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.

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:(John MacMillian) 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: 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: 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: 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: 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: 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.