cob/straw bale house plans

I'm in the planning stages for a cob/straw bale house I'm going to build next year. I thought I would post some thoughts here to get some input, since I love reading this forum and I have gotten a lot of ideas from you all!!

Some context:

I live in central NC, and there's plenty of clay around here. The climate is temperate (leaning towards the warmer side). It's also very humid. My parents have six acres they were dong nothing with, so my husband and I moved in with them and started a two acre permaculture forest garden (well, it will be two acres when it is finished). Now we want the eco house to go with it! (We're pretty tired of living with my parents, anyways) The site is at the top of a gentle slope that is a big open, grassy field (the field we are filling in with the forest garden)... I'm really scared of digging down and hitting bedrock, since the property is sort of half floodplain and half rocky outcrop.

I own the $50 and Up Underground House Book and the Cob Builder's Handbook... as well as this old book I found at a used bookstore called Passive Solar Energy written something like 30 years ago. That and the two cob houses I have been in are my source of info of how to build something like this. I thought I was going to have $3,000 to spend on the house... but I just had a baby and long story short... my savings have been depleted more than I thought they would be (thank you insurance company). So now I have to do this house on as little as possible.

I have a friend who recently finished building her own house this year. Here is a link to a big set of photos of the build start to finish (or I guess there is still some plastering left to do, so there may not be "finished" photos yet!):

http://www.flickr.com/photos/danielleackley/sets/72157623078007101/

One of the guys who helped her build this place has taken workshops with Ianto Evans in Oregon... boy am I glad he flew all the way out there so I don't have to!! He is currently building a cob/conventional home hybrid for his parents to live in. He is blogging about it here:

http://cobandon.blogspot.com/

I'm thinking I want to start with just one room.. with a loft space above. I think the loft space will open it up and give the downstairs a big, open feeling even though it's a small space. I also know that roof/foundation are the biggest expenses, and building up takes advantage of that. I'm thinking upstairs can be a nice cozy sleeping loft in the winter, when warm air rises up. But in the summer, I was planing on trying to create a solar chimney effect... is there anything wrong with this plan? Or is this just "strategically placed vents" and not a solar chimney?? See attached photo. Notice my roof slopes downhill... thank you Mike Oehler!

For the roof, I understand pond liner is a huge expense, so I don't want to do a living roof... but that is by far the most attractive option. I don't mind just a tin roof, but will I have to replace it often? Can I put a layer of dirt under the tin roof (for insulation)? Or a layer of straw? Is that a stupid amount of flammability?? I would rather have a living roof for the strength and insulation, if there is an affordable way to do it. How does evaporative cooling work with pond liner? And if it does, how does it not drain the house of heat in the winter? I do have plenty of woods on my land, so I can harvest some trees for the roof.

I want the bale walls to be load bearing, because I don't want the expense of timber/nails/labor (I am no good at woodworking so would have to hire someone). Does this mean I have to have straight walls?

I want to build a rocket stove... is there an advantage to putting the heated cob bench on the south vs. north walls? Can I run the exhaust pipe through a straw bale wall or does it have to be a cob wall? I'm thinking most of my walls will be straw bale since it gets pretty hot here and I don't want the thermal mass of the cob overheating the house in the summer. Or is cob supposed to cool in the summer as well? My friend's cob house is straw bale only on the north wall, and it gets a bit toasty in there this time of year. There is not sufficient overhang though, so I'm not sure what it causing it exactly.

Also, if I have some leftover dirt, I was wanting to make a cob wall sun trap. My understanding is that it's just a cob wall in a cupped shape, facing south. And then you plant stuff in the curve on the south side. Right?

-Giovanna

     Since you are starting out nearly broke it would be wise for you to first deal with all site prep issues such as drainage and any major sculpting of the land surrounding your home. If there's still money left get posts and a roof up.

     It would be a huge mistake to build your walls and exhaust your resources before you have a means to keep it all dry. If the money runs out after the roof is on you will now have as much time as is needed for completion without the danger of rain destroying your efforts. If you can't afford to build a roof to keep your home dry, then you simply can't afford to build right now. Mother nature doesn't really care how much money you have.

