with tar, please examine carefully the reinforcing steel bars inside the concrete for rusting. The structure depends mostly on the integrity of these steel bars. A little rust is OK, but if they are rusted more than 1/3 the way thru, then you have a structural problem. Concrete does well in wet conditions so usually no problem there, unless it is eroded by bad (hard) water. New drainage system while the dirt is removed will be a great idea: drainage pipes top and bottom and one ft crushed stone up along the walls, on top of foam board insulation = good idea!

Q: I am a civil contractor in the Houston area. Although I have plenty of construction experience, I need to know the best means and application for water proofing the exterior walls and roof for an underground house. I want to keep the structure free from mildew and associated odors. The soil is a very high Ph in the 50 range and the land is very flat with poor natural drainage. Any suggestions?

A:(Paul Shippee) I would apply two coats of regular asphalt then ice and water shield adhesive membrane over that.

Q: I'm building an earth-berm, high thermal mass (masonry) passive solar house. My earth berm walls are 10" thick concrete and 9' tall. I plan on putting a 4 ft. wide band of 5" of EPS insulation with a 1/2" square hardware cloth. This band will be then be covered with a coat of surface bonding cement and then 2 coats of heavy-duty masonry coating. Of this 4' band, one or two feet are exposed to daylight. The next 2 feet are covered with dirt. Then an insulated watershed umbrella 16 feet wide around the 2 earth-bermed walls. Below the insulated umbrella watershed, the dry earth is next to the concrete wall. My question is: What is the best finish for the interior walls? I'd planned to use surface bonding cement with a traditional stucco. But I'm now wondering if a more natural/breathable material would be better? Any advice on my exterior stucco and my interior finish would be greatly appreciated.

A: (Paul Shippee) I like your interest in making the walls breathable. However, I’d look to seal the interior from moisture that might enter from the interior, and also find a breathable finish for the exterior. I like that strategy. The wall breathes outward only. In searching for breathable coatings, ask the mfg for the perm rating (which tells how much moisture passes thru).

Q: We have a 3 sided bermed house - built in 1982. We just bought the house in December. The spring rains are coming into our house -- help!!

A: (Paul Shippee) Well, it’s simple but not so easy...you simply have to find the source of the leaking water! Is it from rain, are gutters broken or missing, how is the water getting in, and from where? Start there...

Very funny Paul : ) Okay, it's coming in through the topsoil/ground - the rear part of the house - the bermed part does not have a roof, and instead is covered by dirt. The water is pooling above the structure. Here's the question: what can be done to fix it? A french drain? The area can't be regraded.

This is a difficult fix...with dirt on the roof as well as bermed up on three sides. Chances are, like one of my earth–covered homes, water is penetrating the roof soil and flowing under the roof soil back toward the north side, and then down the wall to find some entry into the home. Without a sketch or picture it’s difficult to imagine all your roof/wall details, so I can’t say for sure. And I was not being funny when I said to determine exactly where the water is coming from. This first step, it seems, will take time and effort, and maybe some digging, maybe a lot of digging :)

Q: I am going to build an earth home aprox 36 x 40 ft, 3 sides and roof will be underground; walls either ICF or poured concrete walls. The roof I am thinking of doing is hollowcore slabs, 12in, with some kind of covering for sealing the top of the slabs [with what product?], then 2 in or 4 in of polystyrene 4x8 panels, then another covering over the foam, [what product?] then 12 to 18 inch of soil , sloped with sod on top. What would be the best products for sealing roof so it wont leak?

A: (Paul Shippee) You can use a product I used on my roof called “ice & water shield” (and possibly by other local names where you live) It is a asphalt ic membrane that comes in rolls with adhesive backing (not sure how it will interact with polystyrene foam boards). Then on top on that I would place EPDM membrane. So, a combination of these two (although an extra expense for two layers) will last for centuries as a waterproof covered by noncorrosive earth.

