Increased insulation Cob

Hello,

I was wondering if there is any technique to build a cob wall in which one of the sides would have something to increase it's insulation proprieties. The idea is to build a greenhouse with double glass facing South and cob facing North but I wanted to add something on the outside part of the cob wall in order to prevent the warm that's on the inside going out but still keep the thermal mass proprieties.

I'm in a 8 hardiness zone and I'll need a year-round stable temperature in the greenhouse in order to keep an aquaponic and BSF composting system running. The greenhouse will have a rocket mass heater to help when the temperatures go really low.

It's the first I'm building something like this so any help would be greatly appreciated.

Thank you

Great thinking Bruno. You definitely will benefit from some exterior insulation. Foam sheathing is the traditional choice. Research Mineral wool board sheathing and foam glas for more natural alternatives.

I have never built with cob but want to someday. I was wondering if you could earth berm up against it?

Hey Bruno,

Great plan!!!

Think of it this way, it might help:

Average cob R factor per 25 mm is about 0.65. So a 300 mm thick wall has 7.8 R's, that doesn't seem like much but we are not considering the thermal mass properties of the material. That is why a log cabin might only have R 11 but still be very efficient if well built and not drafty. Now add 25 mm of polyiso foam to the outside of the cob wall. You haven't added any great mass, but you have added the equivalence of almost 250 mm more cob thickness to the wall.

I would extend the roof drip line in the area of mention. Embed some heavy horizontal furry strips in the cob about every 500 mm, then attache you r foam to that. Now attach some vertical furring strips through the foam to the horizontal furring strips, then attach another layer of horizontal furring strips. Onto that attach some nice vertical wood board, and you have an insulated cob wall with a wood sided breathing wall. The foam is protected, by the boards, the boards by the overhang and the air circulating on both sides will keep the boards in good shape for about two hundred years of so, maybe more.

Good Luck,

jay

I'm doing this in a farm where I'm volunteering and the budget is short so I was thinking about using straw since we have a lot of it here.

I also wanted to make some kind of curtains for the windows so maybe I can attach some straw to one of the materials you mentioned...

We have wool from the sheep so I think I'll use it to insulate the north wall. About the curtains for the windows, if there's enough wool I think I'll try to do something with it. Latter I'm posting a sketch of the greenhouse here so that you can see how it looks

I would felt the wool first.

I don't see a sketch...where is it?

Wool will work but you will have to treat it with borates and probably some other materials as well or it will get eat'n and chewed to bits by the wee beasties. It is a very demanding insulation, good, but very demanding.

Thanks for the heads up about the wool. I'll discuss it tomorrow with the farmer in charge here.

I hope the sketch is understandable. The brown wall is the one being made with cob and the North facing part of the wall is the one that I want to insulate.

I also wanted to know which is the best angle for the roof. I drew it in red and with an "X" for you guys to know what I'm referring to. This is an area with snowy winters so I need to make that roof somewhat steep.

I didn't draw any overhang because, using this design, I don't think I'll need any.

Also, I need to think about the best place for a door and another opening in order to let the inside breath when the weather gets warmer.

Hi Bruno,

You always should have an overhang to protect the wall beneath.

North Wall will need a pent roof with oppisit slope or the roof should have an offset ridge, making it look like a "cat slide," or salt box style.

I would go with a 16/12 pitch (also called a 4/3 pitch.) this is easy to cut as it is a 3/4/5 triangle and the trig that goes with that is really easy. You should, just for info sake, know you equinox winter/summer angles for your area.

I like the wool, but do mind those beasties I mentioned, they can ruin it.

Regards, jay

I think I saw somewhere that you want the angle to be the latitude plus 10 degrees? So, if you are at 33 degrees north, then your angle would be 43 degrees?

Hi Rane,

I think you are correct, but I believe that is a minimum rule, meaning that is the lowest you would want a slop for a specific region, that is why 4/3 pitches work in many areas. They just aren't build because for what ever reason, folk seem to be intimidated by this pitch's steepness. It is called "Gassho," pitch in Japanese, or "Praying Hands." It is very functional and extends the life of most roofing types.

