Subterranean Cob?

Hey all. I'm new to permaculture and natural building, so I'm trying to get as much information as I can as I begin planning my next phase of life, in which I hope to obtain my own land to start heading to full sustainability. In doing this research I am planning on trying to attend several workshops in my area, but I have a question I hope someone on here can answer.

Can cob homes be covered with earth?

This may sound like a stupid question on the surface, but here's my reasoning; All the cob I've seen is made with some sort of a roof to keep the rainwater washed well away from the structure to ensure it stands the test of time and natures erosion patterns.

So what if there is no actual roof? What if you want to bury the house in a hillside and cover it with a foot or more of soil and grass? Can a cob home support that weight even if the roof is supported with strong timber? What do you have to do to seal it should rainwater seep down?

Why do I want to know this?

Because aside from being a self reliant person, I am an unashamed Tolkien fan........ You do the math

So if anyone has any ideas or helpful information, please share! I look forward to reading what ideas or knowledge you can drop on me!

Phil Smith : Welcome to Permies, with ~25,000~ Fellow members world wide you can be sure of coming here and finding someone who wants to talk about
what you want to talk about, generally 24 /7. You will find that they will bring different ideas to the discussion and will stretch your mind as you will stretch
theirs !

A little housekeeping and we will move on ! Location, Location, Location ! Look at your 'Posters Name space', and L@@K at mine ! When you tell us where
you are generally located, altitude, Climate zone and average rainfall you allow us to better help you with answers more likely to be useful for your location!

At the top of this page Between the Permies Ad Banner and the Permies Video of the Day/week is the Permies toolbox ! Find and click on the 'My Profile '
Button and you will be coached through setting up a profile to share ! This may very well help you find a Fellow Member and near neighbor with experience
in the areas you are interested in !

To attempt to answer your question - Yes, But ! The ideal underground home Has an Umbrella like Zone of warm and DRY Earth immediately surrounding
it, and for say 20 feet out from the structures exterior walls ! This is Then capped with a permanent heavy-duty waterproof membrane ! With careful
landscaping details to keep all water away from the perimeter, this is essential to allow the earth to act as a large positive Thermal mass and not a Negative
heat sink !

Some care must be taken to not introduce more water vapor into this envelope from the Cooking, cleaning, and bathing, then the Cob itself can hold and cycle!

Great care must be given to building in an area with excellent drainage, improving on that,and a having an understanding of what you can receive in the way of
weather from a 100 year event - remembering that old saw, Climate is what you expect, weather is what you get !

Some great sources to check for more information Would be the books by Rob Roy, and The ' 50 dollars and up Underground house' book by Mike Oehler!
Underground housing.com

From the Permies tool box you can also do a Permies wide search for Underground housing Passive solar, and membrane roofs, !

Also when you create a new topic our Permies Computer looks for 'Key Words " and posts a listing of "Similar Threads " That will appear at the bottom of this
topic And the bottom of every new topic of all of the many Permies threads ! Happy hunting, For the Good of the Crafts ! Big AL !

Thanks! I thought I had updated all that stuff but I guess it didn't take! I'm in West Tennessee along the Cumberland River.

Yes, I was wondering how all that worked! I have that book on PDF but I guess I have a hard time figuring out what kinds of membranes would work best for the design I have in mind! I'm still very new to all this and looking to get all the info possible that I can Thanks so much for the tips I'll keep scouring and maybe by the end of it all be a little wiser for it!

Phill Smith : Many people find that they can get the heavy-duty big old Billboard signs when they get replaced at the end of the contract,
most are in near perfect condition And are quite suitable Larger sizes and the ones on the side of major highways are already huge- can
be made up by joining up sections with a heat gun! For the good of the Craft ! Think like fire, Flow like a gas, Don't be the Marshmallow-

As always your comments, questions are solicited and welcome !

hi phil
i just wanted to say hi and say i understand what you are looking to do and if you need help in the future i am right in your back yard willing to help just for the fun of it

some problems i can see just looking for someone only doing allot of study on the subject of cob would be how much humidity we have hear could be a problem being underground there are others hear in nashville area that are building with cob (might be a good group to contact about how cob in that form would work in this area) they are in the permies site might want to look for them and contact them i can see a possible problem with cob working the way it should with needing to breath

is it strong enough ? might be the way it bind togeather from what i can see how the staw holds it all togeather its very strong but the problem there could be the undergroud factor again if it got wet in any way it would basicly turn back to mud and would fall back onto itself