Q: We have an earth sheltered home which consists of 3 concrete domes and flat roof section. The domes are 28x28 feet and the flat roof section is 28x16 feet. We have SolaTubes in the roof which we really enjoy. However our roof leaks around some of these tubes. We covered the roof with EPDM rubber covering and had to cut holes for the SolaTubes. That seems to be the source of leakage. What do you recommend to solve this problem?

A: (Paul Shippee) You will have to expose the Sola tubes and examine and find where the leak is. Probably at the roof intersection. Then using the same glue and EPDM material create a way to reseal all around the tube/roof connection and continue that patch up to 6-12 inches above the dirt.

Q: I was wondering if you could help me decide what plastic roofing would be necessary to keep the roof waterproof (there will be a thin layer of straw as insulation in the roof sandwiched between two plastic membranes. On the top layer of plastic will also be a layer of dirt). I have been looking at polyethylene and pond liners.

A: (Paul Shippee) Not knowing how large or the shape of your roof…I would check out EPDM, and pond liners for cost and durability in contact with wet earth (not P/E).

Q: I want to build a 40’x40’ cylindrical silo with 26’ of it underground to be located on the top of a hill (not on the side). I live in Ohio and there are companies that can pour these structures for cow manure storage. My big question is for easiness is there a water barrier that can be sprayed on the outside followed by an insulation that can be sprayed on top of that that will be ok when it gets back filled? Would there need to be insulation on the inside walls also or could drywall or paneling be attached right to the concrete?

A:(Paul Shippee) I would spray or paint two coats of asphalt conventional concrete moisture barrier on the outside of the concrete, then stick a layer of EPDM membrane (if you can afford it) on top of that…then two inches spray foam insulation below grade and 4 inches of same above ground for a good thermal barrier. Leave the inside of the concrete with no insulation; a good treatment is to acid stain it your choice of colors.

Q: I am building an earth bermed home in south Mississippi. We do not have a water table issue as we have built above grade and will be backfilling/covering the roof with approximately 2' of earth. We are concerned with water control. Construction is 12" ICF with 12.5" Insuldeck roof. We are planning to cover the flat roof and walls with a rubberized roll-on product. Walls will then be covered with dimpled roll-type drain board. French drain surrounds the entire structure below the wall foundation joint. We are also planning to install an umbrella structure (after about 12-14" of fill) consisting of a layer of plastic sheeting followed by a 2.5" layer of ICF forms then repeating that layering system to a total of three layers of sheeting and two layers of ICF foam. Final fill would be only enough to support grass growth. Is there anything else I've missed or should do?

A: (Paul Shippee) Sounds like you've got the water leaks issue covered! Two layers on the roof will be plenty. I wonder though, about the structural loads…both against walls and especially on roof. With 200+ psf up there you best have two structural engineers agree on failsafe roof structure.

I had to have a PE sign off on my plans (no changes). The roof has 10.5 in deep beams every 12" with 5" slab thickness. Each beam has 2 #6 and 2 #9 rebars with mesh reinforcement in the slab. Poured monolithically. Walls have #6 rebar every 2' vertically and every 32" horizontally. Hopefully I'm good. We are backfilled about 8' high with no issues after sitting for 8-9 mos. This is a long project as I'm basically doing it all by myself.

Q: I am a senior interior architecture student at the American University in Dubai. I am working on designing a geological discovery center that is partially underground and that is because it is the perfect solution for my design features. I wanted to know if having underground facilities in a climate like Dubai sustainable?

A: (Paul Shippee) I would think very dry climate like Dubai would be quite good for underground structures; because water is usually the main problem.

Q: We are in the process of repairing a bermed home we bought. When the spring rains began we noticed that the water was coming in over the concrete floor under the walls.. We had someone come out and dig the berm away and are trying to get it fixed the best and cheapest as possible as we are on a limited budget. The walls are nothing but wooden beams and boards, there is no brick or anything. There was plastic but poorly done and no drain. We are going to install a drain pipe along the back of the house just below the floor level and put new plastic up along back wall.. The terrain is a mountain side that drains towards the house so what else should I do to help?