Regards, jay

About the roof steepness, I'm living in 53º North so if I use the rule the roof should have a 63º angle. If I use the 4/3 pitch, the angle would be 53º. There's quiet a difference there... The steepness is mainly to let the snow slide off and to increase sun exposure so I'm guessing that the windows should be fully perpendicular to the winter solstice sun. From what I figured out, in the winter solstice, the sun will be at 14º so, in order to have perpendicular windows, the angle should be 76º...

About the equinox, I don't really understand why it is relevant. I would appreciate if you could explain it.

About the overhang, I'll post a new sketch including a possible overhang as soon as I get the right degree.

Morn'n Bruno,

I have read years worth of research and I still get conflicting information on this subject of roof angle vs. winter equinox, (sun at it lowest.) What a lot of green house growers have pointed out to me, is I should note what many professionalism growers use, which is just a hoop house shape orient toward the sun's general directon

I think I saw somewhere that you want the angle to be the latitude plus 10 degrees? So, if you are at 33º north, then your angle would be 43º ?

Another point the make is the angle of the building to the sun. Which also has, according to some research, some things to do with the equinox, but again, it's about averages. I still don't consider my knowledge of this complete, just because I meet folks with such extreme views opposite to each other. I myself would love to read what others here on Permies forum think about building/roof angle to equinox. I think, once you get past 45 degree roof pitch you are in a close enough range to be efficient. Then I just start looking a function of the roof to shed precipitant, ease of construction math and aesthetics. Whether you go for a 3/4/5 triangle (53.13010235º) or an Isosceles triangle (60º), I don't think at those pitches the statistical heat gains are that different, that is why I just go with 53º.

I am in no way a cob expert, but I think that sheathing cob with foam may cause problems with moisture retention. My (limited) understanding of cob building is that you need to have a breathable finish, but I could be wrong. A greenhouse is going to generate a whole lot of moisture.

Hi Julia,

I don't think your concerns are unwarranted. That is the main reason I said to put the foam board on furring strips with a breathing wall of wood. I think if you spray foamed the cob you could end up with a big lump of soft clay and chips. Matter of fact, I don't see how that wouldn't happen. Wool felt would be the next best thing. Me personally, I think the foam may need to go on the inside of the green house if it is used at all. Otherwise I think just cob alone would be best and maybe just extra of it to accommodate the extra cold outside.

Regards, jay

Hello Bruno

Just a thought and not sure if anyone else has mentioned it but why not build the north facing wall out of straw, this will meet all your insulation requirements and you could use the wall as a load bearing to hold the roof, plus, you still can use the cob on both the inside and outside of the wall to keep the same look as the rest of the building. The roof angle degree question may be answered here http://earthship.com/ , from what I have seen and researched you want the angle of the roof to be somewhere near the middle of the highest sun during winter and summer, this should allow for you to get the most sun. I think its wonderful that you are trying to build a greenhouse using the earthship concept.

Good luck

Christian

I might be wrong but the day when I get less sun is in the winter solstice (not equinox), right? The equinox is when the sun is perpendicular to the equator and the days have 12 hours everywhere in the world.

So, about the roof angle, I guess that I'll just go for the steepest possible. I still don't know the dimensions of the greenhouse but if I use an angle of 60º or more I guess I'll have to build a gigantic North wall and I don't know if there's any limitation to how high can a cob wall go...

Thanks for reminding me that the cob should breath (I'm really felling like a beginner here :p).
Christian, I thought about that also but then I would loose the thermal mass factor. Maybe the best solution is to compliment the cob wall with an outside layer of straw. This way I'd keep the warm trapped in the cob instead of loosing it to the exterior and still have a breathable wall.

And, while we are talking about isolation, what do you think would be good "curtains" for the outside of the windows? Something with straw? Wool? Or maybe something made with both?