Good advice. I will concentrate on site prep and the roof.

Is pond liner the only way to do a living roof?

     Pond liner is EPDM rubber. But it is a more expensive type since it is fish grade. Your cheapest source will be from a large roofing contractor. You may be able to use remnants as long as your roof has some slope. On a 3/12 pitch 1 foot of overlap should be fine. Also if you see a gas station or other flat roofed commercial building being demolished stop in to see if they have rubber roofing.

    This material will also work as a vapor barrier to separate your straw bales from any moisture which wicks up from your foundation. Give the building a good-sized overhang and be sure that rainwater is rooted so that it doesn't splash back against the base of your walls.

    Rubber liners from above ground pools can make a poor man's roof membrane. Not as good but probably free. I would double it up if there's enough.

    The 60 mm stuff is best. Rubber roof membranes are easily damaged. You can protect your membrane by placing recycled carpet against the wood to prevent sliver punctures. Wood that is nice and smooth when new may develop sharp checks over time. Another layer of recycled carpet could be placed over the membrane before the soil is added for additional protection. Be sure that the carpet does not contain any Staples or other sharp materials.

You could always use wooden shingles as your roofing material (with a waterproof layer and insulation underneath). You could make them yourself saving some money and they last up to 40 years depending on the type of wood used.

Thanks for the ideas, Dale.

Sam wrote:
You could always use wooden shingles as your roofing material (with a waterproof layer and insulation underneath).

But wouldn't that waterproof layer also be something I could use for a living roof?

Sure, but a shingle roof is a lot lighter than a living roof which has obvious implications when it comes to design. Just a suggestion

What is a natural solution for insulating a roof, other than soil?

Hi Giovanna
Suggestions for roof insulation...sheep's wool or clay slip n straw
For the latter check out www.evalarevolution.blogspot.ca and review the 'FUDDY' entries.
Sounds like it will be wonderful home.
All the best.
Kate

giovanna.ash wrote:
What is a natural solution for insulating a roof, other than soil?

Katee mentioned wool and clay slip/straw. Others include cork, woodfibre, hemp, hemp/lime, flax, woodwool and cellulose. You could also consider thatch made from reed or straw as this insulates as well as forms the roof itself.

Oh, and you could recycle/reuse artificial/synthetic forms of insulation if you're ok with having that kind of stuff in your house.

    Cellulose insulation which is made from recycled newspaper is a fairly natural product. It's treated with Borax as a fire retardant and to make it unpalatable to vermin. I have given away enough of this to insulate many homes. Check to see if there are any demolitions happening in your area.

    You can also salvage used fiberglass but it is much more prone to vermin damage since they prefer to nest in fiberglass.

  No matter what type of insulation you choose, wear a good mask.

So MUCH to think about at this stage.  I took a class with Ianto and Linda a few years ago, have the hand sculpted house book and earlier this year as I began the cob green house, I was overwhelmed with all these questions.

I finally began, and have learned so much already.  The cob wall is half its finished height, and the materials for the roof were delivered today, rafters mostly, and 4 posts to hold the ridge beam up.

Anyway,

Since you are on a tight budget, I wonder about why you want to make a straw bale/ cob house.  I may be wrong, but I don't think bale walls can be load bearing.  Someone fill me in of this OK?  In any case, the straw is going to cost you, where cob is cheaper, and can be load bearing.

You should be plenty cool in a cob house, if you vent it as you are planning, and have overhangs that shade the walls.  This time of year, late summer going into fall is tricky in a passie solar house.  In the spring, we WANT the sun shining in to help drive the winter chill away, but as the fall approaches, the sun is at the same angle as in the spring, so there is a trade off to be made, a choice that needs to be considered, more solar gain in the spring or more shade from the sun in the last heat of summer.  ONe thing I've heard of people doing is making the overhangs wider than needed and trimming them later, as they see how they want to do it.

If you create a current which rises through the house, you could have something shady green and leafy that cools the breeze before it enters the house to carry heat away.