A: (Paul Shippee) Use a waterproofing coating instead of new plastic on the wall, something that is suitable for wood siding, two coats. Lay the new drain pipe in a washed gravel bed beside the concrete footing. Dig a diversion trench ten feet up from the building wall to direct runoff around the building.

Q: I am rebuilding an Earth Lodge, constructed of logs and clay. I am looking for a barrier to put on the roof to water proof it. The liner would be layed first and dirt put on top. Do you know where I can get a liner? I am hearing to use an oil field pit liner, but I am researching what would be the best for this project.

Q: (Paul Shippee) An oil field pit liner sounds good as long as it is not used and has cracks or holes in it and is not expensive. Otherwise, I recommend looking into EPDM roof membrane materials commonly used on residential flat roofs (check compatibility with your earth), or you could go with a 3-layer mopped tar surface that is commonly uses on commercial flat roofs with gravel stones layer to keep the sun away.

Q: We are building a cordwood house with a 4 ft high crawlspace made from concrete block. Most of the crawlspace will be underground. I was given several rolls of epdm roofing that I want to use that to seal the crawlspace block walls on the exterior. I want to use asphalt and then the epdm but someone told me that epdm and asphalt are not compatible. Is this true? Also where would I find information as to wether our soil would be corrosive to the epdm. It has approx 5.4 ph.

A: (Paul Shippee) EPDM is definitely not compatible with asphalt products. It’s the solvents and light oils that corrode the rubber. However, if the asphalt is old and solvents are dried out then it might be OK. I suggest coating the block walls with a plaster product called Q-Bond which can be applied after you dry stack the blocks…i.e., no mortar joints needed. as to the soils compatibility with EPDM, probably OK, but you’d have to know what’s in the soil as far as elements & minerals & chemicals. Here’s chart to use once you know what’s in the soil: http://mykin.com/rubber-chemical-resistance-chart

Q: How would you normally shield houses build (partially) underground from water?

A: (Kelly) The typical way to shield an underground house from moisture intrusion is with a moisture barrier, such as polyethylene sheeting, EPDM, pond liner, or a bitumen compound. In any case the wall should be constructed with materials that can withstand moisture if it does happen to get through. Earthbags, and masonry materials are good for this.

Q: We had an earth sheltered home built in 2004. It has since developed several leaks, inside and outside on the porch overhang. The home exterior is synthetic stucco. The leaks are causing the stucco to peal away from the concrete. Is there a way to waterproof from underneath or do we need to address the problem from top?

A: From the top.

Q: We are two students of Architecture at the University of Amsterdam. Our study mainly focuses on the possibilities of sustainable and circular building. Our graduation research focuses on building with earthbags or recycled car tires on the Forteiland Pampus in the Netherlands (UNESCO world heritage). Since this island belongs to UNESCO, the issue is to build underground. As you may know, the climate in the Netherlands is wet and cold. In the literature we read that (underground) construction with earthbags in areas with a high groundwater level and in wet and cold areas is not recommended. Solutions are given such as water retaining foils or bentonite clay. Car tires are often given as an alternative.

A: (Kelly) Building underground in areas with a high groundwater level and in wet areas is challenging no matter what building materials are used. In many cases it is simply not advised. Earthbags can actually be used in many circumstances where other materials would fail, especially if they are filled with materials that will not degrade or distort with moisture, such as gravel, sand, etc. It is possible to incline walls into the bank to help stabilize them. Careful drainage measures must be in place to keep water out of the structure, such as a moisture barrier, French drains, exterior swales, etc.

While tires do not degrade under these conditions, they are very awkward and labor intensive to install, and they inherently do not provide very smooth walls; I would always choose earthbags in preference.

Apparently the underground year-round temperature is about 52 degree F (11 C.) in Amsterdam,  so you would definitely want to employ some passive solar heating into the design to be comfortable without high energy inputs.

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