Hey Bruno,

I might be wrong but the day when I get less sun is in the winter solstice (not equinox), right?

Thanks for good questions, you are keeping me on my toes and making me feel like I teaching Science again. I have always reversed these in my head, (dyslexia is a pain in the back side,) the vernal equinox, summer solstice, fall equinox, and winter solstice. Thanks for catching that.


I just won't use straw outside of arid climates, there is just to much reporting of mold and moisture issues beginning to pop up. I have been to straw bail homes here on the East cost starting to have issues, and when moisture ready in the wall indicated 80% we new there was a problem. Now they do use hay bails in the winter to on north walls and in attics. then use them as mulch in the spring, summer, fall of the next year. Then start the cycle over again.

I would not bother going with a pitch beyond 53º, it is more roof material, I don't believe will make a noticeable difference in efficiency, and has less easy math to deal with in design work. We are planning a green house for next year. If I can find the design file, I will post it here. Do you use, or can you download Google Sketchup?

Regards,

Jay

Jay, I had to do an half-hour research to get the equinox and solstice right :p

And now that's another problem! This is a wet area and, in top of that, I'm building in a low, wet land. I'll probably make a big pile of dirt and build on top of that. I'm only afraid that the pile collapses over the weight of the greenhouse...

I don't use it but maybe now it's a good time to start (sketching with Paint was much more fun when I was a kid :p)

Hey Bruno,

Since you are asking great questions and coming back for more, I will start giving you some answers that are more like I do my students when I teach. You are doing some good research, and I do better with those kind of well thought out "what about this or that," questions.

So you are building in a wet area...how wet?

Maybe a dirt pile...what kind of dirt?

Any clay?

Look up "angle of repose," that will answer some of you questions.

Great Wall of China is nothing but a big pile of dirt..."rammed earth and stone," to be exact.

You do need to learn Sketchup, I will send you my green house design when I get a chance and we can modify it to your needs.

We can also look at some other insulation alternatives, recycled carpet or carpet felt padding, sawdust, just some wood chips...the list goes on.

Till later,

jay

Ok Jay, thank you!

I'm going to explain the situation. I'm Portuguese and I'm in The Netherlands doing volunteer work in a farm. I'm with another young guy (I'm 25 and he's 23). I have always lived in a city and never really got hands on with any type of building or agriculture. My friend has more experience and he's helping with the project.

I had the idea of doing Black Soldier Fly composting and an aquaponics system that would use the waste from the farm to produce fish, vegetables and larvae for feeding. Because this will need a temperature of about 25 ºC, I decided that the best way was to build a greenhouse. This was when I started to search about green ways of building and found about passive solar and cob.

Long story short, I ended up imagining a greenhouse that would be warmed by the sun and would have a rocket mass heater inside. I though that this would be enough to keep the temperature on the inside good enough even during the winter but I'm now realizing that it's probably going to need a lot of wood for the rocket mass heater and therefore will become not economically viable (I'm not doing for profit, because I won't get any, but there's no point in building something that consumes more that what it produces). Adding to the frustration of seeing my project, and dream, not being possible, I still have to deal with the fact that I'm a complete beginner in all of this and my ideas are always wrong!

Anyway, this is low, really low. The village is 2 meters above sea level. When there was high tide, the water used to come until the village but they build dikes to stop this from happening. Initially we were going to build the greenhouse in a piece of land that gets very swampy during the winter (it even creates a lake in the middle of the field), but we decided that it's best to do it in higher ground so now we have a place that's like 1 meter above the previous land. This place doesn't get so much sun but it's more stable for building it.

So today we thought that maybe we should drop the idea of building just one wall with cob and do three walls instead. If we do this, we'll have sun coming only from the South and the roof. The idea is that we'll have less heath going out but with the down-side of having less sun.

