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Community and Q&A

New earth sheltered PAHS construction

Laurel Davison | Posted in Plans Review on

I would like all opinions regarding my new home construction project. I have a full 3 days experience learning Sketchup and have managed to put several ideas on paper. Please review my plans…and pick holes in them :). I don’t guarantee to change my mind, but I do promise to research varying opinions thoroughly and supporting data would be appreciated.

Assumptions:
Mixed humid climate Missouri 39 deg latititude
CoMo HDD 5173
CDD 396
Sunny days 195
Humidity ave 70% 83% AM, 56% PM
111 days rain for 46″/yr
10 days snow for 14″/yr

Shell: 28’x28′ Terra-dome module x 3 living + 1 garage
Southern unimpeded exposure, gentle slope estimated 5%, Soils Keswick and Leonard silt loam, minimal rocks, but I ran into a thin line of shale when drilling homes for fence posts a few years ago. Goal-one or less ceiling fenestrations per dome module.

HVAC: Passive solar for primary heat source, 19 3×5 windows (12 fixed, 7 casement, u<.3, SHGC>.5) 1.5 feet under 3 foot overhang for summer shading. Mini-split (one or two?) for back up heat/cooling. Anticipate needing external heatsource for approx 2 years while PAHS envelop heats up.
Dehumidification: needs lots of help here: Again 2 years standalone dehumidifier while concrete shell dries more thoroughly, minimal external exposure to outside moisture due to PAHS umbrella. Goal to manage open windows/passive ventilation to lower humidity seasons/times of day.
EAHX (earthtubes) with spot exhaust mandatory at stove/bathrooms, need additional passive stack ventilation or can an unused power exhaust be ‘leaky enough’ to stack ventilate?

Super low energy use electrical appliances with goal to solar addition after 1-2 years of data collection. DC wiring above waist high, capturing DC ambient lights, task lights, circulating fan, maybe $3000 DC fridge, with AC wiring below waist for plug in appliances. DC floor lamps are hard to find!

Air heat pump waterheater because SO already has one, and the cool air release can help freezer and fridge be more efficient, swapping heat with the WH.

So please chime in on my project. I have an interested builder, land, $ and a quiet winter to get all the details addressed.

Thanks,

LTD

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Replies

  1. Laurel Davison | | #1

    OK, export to pictures will be unsatisfactory, Try to access the Sketchup here:
    https://sites.google.com/site/terradomehouseproject/home/sketchup-houseplan-11-27-16

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Laurel,
    I hate to be the bearer of bad tidings, but I am giving it to you straight: you have decided to adopt a discredited approach to residential construction.

    The design approach you are discussing (the passive annual heat storage approach, which is abbreviated PAHS) has passionate advocates but little data to back up its claims. In those respects, it resembles the earthship approach.

    We know how to build energy-efficient houses. The physical parameters for good designs are known. You need a compact shape. You need to avoid excessive glazing area. You need to reduce air leaks in the building envelope to the bare minimum. And you need a high R-value in your ceilings, walls, and floors. That's basically it.

    You don't need many cubic meters of soil around your house, nor do you need to install buried insulation to isolate this soil from other soil. Sorry, but it's true. The PAHS approach requires large investments in measures that are irrelevant to your home's energy performance -- expensive measures with no energy yield.

  3. Laurel Davison | | #3

    Martin, thank you for the response. Do have data to back up your conclusions? My structure is not intended to be an earthship, having more thermal mass, And I did not state my additional goal of minimal to none exterior maintenance costs which could not be satisfied by the wonderful above ground supertight houses available today, ideal for suburban lots and traditional families spending most time outside the home. Another goal is tornado protection which is a fair threat in Missouri (and I used to live in KS...I get rather twitchy during tornado season).

    I very much am interested in data supporting the measures that provide no energy yield.

    Thanks again,

    LTD

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Laurel,
    1. I understand that you are not building an earthship. My point is that PAHS advocates have the same fervor as earthship advocates. It was a social observation.

    2. If you don't want to maintain siding, it's possible to build an underground house -- but the cost of the waterproofing details to build an underground house are likely to exceed the cost of siding maintenance.

    3. If you want to build an underground house, you should insulate the walls and roof of your house, instead of locating the insulation a long ways away from the house as is typically shown in PAHS illustrations. (See illustration below).

