10m2 cob roundhouse den for cold climate

Hi there,

I’m planning a small outbuilding (under permit sized) on our rural property, which is located in southwestern Ontario, where we get a couple of very wet months in the spring, and 3-4 months of icy and snowy weather.

We have a beautiful thick layer of clay about a foot down into our soil, and a pile of double glass window panels, 2’x6’. We also have access to lots of clean concrete fill, and possibly some sheep’s wool for really cheap. Piles of old brick and foundation stones. And about 4000 sf of old barn steel.

Our house is straw bale, heated by a mass stove, and i realize that this is probably the most energy efficient natural building technique for this climate (Or at least the technique that is most efficient but still balances out labour and cost factors). We have almost everything we need for this, plentifully. But, the thickness of straw walls limits the interior size of the shed, given the zoning constraints.

For the moment, I’m leaning toward a 1.5 story (small half loft) round cob structure framed on a henge, with a reciprocal roof. I would frame at least 6 of the glass panels vertically along the south facing wall, to create a sunspot effect in winter. And then building a mass stove from recycled brick or maybe more
Cob.

I realize cob has low insulative value, but high mass. I wonder if anyone has experience with cob for small, simple structures like this, in a northern climate. This isn’t intended as a structure for living, but more as an occasional daytime workshop/studio, and in the summer maybe an overnight guest space.

Thanks in advance for your feedback!

By 'henge', do you mean this;
The word henge refers to a particular type of earthwork of the Neolithic period, typically consisting of a roughly circular or oval-shaped bank with an internal ditch surrounding a central flat area of more than 20 m (66 ft) in diameter?


You list some materials, how do you propose to use them?
If the area is very damp for some of the year, what are your plans to ensure it does not ingress the structure?
If you have very thick walls, 600mm to 800mm replicating what the Northern Europeans built
their experience will help you. What floor material do you propose to implement?

John C Daley wrote:By 'henge', do you mean this;
The word henge refers to a particular type of earthwork of the Neolithic period, typically consisting of a roughly circular or oval-shaped bank with an internal ditch surrounding a central flat area of more than 20 m (66 ft) in diameter?


You list some materials, how do you propose to use them?
If the area is very damp for some of the year, what are your plans to ensure it does not ingress the structure?
If you have very thick walls, 600mm to 800mm replicating what the Northern Europeans built
their experience will help you. What floor material do you propose to implement?

Hi John,

Thanks for your response. Good questions!

A henge in the context that I’m using it is a circular post and beam frame that looks like the Stonehenge you were referring to. Apologies for the jargon, this is how I’ve seen it used frequently in roundhouse building information, much of it rooted in Wales/ the UK.  The frame provides load bearing capacity for the reciprocal rafters, which can then support a green roof (and perhaps in my region some extra insulation).

The posts are generally buried in a foundation trench, around the perimeter, which has been backfilled with rubble/stones/gravel to provide drainage for the structure. (Apologies if you know this already, attempting to answer your questions). However for my application, I will dig down past the frost line. I am also very interested in hearing from others who have built in this style, about how they have modified for the climate (given that this is an occasional structure and not a house).  My Straw bale home was built slab on grade with a wide margin of gravel for drainage, around the perimeter. It’s been standing for 20+ years with no moisture or ventilation issues, leading me to think this approach works for the site.

I’ve not committed to a flooring approach though generally in my research, rammed earth is often used in this type of dwelling.

I tend to agree about the 2ft deep walls, they would insulate well in this climate (whether straw bale as in my dwelling, or the thicker cob you reference). But that won’t make a useable building as it takes 4 feet off the diameter of my Round shed.

And I guess that’s the gist of my entire post. I wonder how cob or a thinner hybrid substance might work for an occasional use building that’s not being slept in over the winter.  Would it be adequate for my purposes (storage and daytime puttering in Winter clothing) if I incorporate a mass heater? Are there other ways that folks have experienced to use found-in-place materials to create a better insulated but still thinner wall material?