My friend did some drawings on a paper explaining me how he wants to build it, mainly covering the foundation part. I'll try to use google sketchup to show you what's the idea. Once again, thanks for helping! We really needed someone to guide us but since there's no one I guess I'll have to rely on the good will of guys like you.

Bruno,

Give my your Google Map Location so I may study it.

Also, it is normally best, even in modern times, to look at how people of that region live traditionally. The further you get away from traditional living modalities, the more work and money you often spend.

Regards, jay

http://maps.google.pt/?ll=53.204647,6.110711&spn=0.000398,0.001321&t=h&z=20

That is the place. The white spot is, more or less, where we want to build it now and the field on the left is the place where we first thought about placing it

Bruno,

I know that you will not see the brutal winter temperatures we can see here in the US midwest, however you might find this thread useful; it describes a low mass sunspace:

http://builditsolar.com/Projects/Sunspace/LowMassSunspace/TestsLowMassSunspace.htm

Decoupling mass from the (insulating) north wall may be helpful...

Look around a while on the builditsolar site, lots of good info on green house design for northern climates - I spent some time in Holland during the winter...

Andor ak mechartnik

Hello Bruno,

I have more questions...

Do you read and write english or are you using a translator?

I took the location and saved it to "My Google Maps," I just spent the last hour taking a virtual tour of the surrounding country side in your area. It is very green and lush. I also note the pitch on many roofs are 3-4-5 or 4/3 pitch. I also noted a far number of "thatched roofs." Thatch could be of use to you.

I also noted a number of large forest within one hour of your location, and there would appear to be trees everywhere. You have many small saw mills in your area, so lumber should not be an issue.

I was also pleased to find a thriving timber framing culture is still active all through the Netherlands, so it will not be considered unusual for you to build with timber framing as your method.

Do you have access to a tractor on the property and does it have a backhoe attachment?

If No, how do you intend to move dirt around?

Have you checked the clay content of your soil, to see if it is suitable for Cob work?

What type of budget do you have for this project?

Your idea for only having the front and roof is somewhat valid, except most heat loss is through the roof.

Green house (Glastuinbouw )growing seems to be everywhere in the Netherlands, so getting supplies should not be that difficult.

Regards, jay

I don't use any translator. Is my English that bad?

Yes, the roofs here are much more steep that in Portugal, I guess it's because of the snow. About the thatched roofs, actually we have one here on the farm. The big building is where we keep the cows in the winter and in the roof there are some plants.

Yes, we have lots of trees here. In fact, just some weeks ago we were cutting trees in the fields to send them to a friend who has a wood shop where they cut the trees into small pieces for the fire place. They don't work the wood, they just cut it into smaller pieces.

We have a tractor but not a backhoe. We have the front part but not the part that's on the back:

http://www.google.pt/search?hl=pt-PT&q=backhoe&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.42080656,d.d2k&biw=1366&bih=598&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi&ei=kIITUa_3B--10QWktoCICw#imgrc=UHvwhsWyYG4EIM%3A%3Bt3QM9CypUwzFJM%3Bhttp%253A%252F%252Fwww.foxheavyequipmentoperator.com%252Fimages%252Fbackhoe2.gif%3Bhttp%253A%252F%252Fwww.foxheavyequipmentoperator.com%252Fbackhoeoperatortraining.htm%3B760%3B620

Anyway, if we really need I think there's someone around here from who we borrow one.

I still haven't checked the soil for the cob work but I'm starting to think that maybe straw bale would be a better option because it has better insulation.

About the budget, we don't a one. We just go and ask the farmer for the things we need and he decides if it's reasonable or not.

I realized that the roof is the part of the greenhouse that lets more heat escape (I had a thermodynamics subject during my bachelor so I'm familiar with some concepts about this). I just don't know how I can insulate the roof without blocking the sun. We do have a big supply of double glass that a friend gets for free from old houses that are being renewed. Maybe it's possible to combine those with some kind of special industrial plastic film...

EDIT: Some bad english needed to be corrected :p

Hi Bruno,

Your English is very good.
Do you speak other languages as well?