    4. If you want tornado protection, using insulated concrete forms (ICFs) is a good choice for wall construction. An ICF wall doesn't have to be underground; you can build a tornado-resistant ICF home that is above grade.

    .

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Laurel,
    There are so many aspects of the PAHS approach that make no sense that it's hard to list them. However, it's probably worth warning you that many homes with earth tubes have abandoned and sealed the earth tubes because of problems with air quality and mold. More information here: Do Earth Tubes Make Any Sense?

  6. Brendan Albano | | #6

    If you're really keen on annual heat storage and want to investigate something really wild, check out the German "Eisspeicher" technique which uses a huge underground watertank as a heat sink. Over the heating season, a heat pump heats the home while freezing the water. In the cooling season, the heat pump cools the home while melting the ice. The energy stored in the phase change from water to ice is humongous (much much greater than the amount of energy that can be stored in a chunk of earth).

    Whether or not this is a "good idea" is a totally different question. But if you google image search "eisspeicher" you can see some pretty impressive pictures of giant frozen tanks and coils!

    There's not a lot of English info on this idea (and I don't read German), but there is this paper: "HEAT PUMP, SOLAR ENERGY AND ICE STORAGE SYSTEMS -
    MODELLING AND SEASONAL PERFORMANCE
    Christian Winteler, et al" https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&ved=0ahUKEwjgo9Pbsc7QAhXHbiYKHQmnA6EQFgg-MAk&url=http%3A%2F%2Fwww.fhnw.ch%2Fhabg%2Fiebau%2Fdokumente-1%2Fpublikationen%2Ffachpublikationen%2F2014%2F2014_Winteler_IEAHPC2014_IceStorage.pdf&usg=AFQjCNFIkaKY4pP_6swlsn6Uo2NcvZ-Qvg&cad=rja

    If you don't have a German PHD on hand to implement the eisspeicher (ice storage) system, Martin's advice is probably a much surer route to a high performance home! ;)

  7. User avatar
    Dana Dorsett | | #7

    Only a few in the US have taken the Eispeicher technique to heart and run with it, including the low-temp solar thermal for wintertime re-charge, but there are some eg:

    http://www.thermalbatterysystems.com/featured-systems/laramie-wyoming-thermal-battery-system-example/#.WD3i1_nR-Hs

    The thermal heat of fusion of water is about 144 BTU per lb, which gives the tank a 144x larger "apparent thermal mass" than it would be as liquid water, with it's specific heat of 1 BTU/lb per degree-F. But once the tank is fully frozen the specific heat of ice is barely half that of liquid water, and the storage temperature plummets FAST. It's important to design the system to keep at least some of the tank liquid at all times. The amount of heat you get from the soil for those purposes is pretty small, given the small amount of heat exchange surface area of the tank, and the R value of the soil. That's why low-temp solar-thermal is critical to keeping the tank size bounded.

    The solar thermal efficiency of flat plate collectors is pretty high when you're dealing with 30-40F water in the tank, even at an outdoor temp of 0F, and much better when the outdoor temps are above freezing. The COP efficiency of water-to-water heat pumps are also still pretty good a 32F tank temp, and typical radiant floor (or tepid-air air handler) temps used in ground source heat pump systems.

    It's quite the Rube Goldberg contraption, and can be fairly expensive to design, implement & maintain compared to say, rooftop PV and an air source mini-split heat pump.

  8. Bruce Lepper | | #8

    Laurel,

    For some reason Martin Holladay regularly discredits (his word) the PAHS idea and adds "social observations", but he's right that there's little data available and that earth tubes can be problematic. Many leave them out. I am building a PAHS house (www.blepperblog.wordpress.com), but only because I happened on a Virginian who's been living in one for years and is, to say the least, happy with the experience. He says it's the most comfortable house he's ever lived in, and his neighbor, who ordered a copy from him, says the same.

    BCE (see BCEweb.com) is the largest Structural Engineering firm in Montana. It was founded as Beaudette Consulting Engineers, Inc. in 1990 by Tom Beaudette, one of the designers of the first PAHS house. He later constructed a PAHS house for himself, you can see an image of it in Hait's book. In 2013 I asked him how it turned out. His reply:

    "We lived in our PAHS home for 20 years. It performed as expected. With growing family, we ended up bumping up around half of the house above grade with a super insulated 2nd level. We sold in 2005."