In the climate where I grew up, people had the habit of opening up the house in the evening to let the days heat out, then in the morning, closing windows and keeping the cool in for the day, but I don't know if your climate cools down a t night, os that might not work for you.

I like the idea of starting with the site prep and getting the roof up, then getting to the rest of it. 

You've talked about all kinds of roof options.  Keep in mind the weight of the roof, and be sure to build something capable of holding up what ever kind of roof you choose.

Though it is a mixed blessing, you could get some help with the cob mixing by becoming a wwoof host farm.  I have to say, I am still developing my screening techniques and questions, but you do get some good hard workers and some interesting people.

good luck with it all. 

Thekla

Sam wrote:
Katee mentioned wool and clay slip/straw. Others include cork, woodfibre, hemp, hemp/lime, flax, woodwool and cellulose. You could also consider thatch made from reed or straw as this insulates as well as forms the roof itself.

Oh, and you could recycle/reuse artificial/synthetic forms of insulation if you're ok with having that kind of stuff in your house.

I've actually been looking at green roofs recently as part of my MSc and I've kind of had a mini-revelation... In fact, I think I'd choose a green roof over any other form of roofing (assuming it's not inappropriate for whatever reason)! There's just so many different functions that a green roof performs... Just avoid using sedum, which green roofing companies are keen to push, as the 'green'.

about the overhangs,  you can build the overhang to suit your early spring heating needs, and have an extension at the edge of the overhang.  The extension would be to support a vine such as passionfruit or hops, or anything else that grows vigorously through out the season, but dies to the ground in the winter. 

In the spring the extension/trellis is bare and lets the sun shine in the windows and onto the wall, but come late summer when the sun is getting lower in the sky, and can hit the south wall and windows, the vines have provided shade.

Thekla

Thekla- straw bale can be load bearing. But I don't think it is strong enough to hold up a living roof, or a second story. I had decided on straw bale for two reasons... one, it's faster to go up than cob (next summer a ton of people are moving into the house we are living in, so we want OUT as fast as possible). Two, I think insulation is a bit more important than thermal mass where I am... right now we are staying in the upstairs bedroom of my parent's home. It has two northwest windows and one southeast window. At night we would open them up (it gets down to about 70 at night in the summer) and it would stay comfortable in the room all day long (into the upper 90's). In the winter (which isn't too terrible, maybe average temp mid 40's and 30 at night?) I thought having all that insulation would work out well with a rocket stove heating the straw bale house easily, rather than solar gain heating it, which would overheat the place quickly in the summer no matter how man overhangs I had... and I know this is a problem here from being in other cob houses in the area. I could get really clever with the overhangs but that seems more difficult and more likely to fail. Am I wrong? I figured I could make the interior north wall have a thick cob layer to provide some thermal mass for the sun to hit and heat up in the winter (I plan on having some south facing windows) but since it's inside and on the north wall, the summer sun won't reach it easily.

It was actually suggested to us that we run a cob workshop (for labor and to help cover costs a bit). Sounds like a great idea.

I like the extended overhang/trellis idea. The only downside is not having that shade if something happens to the plant (I imagine living on the south facing, thermal mass wall of the house can be a bit hot for some plants). I have planted apricot and cherry trees on the south side of my build site.

Sam- I also like everything I read about living roofs.

If I'm going to build the roof before the walls... I think that will defeat the purpose of building load bearing straw bale walls in order to save money on timber? Because something has to support the roof. I do have some nice cedar on my property I can use, like I did when I built my tool shed:

http://www.flickr.com/photos/genissimo/5711837448/in/set-72157626858101281

Giovanna,  sounds like you've done a lot of planning and have some good resources.  I did take a week long cob workshop with Ianto Evans, and Linda Smiley.  It was great, and the walls went up pretty fast.  To the best of my memory, we got the first story walls up, including and interior room divider.  That is a LOT in one week.