How old is the thatched structure on the farm?

Is it originally a barn and is it timber framed?

Describe the farm and it's livestock?

Is this a hobby farm or a real working farm?

Your link took me to a page I could see tractors for a second then they switched to a blank Portuguese Google search page???

What size and type tractor do you have access to?

This project will take at least 6 months to a year to complete, with proper planning, design and your learning how to build it. Are you up for that?

Do you have any timber framing experience at all and are you willing to learn?

This will provide the frame work for the green house.

Stay away from straw bails work in that climate for a green house...it's just too wet. You would be better off making a thick thatched wall than straw bail, and the cob would be just as easy and last much longer.

Do your soil check for clay.

Do you have access to sawdust? It can be used as an insulator and additive to cob.

Look forward to hearing form you..

jay

I speak, of course, Portuguese, a little French, Spanish and Italian. I'm having Dutch lessons but I don't think I'll learn it :p

The farm was made 20 year ago so I guess that the thatched roof as the same age.

Actually it's more than a barn. Inside the structure there's space for the cows but also 4 bedrooms, one kitchen, two big bathrooms, one small living room and another room with computers, books, etc. I think it's fully made with wood with the exception of the foundations which, I guess, are made of concrete and some iron.

It's an organic farm with rabbits (~10), goats (5 - 6), peacocks (2 - 3) , ducks (2 - 3), goose (3 - 4), cows (~15), dogs (2), cats (2), fish (3 aquariums), pigs (2), chickens (~15), deer (~13) and horses (4). Maybe I'm forgetting some but there are so many animals that I get lost.
It's a real farm. It also works and a social school for kids with learning difficulties and kids with mental illness. Also, young kids come to spend the weekends to play in the farm and we have elderly coming just to spend some time outside feeding the animals and taking a walk.

Try this image: http://www.lojadosbrinquedos.pt/sites/default/files/styles/product_full/public/4006874036523_john_deere_com_carregador_frontal.jpg
The tractor is similar to that one.

Well, I think I don't have that much time. The project is over in November but I don't if I'm staying until the end because I applied for a some Master courses and, if they choose me, I'll start sometime in August or September. I don't know what you have in mind but I don't see why we'll need so much time. It's a complicated project, yes, but it's not that big. We're talking about something maybe 4 per 3 meters...

I don't have any experience but my friends does and we have a small wood shop here in the farm with two guys who can help us.

Yes, sawdust is not a problem.

I'll do the soil test tomorrow. Testing the soil is digging past the organic matter part of the soil, collecting a sample, putting it in a jar with water and shake it in order to see the proportion of different constituents, right?

Finally I think we've decided that we're going to to use timber framing with slip straw inside for insulation. Opposed to what I've seen in some videos, we won't take the wood from the wall and leave the slip straw exposed. Instead, we're basically going to build a wall with both sides made of wood and with slip straw inside. If you see any disadvantages please let me know.

Also, if we have space, we'll do a smaller wall on the inside with cob just for the thermal mass. It will be some centimetres away from the wood wall to allow air to flow freely.

Hey Bruno,

Boy you have talent in the language department. I like your build idea. Cob wall inside isn't bad, but it takes up valuable growing real estate. Water takes for aquaponics would serve the same purpose and store more heat. Keep us informed of you progress. I like all the animals around, sounds like a nice place. What are you going to major in for grad school?

Regards,

jay

Thanks Jay. I'm taking an English test this Saturday so I have to get my English tuneful.

I did a bachelor in food technology and now I'm applying for three different masters but all of them are related to sustainable agriculture and/or rural development. I still don't know if I'll get in though.

We also changed a little the design of the greenhouse. I'm currently working on a excel sheet that will allow us to calculate all the wall lengths provided you give one or two parameters. For example, if you give the angle of the roof and the length of the biggest wall, the excel sheet will give you the all the other lengths. I did it for the first design (the one that I posted here) but now, because we are changing it, I need to do another one. It's a little bit more tricky but it will pay off, I hope, because we're getting the windows for free but we still don't know their size.