    Here on GBA (http://www.greenbuildingadvisor.com/homes/passive-solar-home-1980s#ixzz4AMmjVNB0)
    Joe LaCour describes a PAHS conversion he did on a 1970's solar home in the Pacific NorthWest. He comments:

    "The results after 5 years are impressive. We only have a wood stove for heat, and it only is used when someone is cold, which is usually when we have a few days of overcast and it is cold. The inside temperature is heavily moderated by the thermal mass, with the concrete structure staying in the mid to high 60's. The air temperature fluctuates from low 60's to hi 70's, depending on the sun, season, outside temperature, etc. In the winter when the outside temp is in the the 30's it is quite warm on any sunny day. In the summer, little sun comes in so we have no need for AC. Yes, it all really works and quite well at that. It is so cool when guests arrive on a cold evening, walk in, and are sure we have a heater on."

    There are many other isolated examples, but as Martin would point out, it tends to be anecdotal. I'm hoping to add a little data with the buried temperature and humidity probes around our house, but that will be in the future.

    Fire away Martin :)

  9. User avatar GBA Editor
    Martin Holladay | | #9

    Bruce,
    If you want to build a PAHS house, you are of course free to do so. You're probably aware that the cost of this approach is much higher than other forms of construction, because (a) it's an underground house, which means you'll need lots of concrete, (b) it's an underground house, which means you'll need expensive waterproofing details, and (c) you'll need more rigid foam than usual, because you are trying to insulate the whole neighborhood, and (d) you'll have to spend much more for excavation and earth-moving than is required for the typical single-family house.

    If you have a fat enough budget to cover these expenses, and you trust the veracity of the anecdotes you've heard about PAHS comfort, go right ahead.

    Readers of GBA who are thinking about different approaches to energy-efficient construction, however, deserve an analysis that considers whether these added expenses yield any energy savings. In my opinion, they don't.

    One person who you quote said, "We only have a wood stove for heat, and it only is used when someone is cold." Well, that applies to any house with a wood stove.

    The testimony continued, "In the winter when the outside temp is in the 30's, it is quite warm [indoors] on any sunny day." That's true for almost any house with south-facing windows, including my own (very leaky) home. A sunny day in the 30s is nothing.

    Anyway, I don't doubt that these homes are comfortable. Lots of homes are comfortable.

    We know how to create a home that uses very little energy for space heating. Make it as airtight as possible. Create a thermal envelope with above-code levels of insulation. Choose high-performance windows.

    I don't think that physics can explain why it's necessary to raise the temperature of the dirt around your house. It isn't.

  10. Bruce Lepper | | #10

    Martin, I'll try to comment on your assertions in more or less the same order as you made them.

    You are wrong with your sweeping statement about PAHS houses being "much more expensive than other forms of construction". That's like saying that rammed earth houses are expensive. They are if you have them built by a professional in California.

    You are wrong about PAHS houses needing lots of concrete because they are underground, for similar reasons. PAHS houses can be bermed, they don't have to be underground. Mine is on a south-facing slope, and digging out the hole for it was certainly no more expensive than pouring lots of concrete to make the foundations for an above-ground house on a slope. By the way, I chose rammed earth infilling between concrete pillars and beams and no slab, so concrete use has been minimal.

    Your "underground house" belief thus leads to your next mistake about extra waterproofing costs. A bermed house needs no more waterproofing than the average under-house garage or cellar. And in the case of a bermed PAHS house it doesn't really need any waterproofing at all, because the dirt surrounding the house (usually on three sides) is under a waterproof, insulated "umbrella". The umbrella even permits below-grade rammed earth walls.

    Your next assertion, that "you'll need more rigid foam because you're trying to heat the whole neighborhood" makes me wonder if you are really interested in a serious discussion about PAHS (which is what Laurel was asking for). This remark, and your vague reference to "physics", seem to indicate that, despite the evidence from homeowners, you think heat cannot be stored in the vicinity of the house and is just going to dissipate around town. If that was the case those homeowners would be shivering for six months of the year. Why do they say their house is the most comfortable one they've ever owned?

    Finally, landscaping costs. When you create a flat area in a hillside you get extra dirt piled all around the house and a terrace in front, without even trying. It's there waiting to be bermed. Not a big job, and perfect for PAHS.

