Cob or Strawbale? hybrid earth sheltered/earth bag/straw bale/cob home

First time poster, long time fan here...This site is a GOLD MINE. My wife and I just purchased 10 acres and are building an earth-sheltered home this spring as soon as the ground thaws on the south shore of Lake Superior in NW Wisconsin. I've been planning the house for months. We were originally planning to build a dome home and came across Simon Dale's site during our research and fell in love with the idea. Although we've had to alter many parts of the design to try to stay within the confines of the building code, we have decided to do the timber frame and reciprocal roof living roof, rubble trench/frost protected shallow footings with 20 foot wing insulation on the south side, leading to the drain tile that encircles the entire umbrella of the building. We plan on using earthbags to build a retaining wall for our earth berm on the north, east and west sides and using them for the foundation/stem wall on the south side. Everything inside will be on a gravel pad with a cob floor (similar to slab-on-grade) for thermal mass. My question (if anyone can help) is whether or not to use strawbales with cob plaster or just plain cob on the south side? We have long, cold winters and we'll have a LOT of windows on that side of the house, so I don't know if there will be any REAL advantage as far as insulative value (not to mention the cost of shipping and trying to find them this time of year) to using straw bales as per my original design. We also plan on building a green house on the outside of that wall probably next spring. With cob, we could run a rocket mass heater through that wall...Anyone have any insight? Thanks for reading!

Hi Jason,

This sounds like a wonderful project, I look forward to following along as you progress. I would lean toward just cobb, but would love to hear what others think. Also, will your timber frame section be traditional or actually just large timber construction with metal fasteners and joints? Thank you for share this project with us.

Regards,

jay

We will be doing it as authentic as possible. I'll use traditional joinery techniques around the top of the frame but we'll use metal fasteners only to attach the roundwood timbers to the pylons so we won't have to embed the wood in the ground, or possibly setting metal tubes into the concrete that the butt ends of the logs can sit in. The pylons are only place we intend to use concrete. As for the rest of the structure, we don't have enough suitable trees on the land to do the frame, so we'll have to order those from a local guy. On the other hand, I can eliminate the timber framed section on the south wall if we use cob, so that'll cut down on the outsourcing of materials. And our location gives us an abundance of clay, so that's a great resource. Thanks for your input. I've been leaning that way myself, but it never hurts to have a few other brains weigh in.

I have zero hands on experience with cob or strawbale but I vote strawbale given the choice in your climate. Your Building climate zone 7 is pretty extreme. I recommend AT LEAST building to the current International Energy Code minimums (IECC 2012). Your wall R values should be close to "“R-20+R-5” insulation or “R-13+R-10” insulation". With strawbale this would mean R25.

If there was a good way to give COB some exterior insulative sheathing (the "+" part) then COB would be more appealing. The thickness of either wall system is a huge negative to me. Probably want to splay your window openings if possible.

More importantly you should be building airtight enough to achieve an ACH50 of 3. The IECC 2015 will probably require an ACH50 of 1.5 in your climate which would be a much higher performing target and have a much bigger effect on your comfort and heating fuel use.

Not sure how hot your summers are but building a green house right up against your living house can cause more problems than the unwanted heat gain. Excess moisture and humidity is not good for structures that are lived in. Your building site sounds pretty amazing!

Hi Brian,

I don't want to steal Jansno E's Post Thread, but I would like to here Janson address your thoughts, which I know you are a big proponent of "Air Tight" houses. Which is opposite of many of the "traditional green" builders. I have been reading and following along on several fronts, I was of the opinion on that cobb would better serve, but as you pointed out the walls are thicker. I never design a wall thinner than 300 mm or thicker, so our design foundation is different in that regard.

Regards,

jay

Whats a millimeter?

I probably shouldnt pick on COB in this particular forum but I do think its a tough choice for cold climates. I think it could be a better choice than strawbale depending on the particular situation. Even if it ends up with a lower R value, I think its easier to make airtight and if the site has the right materials or the builder is more comfortable with it, then COB may be the better choice.

I think I disagree with most "traditional green" builders not wanting an airtight enclosure. A better term might be "natural" builders. I think most natural builders favor airtight construction but certainly not impermeable.

Permeability is the drying potential of a material. Generally, the more permeable the better (above ground). However, air leaks are entirely different and undesirable. Air leaks can introduce moisture and inhibit the drying potential of a material.

Air leaks are also responsible for about 1/3 of our country's energy use from our homes and buildings. Even those that burn with wood should do their part to reduce that statistic.

I've heard of R values being much higher with strawbale walls than you're suggesting, Brian, a minimum of R40, anywhere from R1.5 to R3 per inch of strawbale thickiness (from a study done by the State of Michigan). The greenhouse will be similar to the Undercroft, another of Simon Dale's houses. We like the look and the accesibility, and we'll be able to open them up for air flow and help to keep our heat inside in the winter, essentially turning the wall in question into a trombe wall (this term I found in "The Hand Sculpted House")...with our passive solar design the sun won't shine on that thermal mass during the hot part of the year. Also, regarding the air-tight-ness, we'll be installing earthtubes for fresh air circulation, so not air-tight for air quality and the draw of the R.M.H. Keep in mind that this will be an earth-sheltered home, so we'll only be heating from around 50 degrees or so, much more economical than the 2X4 stick frames in the area. Thanks for the input, every bit helps!

Another thing that just occured to me. While trying to find the answer to my own ? yesterday, I saw a statistic that a basic cob mixture has a value of R30 by itself, and you can add some other things to increase the insulative value. Another thing is the windows....with that much glazing, it sort of makes no real difference as to which method we choose...thoughts, Brian? You seem to have pretty broad knowledge on the insulating subject.