maybe and i only say maybe as a infill with a strong frame to hold it all up could the cob hold up would be my guess

i would say you would have to be 100% possitive no moisture would ever get to it

now all of this info is from someone only doing allot of study on the subject no real world experiance at all

i know for a fact because others have built cob home's in our area and are living in them that it can be done above ground with a great hat (roof) and shoes (foundation)

please keep talking about it with others i would be very interested about more info on this subject and have been looking for other wanting to build in a natural way like this would for sure love to help eather way and make a connection with someone for when i am looking to build something in a natural way like this also i am looking for land to start with so when i get to that point i will be building something most likely cob infill something along that line

hope the best on your path to hobbiton may your doors be round and your feet large

ken

Welcome to the fray Phil! I share your admiration of shire architecture and find myself gravitating towards bermed walls and living roofs. While I know it can work just fine for the right projects, I regularly question Cob's feasibility in our Mixed-Humid climate above grade, mainly because I dont feel it offers enough R value.

The nice thing about building below grade is that wall R value becomes less important. However, the Delta T concerns are replaced with moisture concerns. As Al said, interior moisture is already a challenge for thermally efficient homes but when moving below grade it comes at you in even greater force from the exterior. I dont really think the "umbrella" techniques have proven themselves yet and think it takes very dry sites for this to even work in theory.

I think any below grade portion of a home or building needs to have a vapor barrier. I dont think cob and vapor barriers play together nicely but I do think a below grade cob wall would need one if it was going to have an average lifespan.

Hi, Everyone

Definite newby here, and I'm happy to land among my fellow lovers of Bag End. I hope you don't mind me jumping in to this thread; I was doing some searches for 'underground cob' as I'm mulling over possible designs for combining the $ 500 house with cob, and this thread showed up in the top ten. The whole site looks totally brilliant!

In England, where I learned to build with cob, green roofs are frequently put on cob buildings. The walls will support any amount of weight, and the maritime climate is perfect for growth. They use a lot of thatch, too, but the trend is toward living roofs for cob.

I've been thinking about your other question, about using cob walls in an underground house, and it seems to me that it would be feasible following the same technique as with wood. That is, having a layer of plastic or rubber on the outside of the wall, between the wall and the earth. That should keep the moisture from the cob in the same way it's kept from the wood.

With regard to the breathability of the cob, assuming it was allowed to dry before the plastic layer on the outside and the earth backfilling, I can't see why that would be a problem. It won't rot, because moisture will be kept away thanks to the plastic or rubber membrane, and it will breath on the inside side of the wall. At least, that's what I'm thinking right now; if anyone knows better, don't feel shy to say so.

A question for Allen Lumley, above: you say that the ideal underground structure has 20 feet of dry earth outside the walls. How do you get that much of a belt around the entire building to stay covered and dry?

Cheers

Welcome El! I agree with your observations and questions. In regards to the underground cob "breathing" I would adjust the terminology to say that the cob could dry to the inside with the vapor barrier (plastic or rubber) to the exterior of the wall. The warmer and drier the interior air, the better it would stay safe and dry with the right water management details on the other side and beneath of course.

I share your question about achieving dry dirt around the structure in wetter climates. Cant remember what thread we theorized about it before but I believe you would need to isolate the dry dirt from all sides including below. I dont think an "umbrella" above would keep things sufficiently dry in wet climates. It would probably need to be more of a complete sphere or cube of protection. Even then, I think temps are going to average closer to the underground temperatures rather than the human comfort temps without some heating or cooling input.

El Rowlatt : By using Exactly the same techniques as used to make a 'Living roof' Watertight and then extending the area out around the perimeter
to create an umbrella ! Obviously you must extensively use below grade drainage to daylight, and accept the use of plastic membranes !

I can recommend Rob Roy for further Reading on the Subject, His understanding of underground/earth sheltered dwelling is Encyclopedic !

Hope this is helpful and timely For the good of the craft ! Big AL !

Hi everyone. I may be off track a little...sorry, but one bit of information may help. We're also new to Permies and I (Ilene who is Max's wife) will exercise my ability to show my lack of knowledge but desire to learn). Max has done a lot of research on building our own hobbit hole with several methods. What we wanted for sure was a green roof. We recently purchased 34 acres in southern Idaho and learned that in order to do a green roof we need to have any plans signed off by a structural engineer. You may or may not need to have this done but it is causing us grief because we can't locate one who is familiar with green roofs. Still looking. If anyone out there can point us in the right direction we would appreciate it. Good luck in your hobbitous adventure. May your feet be forever hairy.