    Well, I guess I'll find out for myself in a year or two, but I find the idea of lots of mass and lots of sun more appealing than sticks and lots of insulation. I'm also expecting natural moisture-control from those rammed earth walls.

    Is Hait a crackpot, or could he be the genius of the insulated umbrella?

  11. User avatar GBA Editor
    Martin Holladay | | #11

    Bruce,
    If you have found techniques to lower your construction costs -- by using rammed-earth walls between pillars and no slab -- I salute you. It's great when owner-builders come up with innovative methods using unusual materials like earth to build a house. I've built houses using similar techniques and materials (dirt-floored cellars, cellar walls built from stones found on the building site, joists made from logs cut on the property and squared by hand). These are great ways to lower the cost of building.

    That said, I stand by my statement that for most homeowners, the construction cost of a PAHS house will be significantly higher than a conventional house -- because these homes are partially or entirely underground.

    I've already conceded that these homes are comfortable. But as I noted in my last comment, lots of homes are comfortable.

    The main question concerns whether rigid foam insulation should be installed contiguous with the below-grade walls and roof, or whether the rigid foam insulation should instead be installed a long ways away from the house, in order to insulate tons of dirt near the house. I stand by my statement that it makes more sense (using physics as my guide) to locate the insulation adjacent to the walls and roof of the house.

  12. Malcolm Taylor | | #12

    Bruce,
    It's impossible to discuss of the comparative costs of various types of construction if you include DIY labour. There are simply too many variables.

    Rammed-earth is a perfect example. Many commercial rammed-earth builders avoid the high labour costs by running seminars which effectively use students as their crews, or enlist their clients to help. So yes, they are expensive "if you get them built by a professional" - and the same distinction is true of every other method if construction.

    One of the real benefits of the recent interest in building science has been the application of measurement to the field, displacing anecdotal knowledge or subjective experience. If PAHS is going to make claims as to its efficiency, it needs to be able to show real tangible results - and maybe in the future it will. But for now relying on the word of those who have built them doesn't provide much comfort for those thinking of following their example.

  13. Bruce Lepper | | #13

    Malcolm,

    I entirely agree with that, like me you have an open mind on this technique. Our house is an experiment, is equipped with temperature sensors under the umbrella, and we may be the first to declare that Hait's idea doesn't work for us. That's OK. We are using significantly more glass than Hait and a bigger umbrella (6m - 18ft), both of which he recommended, but we are not underground and are relying on extra above-ceiling insulation to compensate. Natural heat transfer is very, very slow so transfer area is important. Our floor will be our biggest heat source, but we have designed the house with air movement in mind. The entire back wall has unobstructed air access to the main living-study-kitchen glassed area.

    Martin,

    The physics as I see it is basically the same as a Trombe wall. Hait's umbrella creates a big seasonal Trombe wall around the house, slowly absorbing summer warmth through the walls and slowly releasing it back in winter. He used sensors and found that the soil temperature around the experimental house stabilised in the high sixties.

    I can understand that you can't see an immediate commercial interest in such a construction. We've used a lot of recycled materials such as used pvc swimming pool covers and polystyrene blocks from exhibitions so our costs are extremely low apart from the double-glazing, which we have a lot of. But what is interesting is the physics that you keep mentioning. What, precisely, is the problem with the physics?

    One last point. When you have lots of mass, as you know, you get lots of benefits that don't come with lots of insulation. That's the main reason we're building a half-buried house with rammed earth walls. Which, without Hait's umbrella, would be a more risky business.

  14. User avatar GBA Editor
    Martin Holladay | | #14

    Bruce,
    It all comes down to a cost-benefit analysis.

    Underground houses cost more to build, but they can have lower space heating costs than above-ground houses (because soil is usually warmer than outdoor air for most of the winter). Needless to say, insulation is still an essential part of any house. In most cases, the savings in energy bills attributable to building underground aren't enough to justify the higher construction cost.

    If you have below-grade space, insulating the soil with a large umbrella of insulation will, indeed, raise the soil temperature in winter, further lowering your space heating costs. Again, the important calculation is a cost-benefit analysis: will the drop in energy costs be enough to justify the very large capital cost of installing the umbrella? To my mind, the answer is no.

