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Best sealant for underground house walls

GBA Editor | Posted in GBA Pro Help on

We have purchased an underground house; only south side face showing. We have removed 3 feet of dirt from the top and have had a backhoe remove dirt from all three sides so the french drain system can be replaced. What is the best sealant/insulation to put on these three walls? A company doing open cell foam spray tells us that’s the best but I don’t want that on top as I want to keep it for a patio. What would be the best for the top?

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  1. GBA Editor
    Martin Holladay | | #1

    1. Where are you located? In all but the very warmest climates -- for example, Hawaii -- you definitely want insulation on your roof. Listen to your foam contractor.

    2. The three walls can be insulated with extruded polystyrene (XPS), high-density expanded polystyrene (EPS), closed-cell spray polyurethane foam, or rigid rock wool (Roxul). Of these options, the least expensive will be XPS or Roxul.

    3. The roof can be insulated with XPS or closed-cell spray polyurethane foam. If you want a patio on your roof, you can still insulate. I would use XPS. A contractor familiar with these systems should be consulted. Such a roof needs high-quality membrane roofing (either under or over the XPS), a drainage medium (coarse granular material), and then a good sub-base for your patio pavers.

    4. Your walls will need a high-quality waterproofing system in addition to insulation. Don't try to save money on these details. I would use a three-dimensional drainage or dimple mat over peel-and-stick membrane, followed by coarse granular backfill. Be sure that your new footing drains have risers (cleanouts) accessible at grade, and be sure that the system drains to daylight.

    Good luck.

  2. Expert Member
    Michael Maines | | #2

    There is another way to do underground homes. It is far from conventional but the concepts are sound. You can read about it here:


    I'm with Martin on the roof assembly except I would definitely put the waterproofing membrane under the XPS (I like MuleHyde EPDM from ABC supply) and I would cover it with enough layers of extruded polystyrene to achieve R-30 minimum and then cover that with a dimpled drainage mat such as Ameridrain before adding two layers of Hardi-backer tile backer board with offset seams adhered to each other with modified thinset tile adhesive on which I would thin set a 4x8 quarry tile such as American Olean's Quarry Naturals for a wear surface.

    We just completed a 650 sf R-30 roof patio of this construction that will have raised bed planters and a gas grill on it. The handrails and storm water collection (gutters and scuppers) can be challenging from a design perspective. We used a mixture of stucco parapets with black steel decorative railings welded up from a very affordable set of parts from King Architectural Metals to meet code on this one.(Get on their e-mail list, they have 20% off sales every other month) We've also done cool things with pipe railings and with built-in gutters that had outboard railings made of wood. I also like making railings out of galvanized electrical conduit using a 2" pipe for the top rail and 1" for the rest. 20' stock breaks into thirds that will span 6'8" so you need to design around that post placement.

    We are working on a plan in the office now that has a cantalevered raised bed planter (to create an overhang) that is 20" tall with a built-in bench on the patio side and has a 16" planting trellis centered over the planter which is only 14" deep back from the seat. code requires a 36" railing that won't pass a 6" ball between horizontal elements. 20"+16" gets us to code without looking like a skinny railing at the edge of the roof. It's fun being married to my architect.

  4. iMarc | | #4

    Hardie-backer and other brands of cement backer board are not recommended for cold climate exterior applications where it in contact with grade. I had inquired about use of cement backer board as a substrate for Portland cement stucco finish as protection for foam foundation insulation and was told by Hardie that the material would eventually deteriorate due to freeze-thaw cycle unless it was kept up 2" above finish grade. They said it would tolerate approximately 60 freeze-thaw cycles before beginning to deteriorate, so whether or not you can use it as a tile backer outdoors will depend upon your climate.

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