From what I understand though, Cob Cottage requires that you've taken one of their work shops, and the planning is done more in advance than you are talking bout.  I bet there are some other good teachers out there, and the whole crew under direction of a skilled person is wonderful.  I have had some volunteers (WWOOF) who were not really here to learn cobbing, they wanted a place to stay in exchange for labor.  This did not work out, as the person was not motivated to build the best possible cob, was interested in the easiest for him, using my topsoil when I wasnt't there, in place of the sandy sub soil.  Continued to use it AFTER I had told him not to:  "I thought 1 topsoil to 2 sand would be a good compromise".  Compromising quality of cob, therefore the longevity of my building and my personal safety for his convenience. 

About the plants, passionvine and hops both like heat pretty well.  They need good soil and moisture, and once established, they'll easily grow 20 feet in the season.

Sam<  I am curious about living roofs.  I will be putting up the posts and rafters next week, and would like to know how much understructure is needed to hold up a roof with 2-3 inches of soil.

Current plans are:  spanning a 23 foot space, 4 redwood 4x4 posts set in the ground.  Above the posts a 4x6 beam, which also overlaps a wood frame structure by 8 feet, and is supported at the other end by the cob wall.

Onto the 4x6 beam I have planned 2x6 rafters which span a horizontal distance of 10 feet.  The slope is an inch or two less than 40 inches in the 120, but I could adjust this to be flatter.

I was planning to use blocking to keep the rafters straight and true.

I yesterday, after I read your post about living roofs, I started thinking about things like putting a second supporting beam across (takes out 6 inches of head room)  doubling up the 2x6 rafters, (planned at 16 inch centers).  Then I began to wonder what would hold up the weight of the extra lumber....

On the rafters, decking, a combination of the thinnest OSB (which was used as packing on a delivery I received)  and similar thickness pallet wood.  Then 15 # tar paper, then corrugated metal, the old kind, from a garage disassembled on this property before I bought it 24 years ago.

I would love to throw some soil up there.  Have thought I could make less of a load by putting in a layer of aluminum soda cans, and shoveling loose dirt and straw over the cans.  But, though I will not sleep in this building (it is a green house)  I have gone to a lot of trouble so far, and do not want to do something stupid like putting more weight up than the roof can hold.

I guess I will put this on its own thread too, but I would appreciate any help you can give me.

Thanks
Thekla

OOPS!  That span is not 23 feet it is 13.  big difference.

Thekla

Why do you have to dig a trench, fill it with gravel, and then build the foundation up out of that? It seems to me that if you just dry stack the foundation on the ground, and build up the interior floor to be higher than the surrounding ground... that would keep water out. The uphill side of my build site is a forest, so there won't be much water flowing across the surface, and I could always put a french drain around the house if needed.

I'm only wondering because there are lots of roots and rocks in my soil, and digging down is going to be slow. And I can't see how having gravel directly bellow the foundation does much of anything that an adjacent french drain could do.

giovanna.ash wrote:
Why do you have to dig a trench, fill it with gravel, and then build the foundation up out of that?

I'm also getting things together to build a strawbale barn this month, so I've been thinking a lot about foundations and such.  Here we have clay soil (or really in a lot of places, clay without much soil), and the reason to dig down for a foundation is that otherwise during wet seasons the walls would sink on our mud.  As far as I know, the major purpose of digging for a foundation is to get down to either rock or deep enough into the subsoil that the foundation won't move.

Where I am and with my waiting-to-turn-into-mud-clay-soil, it seems I have to dig a bit.  If your subsoil is rocks and gravel, then maybe that is already firm enough.

what are the zoning or permit situation for building a cob home where you live in NC... i am looking at buying a home around the Charlotte area and was wondering how you were able to build a cob home in your area?

I can't help with the rest, but I can save you a ton of money on the roof membrane. Go to a construction site where they are putting a new roof on a large commercial building, ie a hospital, bank, etc. When they tear off the old rubber roofing, they fold it up and load it onto pallets and move them off the building with a crane. They are happy to load it onto a trailer for you if you are sitting there waiting. My brother works as a roofer and called me the last time they were roofing a hospital. I got 40,000 sq ft of the rubber roofing material just for going and getting it. My friend has a car hauler (trailer) and moved it for me for about 3,000 ft of the roofing. The stuff is pretty much indestructible. You couldn't tear it if you tried. It does need to be protected from UV until it is covered.