    Lots of people have looked at the advantages of thermal mass. (By the way, no one builds Trombe walls anymore.) These analyses show that the advantages of thermal mass can be achieved at lower cost by increasing insulation levels. In other words, a low-mass house with a high-R envelope beats a high-mass house with a low-R envelope every time. And it's cheaper to build

  15. Laurel Davison | | #15

    Thank you Bruce for reigniting the post. I assumed I would get no further response after the pessimism of the board leader. I am sold on the idea, now I need technical help!

    I WILL BUILD A PAHS UNDERGROUND HOUSE.
    And request the engineer/architect/forward thinkers in this board to critique my plans and thought processes.

    Thank you Brenda and Dana for your posts regarding the Eisspeicher technique. However one of my main goals is passive, low input, low maintenance and reduction of "systems".

    Thank you to Martin for your critique. I would like to detail my plans for the evaluation of the readers for further discussion. " (b) it's an underground house, which means you'll need expensive waterproofing details, and (c) you'll need more rigid foam than usual, because you are trying to insulate the whole neighborhood," Terradome comes in dome-shaped modules with valleys between. Typically drain pipe is laid in these valleys to collect and relocate rainwater, on top of the insulation/waterproofing layer (membrane for brevity hereafter). My intention to to put a "minimum" of waterproofing over the concrete, lay the drain pipes to effluent directly out on the patio below signalling failure of overlying membrane, Then I plan to even out the dirt above so that following the slope of the valleys does not require time and materials but is one smooth near flat area to lay the "real" membrane only sloping off at the sides (and front, with an overlying drainpipe draining away to the sides of the house unseen). Hopefully this decreases the "detailing" and only minimally increases the sqft required. Please debate: HOW THICK AND HOW FAR to extend the membrane. Terradome builder says 4inches I think, or that is what my ears rounded up to :).

    "(d) you'll have to spend much more for excavation and earth-moving than is required for the typical single-family house." I am also thinking of putting in a pond in the front yard, so increased dirt moving is not a barrier. That project can absorb or contribute earth as needed. Looked into pond loops to replace the minisplit but that is not consistent with the low input, low maintenance goals. And I do not intend this house to be a "typical single-family house". It will satisfy many more requirements we have.

    The house will be partially off grid. Meaning, lights/fans/fridge/freezer and everything else I can rewire will be DC fed from batteries fed by PV panels just behind the house. High draw, intermittent use items will continue to be grid AC until technologies catch up. Missouri netmetering program is not all it is cracked up to be and I do not wish to participate.

    I have been in contact with a delightful 3rd year civil engineering student from Missouri S&T who may be helping me with ventilation. I feel ventilation and excess humidity are the largest hurdles presently. I have designed the floor plan to have exhaust air exit the house toward the back near humidity sources and enter the house in living areas. Will exhaust air be sufficient to approach ASHRAE standards? Do I place extra "holes" in the roof for a passive system controlled at the inlet source based on outdoor humidity levels? PLEASE DEBATE THESE QUESTIONS AS WELL.

    Thank you, contributors for your time and expertise.

  16. Brendan Albano | | #16

    It seems like you may be looking for advice in the wrong places in some ways.

    If you want to build a PAHS underground house, it makes sense to seek advice from people who have built many PAHS underground houses. On this thread, the question "should you build an underground house" is being hotly debated. But it sounds like that's not your question. Your question is "If I want to build an underground house, what's the best way to do it".

    If you are dead set on building a PAHS home, you need to talk to people who have done it before, who have done it many times, and who have learned from their mistakes in the process!

    For example, your question about "how thick and how far to extend the membrane". Your Terradome builder says 4 inches. If you're going to use the Terradome product, it makes sense to me to get your Terradome-specific detailing advice from Terradome!

    Again, you are working with a 3rd year civil engineering student for help with ventilation. Why not seek out an MEP engineer or contractor or builder with experience building and ventilating underground homes? I'm sure there is a lot of knowledge about these fairly unique homes that is not contained in a standard university engineering textbook, simply because they are not a standard method of construction! The student will be great at the physics and the principles, but might not have the experience you need.

    Have you read Malcolm Wells' books on earth-sheltered construction? You probably should! http://www.malcolmwells.com/books.html Maybe you can search out some of his disciples to get advice from regarding ventilation and humidity issues.

