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Wall types for bermed naturhus

itshowtime | Posted in General Questions on

Hello, I am designing an earth sheltered naturhus for my senior project and would appreciate you thoughts. 

Location: Boulder, Colorado, Zone 5, Granular soil type, South facing mountain, Elevation 8,000

The design consists of a half dome that encases living structures, see section cut in attachment. The N, E, & W sides of the living units are surrounded by earth to help regulate temperatures. From what I understand, these wall types will need to be highly insulating and have a thermal break in-between the ground and wall. I saw on your other comments that this Zone type would require a “~27% of the total R to be on the exterior of the sheathing. So if you have 9.5″ of R3.7/inch dense packed cellulose that yields a cavity-fill R of about R35, which means you would need ~ R13 of exterior R to gain that resilience”. Since the entire structure is encase within a glass dome to create a microclimate, how much effect with this alternate climate have on the insulation requirements? Or is it more so determined by the soil temperature? The greenhouse will create a microclimate between ~60-70 degrees. I plan to use rammed earth blocks for the south facing walls/floor to regulate temperatures through thermal mass. I am trying to decide on materials for the N, E, W walls and roof. Do you have any thoughts on this? I am also trying to minimize concrete use as much as possible. Thank you!

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  1. creativedestruction | | #1

    This theoretical glass dome would have a far outsized effect on the energy demand than any insulation decisions made on the below-grade walls. That's a BIG surface area that needs to be translucent and unfortunately also sees the greatest temperature delta to maintain an interior environment of 60 to 70 degrees and likely high humidity.

    The decision to build below-grade usually precludes the use of environmentally responsible, low-carbon materials. Sure, with enough waterproofing countermeasures you can maybe use a cavity wall with cellulose on the interior but in reality most builders wouldn't go there. Too risky. Especially so if the greenhouse high humidity conditions aren't separated from the rest of the living space. Your best bet might be rammed earth, waterproofing and then rigid mineral wool on the exterior. With all exterior insulation the climate zone ratio for condensation control no longer applies. Best of luck on your project.

    1. itshowtime | | #8

      The living spaces are enclosed and kept separate from the south facing garden. Good to know that the exterior insulation no longer applies. Thank you for your response!

  2. Expert Member
    BILL WICHERS | | #2

    The greenhouse dome is going to be biggest energy loser here, so it's thermal performance will dominate the rest of the structure's insulating materials.

    For the below grade walls, you would want to go eith EPS or XPS, with XPS being preferred for longevity. If your target is "greeness", then select one of the newer XPS variants that uses more benign blowing agents. XPS is less prone to taking on water over time compared to EPS, so it maintains it's R value better over time.

    Concrete is really the best option for the below grade portion of the structure since it will hold up better over time. Remember that any failure of a foundation will require major work to repair, so if you want to minimize materials/energy use, you should consider the life of the structure. If it takes twice as much energy to build, but lasts three times as long, you still come out ahead in the end. I like to remind everyone to consider the entire system, and not be overly focused on any one part of it.

    With a greenhouse-style design, if you will have plants in there, make sure to consider the effects of high moisture on all of your materials. Greenhouses will have much higher average humidty levels compared to "regular" residential structures.


    1. itshowtime | | #7

      Yes, I think if the dome is double or triple glazed with appropriate glazing, and the glazing to rammed earth ratio is correct, it could be successful in creating a livable microclimate. I have found new approaches to concrete that I would like to experiment with. Basilisk, Concrene, and Biomason have interesting materials that would reduce energy footprint. I will keep the moisture levels in mind when speccing materials. Thanks Bill!