I feel guilty for posting anything in this COB forum so Iam really not the one to speak for its R value. I believe that it generally tests out from .3 to 1 R per inch. This makes it more of Thermal Mass TM than insulation. This means your wall needs to be 83" to 23" thick to meet minimum energy codes. I doubt there is very much COB out there that is as high as R1 per inch, that would be about the same as wood. Insulation is WAY more important than TM in your climate by the way. TM is probably more important than usual for your excessive glazing strategy but I would suggest you get your glazing ratios closer to mainstream passive solar amounts.

You should look into the passive solar forum here. Lots of good info and more appropriate place to get into the details. Too much glazing can be just as bad as not enough even with lots of TM.

I do know the difference between insulation and thermal mass. I've also lived here my entire life and know about the climate...that being said...

I've maybe misled you about the amount of glazing in that wall. It's not really excessive, much of the glass will be in the sunspace. My design conforms to many other PS designs out there.

It's very important for that south facing wall to act as TM, because the sunspace/green house on the other side of it also needs to be heated efficiently. The plan is to run a rocket mass heater along that wall as well, on the suspace side with benches for seed starting above, and on the inside, nice window seats. I don't see the benefit of insulating between the spaces when both need to stay warm. We'll also have another mass heater that will double as our cookstove and brick oven in the kitchen for winter cooking (we're building a summer kitchen so we don't heat the house up with our cooking). We could also add some water bottles in the wall as well, while studing for this design I read that water walls make extremely good thermal mass.

I was pretty dead-set on using straw bales until just the last couple of weeks when I started thinking about the sunspace on the other side of the wall and keeping it warm in the winter. We will probably use them on the stem wall of the sunspace, and the cob wall above the 8 ft line. I should mention that the wall is 12 ft high, to accommodate the height of the reciprocal roof in the loft area where the kids' rooms will be. That will help keep the heat that rises from leaching through the walls up there.

I hope someone with real experience in cob building pipes up, as we're both hypothesizing here...

True I would especially like to hear from the cold climate cobbers out there why my concerns are overblown. You have quite a bit of complicated physics going on with your project mainly in regards to the "heated thermal mass" wall that would probably benefit much more from some insulation to separate it from the extremes of the sunspace/greenhouse.

Did you see the threads on Earthships and attached greenhouses? Sounds similar to what you are trying to do. I think the sunspace/greenhouse is a fun idea but the science and real world results seem to rarely make them a good match. What kind of growing do you plan on doing in a WI greenhouse through the winter months or is it just for heating? It might be great on a sunny day but cloudy days and nights will mean that you are firing up the RMH to help alleviate the thermal losses through the wall. I think Iam still stuck on the idea that you have a lot of glazing planned in the sunspace though so I will shut up for a while.

To me, sunspace = expensive energy penalty.

Hi Brian, Jason, et al.,

I have been part of earthship construction on and off for over 20 years. I am definitely not an expert on the subject, just well acquainted with it and the support modalities that build it. (ie. dry laid stone, timber wrighting, cobb, etc.) I am doing a bit of a hit and run here, as I have several projects that got me tied up, but have enjoyed following along. I really want to see Jason achieve his goals and follow his progress.

Brian, you have made that distinction before about "air tight" vs "permeable" in other conversation we have had, sorry I brought it up the way I did. I just lean toward traditional methodologies, versus modern reinvention, like house wrap and spun glass insulation, which neither work as claimed. As for thermal mass versus R factor, I still think we have several more decades of "building science" research to do, especially with a new and vibrant young crop of building scientist thoroughly examining the logic and soundness behind ancient building techniques, like log architecture. This has low R but high thermal mass and evolved in some of the coldest climates of the planet, so even though many miss the logic behind it, the logic is there nonetheless.

As for Cobb, it is as varied as the regions it is used in globally. From the deserts through extreme northern climates, earth architecture with timber framed roofs have be the dominant building form around the planet, in one variant or another for 10 of thousands of years. Just read through some of the UNESCO's World Heritage Inventory of earthen architecture, as one example. Also, as far as R factor, there are variants that can achieve R3/inch perhaps higher, depending on the formation of dead air voids, and/or the incorporation of other methods working in concert with these cobb forms, such as "clay straw" cobb, also called "slipped straw." It is a variant of rammed earth, just way easier and with tremendous R factors.

Jason, I love the 3.6 meter high wall with a mix of both straw bail, and cobb. I think a clay straw cobb, might be good for your application. If you go out on YouTube, you can find examples of it. I hope to have a chapter dedicated to it in the text I'm writing.

Till later,

jay

Jason,Hi
Congratulations on your project. It was as if you were describing my 'planned for' place.
I have some building experience and self gleaned knowledge in cordwood,cob and clay slip and straw building,earthen floors and plasters.
My decision is to have cob ( 18 to 24 inches thick) for the south facing wall ( perhaps somewhat curving to the east and west) with a similar sunspace as is seen on Simon Dale's home at Lammas. ( BTW a pic of it is my computer background)
I wanted thermal mass to catch as much warmth as possible both from the sun on the one side and a planned for rocket mass heater on the other. I have planned for upper and lower vents on this wall which can be shut as needed. Windows, also, as are seen in Simon Dale's place...perhaps not as large but not sure.
One could even consider doubling the sunspaces...as is now being done with cold-climate earthships...I'm dreaming of Meyer lemon trees and papayas already!
I look forward to following your progress.
All the best
Kate

Jay, thanks for your insight. I am planning on using "straw cob", nearly everything i've seen on cob is that form.

Kate, it's good to see others doing the same thing. Simon Dale's houses sure are inspiring. I'm looking forward to seeing what you do as well. I hope it catches on. We should begin breaking ground on the building by June. Our access road onto the property will be started in the next couple weeks, and then the real fun begins!