    And just to be clear, I'm not trying to discourage you from posting here, just pointing out to be cautious about the information you receive if you are asking for experts in one subject (standard above-ground energy efficient building, and off-grid living in these sorts of buildings) for advice about a related but different subject (below-ground PAHS energy efficient building and off-grid living in those sorts of buildlings).

    And just to be clear, my biases would be to build a standard insulated home for efficiency purposes. And if you want to have it be underground for aesthetic purposes, just put it underground! Even if it's just a big pile of dirt, PAHS is still a complex system. I would argue that if you're after simplicity, a well insulated wall and a wood stove is much simpler than PAHS. But if you want to build a PAHS home, definitely seek out advice from people with the expertise!

    One last thought. In theory, a PAHS system is most optimal if you have a relatively similar number of HDD and CDD no? If you're heating way more than you're cooling, or vice versa, your big thermal battery isn't being used optimally to transfer heat from the summer to the winter.

  17. Malcolm Taylor | | #17

    Laurel,

    Brendan has given you excellent advice.

  18. Bruce Lepper | | #18

    Martin,

    OK, so let's say there's no problem with the physics.

    On another subject you say:

    "Lots of people have looked at the advantages of thermal mass. ..These analyses show that the advantages of thermal mass can be achieved at lower cost by increasing insulation levels. In other words, a low-mass house with a high-R envelope beats a high-mass house with a low-R envelope every time. And it's cheaper to build."

    No, thermal mass and insulation are two different animals, you can't compare them that way. The first slowly absorbs heat and gives it back as the ambient temperature cools. The other stops or reduces heat transfer and that's it. Comparing the two as you have done makes no sense at all. Nobody would build a high-mass house with a low-R envelope, just as nobody would build a low-mass house with a low-R envelope.

    The major problem facing low-mass houses is overheating, which is why the passive house organisation recommends opening windows or otherwise venting at night in hot weather if overheating occurs. High mass houses have natural qualities that help alleviate this problem.

    Low-mass heavily insulated houses might (or might not - too many variables) be cheaper to build. But I don't think Laurel asked for advice about costs. He's looking for technical information. As far as that is concerned, Brendan has given good advice, but I sympathise with Laurel for asking here and elsewhere because getting info isn't easy. There is no centralized info on the subject, and Hait's own site about PAHS is long gone. Hait's fairly detailed book is available on Amazon, but it dates from the 1980's and the new edition has unfortunately not added much in the way of feedback from experience. He might try asking on the Underground House list ( http://groups.yahoo.com/group/UndergroundHouse/), it's been pretty inactive recently but I know some PAHS people are on it.

  19. User avatar
    Stephen Sheehy | | #19

    One thing about PAHS is that I suspect such houses, once complete, are probably not saleable at a price that would recoup much of the significant cost of construction.
    So long as one is either rich enough not to care or old enough to be sure this is the last house, resale isn't important.

    Our pretty good house is not terribly unusual, other than its small size. But we have no illusions that we will ever come close to recovering our costs. But we're in our late 60s and built the house to allow us to age in place and we don't expect to ever want to sell it. An underground house is far more unusual and appealing only to a tiny slice of the population.

  20. Malcolm Taylor | | #20

    PAHS, and similar niche approaches, have very limited applicability to the North American housing stock. Whether they turn out to perform as their proponent's wish won't significantly affect the rate at which they are adopted, because they are suited to such a small number of potential projects.

    They are a rural, or at best large-lot suburban phenomenon unsuited to urban or multi-unit construction - which constitutes the overwhelming majority of housing built.

    They also use techniques outside the established construction used by builders, and the end product isn't one familiar to the tastes marketers and culture have instilled in home buyers over time.

    That doesn't mean individuals shouldn't build one if they want to. There are all sorts of alternate types of construction, like log houses, rammed-earth, straw bale etc, that are easy to dismiss because of their lack of general adaptability which make their individual occupants very happy.

    The real elephant in the room is whether, as Martin intimated, they are more akin to double-envelope houses, where the main reason for building them that way turned out to be wrong. I guess time will tell.

  21. Charlie Sullivan | | #21

    With the obligatory discussion about why this is unlikely to be a good solution for most homebuilding out of the way, maybe we should attempt to address more specific questions. The original question said "please chime in" and that invited lots of criticism of the fundamental concept. But it also asked for specific guidance on dehumidification.