Hello Bruno,

I have been working on so many projects, it becomes kind of nebulous sometimes. Did you post a Sketchup CAD drawing of the green house here? I can't find it? That program will do much of your work for you that the spread sheet does and more. I can help better in the design work, as a spread sheet isn't going to do much more than tally amounts. If you really learn Sketchup it can do that also, plus create a cut list. Other wise you will need to do it all manually. I can't seem to attach Sketchup riles to Permies? I also seem to be having problems with attaching photos, sorry about that.


Regards, jay

Try again with attache files...

I've been experimenting with SketchUp but I don't know how much time I'll take to learn how to do what I want. We don't know the size of the windows that we'll get so what I wanted to do was to create a model in SketchUp that would allow me to manipulate variables like the size of the walls of angles of the roof but in relation to the windows size. I don't know if I was clear but if not tell me and I'll try to explain myself better.

I think I do but with out a model it is hard to see. Could you take the file I posted and use that frame, changing it to what you need?

Sorry for the delay in the answer Jay but we can't do anything since we don't have the windows or their size. I've been busy working on what's going inside the greenhouse :p

I think I'll just take the more practical approach and wait for the windows. I already have the design and materials in my head so we'll just start building when we can.

That's the idea and the walls will be with slip straw between two layers of wood except the front side and roof where we'll use the double glass windows.

I've long thought about doing something similar, and have a few ideas.


Firstly, replace the north-facing cob wall with an insulated wall, perhaps straw bale, and shift the cob to the middle of the greenhouse underneath the fish tanks, with the rocket mass heater pipe running through the cob. That way the heat is transmitted directly into the water, which is more energy efficient and also allows the water to act as thermal mass. Adding some insulation under the cob, e.g. vermiculite, will limit heat loss into the ground.

Angle the front glass perpendicularly to the winter solstice sun, as already discussed. The easiest way to work out the angle is to add 23 (the latitude of the tropics) to your latitude. For example, in Friesland the angle you need is around 76 degrees, for East Anglia (where I live) it's around 75, for Portugal it's 60-65.

Add a length of guttering at the bottom of the glass to catch any condensation during the Winter. The warm water of the tanks will result in a very humid atmosphere inside the greenhouse, and that means loads of condensation. For the same reason, avoid right-angled corners if you're using a wood frame for the glass. Condensation will form on any cold surface, however it tends not to stick around if there's moving air nearby. Wide angles will limit the chance of the wooden frame rotting by passively increasing the amount of air movement around the wooden frame.

During the coldest five months of the year (mid Oct to mid-March) cover the roof with insulation, to prevent you losing heat through the glass. Double glazing only reduces heat loss through windows by about a third, and limiting heat loss is your main concern at that time of year.

Build something on the other side of the north-facing wall. Even if it's only a lean-to tool shed, it'll help reduce heat loss through that wall by acting as an extra layer of insulation.

Tim, thanks for your input.

A lot of your ideas go towards what we've been agreeing during our talks:

. We decided to use slip straw on the north side wall. Straw bale was also considered but we ended up decided that slip straw would do better.

. I did some research and concluded the same thing you said, the angle should be around 75º. The problem was that with the first design I had in mind that angle would be very difficult to do but now, with the new design, I thing it's fairly doable.

. I haven't considering the gutter for the condensation because I still don't know what I'll use for the floor. About the use of wide-angles, the design might still change again in order to incorporate a over-hang on the north side wall.

. I also thought about that but I got kind of stuck when I tried to find what materials/system would be better.

. It's not possible to build on the north side wall because there's a fence there but I'll try to add some extra layers of something I find laying around just to give an extra protection.

I made an attempt with google sketchup. It's good enough to get a view of the idea.

That's good Bruno, now we have something to work around. Did you download and look at the one I posted here?

Regards,

jay