  3. Expert Member
    ARMANDO COBO | | #3

    I’ve designed many 3-sided walk-out concrete foundations into the mountains of NM, but never with a live roof system and much less a mountain on top and a dome. Most out of the ordinary building systems are not common for the complexities and/or costs associated with those projects.
    There are several options for concrete-less or reduced concrete foundation systems, like composite, treated wood, timber, precast, stone, ICF, insulated panels and papercrete, among others, but I would talk to an structural engineer if any of these foundations are suitable for a project like yours.
    You also have different options for “live or green roof” systems, which are usually a thin “agro-layer”, usually on top of a concrete roof. I supposed you could do a timber and treated lumber/truss roof system, but then again, to support a mountain on top, you should consult with the structural engineer to know your options. The waterproofing and drainage system must be engineered as well. I imagine that with an unlimited budget anything is possible.
    Also, you can install concrete-less floor system as well, but that’s the easy part.
    As Jason and Bill mentioned, moisture and humidity is a big issue as well, and if there is a way to separate the green house to the living house, I would do it. I know some green houses in Alaska with temperatures of 80°-100°F, due to solar radiation, and the Colorado air at 8k feet is pretty thin.
    I’ve designed passive solar houses with and w/o trombe walls and heat reservoirs, and they are very difficult to regulate its temperature. They overheat constantly, one must install a continuous ventilation system to redistribute the heated air around the house, and a house fan or cooling tower that works on a thermostat to cool the house down. AC would be outrageously expensive.
    I’m not trying to discourage you from doing this project. I actually think this is great class assignment, but only if this is a realistic project and not a “design for Jupiter dream house” futile exercise. This a type of project few people will dare to attempt. You go girl! 💪

    1. Expert Member
      BILL WICHERS | | #4

      Earth tubes might be a good way to deal with excess heat here. If a mountain stream is available somewhere above the structure, it may be possible to pipe some of that water into the greenhouse to help to carry away excess heat as well.

      EPDM roof membranes can be used to build a green roof over things other than concrete when needed. As long as the seals are good (or, ideally, if you can build the roof in such a way that each section can be done with a single piece with no seams), EPDM membranes are pretty reliable over time.

      Lots of options here! I agree completely with Armando about the need for a structural engineer. Terracing your living areas against the side of the mountain might help to limit the forces any one part of the structure needs to handle, so that might be something to think about too. I would not try to do the dome clear span either, use some support columns to allow for cheaper support trusses.


      1. itshowtime | | #6

        The earth tubes is an interesting proposition. I have an earth battery to regulate the temperature and humidity in the greenhouse portion. I am using a wind tower for the living units. I will look more into the EPDM membranes. Thank you!!

    2. itshowtime | | #5

      This is very helpful information. It is good to know trombe walls can have a hard time regulating temperature. Yes, the project is a huge undertaking and would require many live simulation tests, structural testing, rapid prototyping, etc.
      The mountain is not on top, the drawing is misleading. That would really be something!

  4. capecodhaus | | #9

    Design it but don't build it.

    Any atypical design in construction will likely cost more upfront as well as in the long run and fail in ways that normal contractors have no clue or interest to repair. Not to mention most folks don't drift far from status quo and likely wouldn't inhabit a bizarre dwelling. Can it be done yes, should something else be done instead probably.

    I always wondered why higher education focused on projects like these for college credit instead of fixing the world these students were about to take part in. The USPS could use fixing, NPR could use a fix, not to mention poverty.

    1. creativedestruction | | #10

      In point of fact most higher education students actually do choose real world problems. Even hypothetical design concepts are seeded by underlying social, political or environmental realities. The unspoken rule of design critique is to debate the merit of the solution, not the choice of problem.

      Higher education is where we "fail fast". It's where we learn basic and advanced conceptual problem solving in a low-risk environment to improve skills while avoiding costly mistakes. We explore the extreme ends of both our whims and also what can even fall under the umbrella of 'design'. But USPS, NPR, poverty...really? Those aren't design problems even by the most liberal definition.

      1. capecodhaus | | #11

        I stand by my statement.

        1. creativedestruction | | #13

          There are university design-build studio courses that build shelter for the homeless. Tell me how that doesn't address a real world problem. The OP describes what is clearly a design in an early-to-mid stage of DD. "Don't build it", it will "cost more upfront" and stick to the "status quo" is vague and unhelpful critique.

  5. Deleted | | #12


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