    Really, this is the main engineering challenge, because, to the extent that the thermal mass provides seasonal thermal storage and thus the space is cooled by losing heat into the thermal mass during the summer, that cooling can't provide any dehumidification--if it does that means the walls and floor are getting wet and the space will be a better habitat for mold than for humans. So it's essential to provide dehumidification in the summer for the life of the structure, not just to dry the concrete after construction.

    At the risk of repeating myself, to the challenge is this: Even if the outdoor humidity is moderate in the summer, if the space inside is cooler, just supplying ventilation air from outside will result in higher relative humidity indoors. That's before you have any people inside breathing, cooking, and taking showers.

    One concept for how to approach this is to perform the dehumidification on the incoming ventilation air. If that air starts out with higher absolute humidity than the inside air, you can extract a pint of water from it using less energy than you'd need to extract the same pint from the drier inside air. Most such systems are for large commercial buildings, and are called DOASs (dedicated outdoor air systems). But there are some residential versions, sometimes called "magic boxes". http://www.greenbuildingadvisor.com/articles/dept/musings/another-north-american-magic-box
    They claim to do everything well, a claim that doesn't hold up to scrutiny too well. But dehumidifying summer ventilation air is one thing they should do quite well, so they might be worth considering here.

    If you like systems that are elegant in their use of energy, outside the mainstream, theoretically inexpensive (but actually expensive now because they aren't mainstream), you might want to use a desiccant dehumidifier than can be run from solar heat. That's theoretically a good match to the availability of solar heat in the summer, which you need the most dehumidification. Here's the only such product I know of made for a residential-size system:
    http://www.greenbuildingadvisor.com/product-guide/prod/novelaire-comfortdry-250-dehumidifiers

  22. Bruce Lepper | | #22

    Charlie,

    As I mentioned above, Hait's umbrella provides a relatively dry environment which permits the use of rammed earth walls in an underground or bermed house (given a suitable site with a low enough water-table, of course). This opens up exciting new possibilities, as earth walls can apparently act as a natural dehumidifier to some extent.

    Here are a few lines on the subject from Gernot Minke's excellent 2012 book Building with Earth:

    "Loam is able to absorb and desorb humidity faster and to a greater extent than any other building material, enabling it to balance indoor climate. Experiments at the Forschungslabor für Experimentelles Bauen (Building Research Laboratory, or BRL) at the University of Kassel, Germany, demonstrated that when the relative humidity in a room was raised suddenly from 50% to 80%, unbaked bricks were able, in a two day period, to absorb 30 times more humidity than baked bricks. Even when standing in a climatic chamber at 95% humidity for six months, adobes do not become wet or lose their stability; nor do they exceed their equilibrium moisture content, which is about 5% to 7% by weight. (The maximum humidity a dry material can absorb is called its “equilibrium moisture content”).
    Measurements taken in a newly built house in Germany, all of whose interior and exterior walls are from earth, over a period of eight years, showed that the relative humidity in this house was a nearly constant 50% throughout the year. It fluctuated by only 5% to 10%, thereby producing healthy living conditions with reduced humidity in summer and elevated humidity in winter."

  23. Charlie Sullivan | | #23

    Bruce,

    It seems reasonable to expect that earth walls or earth in contact with concrete provides substantial moisture buffering that could help maintain relatively constant humidity over the year, assuming that liquid water is prevented from getting into that soil. The quote you provide doesn't seem to be able to make up its mind, however. It claims as advantages both substantial moisture buffering, and limited moisture uptake (did not exceed ... 5-7%). If you want moisture buffering, you want it to absorb moisture (and later release it).

    If the soil participates substantially in the moisture buffering, the need for dehumidification in the first few years will be increased compared to a design that had a vapor barrier between the soil and concrete walls. Presumably that would be worth it in terms of energy requirements due to the soil helping control humidity in future years and reducing the need for dehumidification later.

    I suspect that in the Missouri climate, which has more humid summers than Germany, the rate of moisture coming in on a hot humid day would be more than could be absorbed by the walls without the surface becoming wet enough to support mold growth. Perhaps there are special surface treatments that would wick the moisture in as fast as it arrives, keeping the surface dry. But I think it would be risky to count on that--I would recommend having the capability to dehumidify the incoming ventilation air to a level to keep the surfaces of the concrete walls dry on the worst hot-humid day, and with the assumption that the walls are nice and cool, as is the design intent.

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