I read a lot of threads where cob is abandoned for straw bale, or where expensive materials are subbed for sand, or where wool is hypothesized. I’m curious about the end results of these experiments. Were they adequate? Did anyone build plain old cob in a similar climate and find that it worked?

I appreciate you helping me to clarify my question.

I think for the use you are intending, a structure that can be used for living during the warmer and cooler months, but not the cold months is great. Especially since you have the materials at hand to build. Of course a bale structure on a rubble trench with maximum insulated roof and a mass stove is best, but this structure is not intended, I believe, for year round living.

If the stove isn't being fired regularly, every day at least, during the cool months, it probably won't work well in the winter. I lived for three years in a 300 square foot cob house in Western Montana, and it worked well for staying warm year round with no insulation in the floor and a living roof with next to no insulation underneath. Of course, your structure is 1/3 that size, and a rocket stove with a big exposed barrel would kick radiant heat off fast for immediate needs and would probably work really well.

Again, for the purpose you proposed above, I think it would be exactly what you want. Love to see pictures when you start, keep us up to date.

Daniel Ray wrote:I think for the use you are intending, a structure that can be used for living during the warmer and cooler months, but not the cold months is great. Especially since you have the materials at hand to build. Of course a bale structure on a rubble trench with maximum insulated roof and a mass stove is best, but this structure is not intended, I believe, for year round living.

If the stove isn't being fired regularly, every day at least, during the cool months, it probably won't work well in the winter. I lived for three years in a 300 square foot cob house in Western Montana, and it worked well for staying warm year round with no insulation in the floor and a living roof with next to no insulation underneath. Of course, your structure is 1/3 that size, and a rocket stove with a big exposed barrel would kick radiant heat off fast for immediate needs and would probably work really well.

Again, for the purpose you proposed above, I think it would be exactly what you want. Love to see pictures when you start, keep us up to date.

Thanks for sharing your experience Dale! I am curious how cold it gets in Western Montana over the winter, and how wet it is in spring.  Would you live in that dwelling again? If you had a time machine, would you change anything about the design before building?

Will be happy to share once I’ve started. Might be spring by then because we’re already getting hard frosts ha

Pretty dry here, not super wet springs. Winter gets down to -20C (-5F), but generally hovers around -6C (20F) during mid winter. We were definitely comfortable with an 8" J rocket mass heater, but burned frequently during the real cold.

I frequently imagine what I would do differently if I built it again, and am now living in a balecob house on a different property that is much better. If I were to do just cob again, I would definitely stick to single story, building up is supposed to save money, but adds a huge amount of labor to get the cob 10-12 feet in the air, and then the timbers too. I would also have built smaller for my first cob house, there was no reason for two people to need as much space as we had, when you look at the really nice compact tiny homes couples are living in now. We could have probably built a max of 200 square feet and been perfectly content. It was also our first earthen floor, and while the mix was good and the floors looked beautiful, we didn't do a moisture barrier underneath and any items that don't breathe like a plastic tub, would condense moisture underneath and cause the floor to bubble.

I still have the house and am planning some changes in the future. I'm going to insulate the roof better from under the ceiling, redo the earthen floors, and change the J stove to a batch.

Daniel Ray wrote:Winter gets down to -20C (-5F), but generally hovers around -6C (20F) during mid winter.

I think we get down to -30C a few times but that’s rare, we hover closer to -15C here.

Daniel Ray wrote: I frequently imagine what I would do differently if I built it again, and am now living in a balecob house on a different property that is much better.

We love our bale house!! But I’m interested in experimenting with something different...

From your photo, wow! That’s gorgeous! Can I ask how long it took to build?  

I really appreciate your advice, it’s nice to benefit from the hindsight of others ha. I have to think over the loft bit. Trying to maximize the ways the space can be used, but maybe there’s another way to cram a bed for guests in there.  

Ps I see my autocorrect changed your name to Dale, sorry about that! I hate being misnamed.

No problem!

Took a solid year to build, but I was working full time on the house digging the foundation by hand and mixing cob by foot. A loft is a good idea for a bed, maybe a crawl in that is just a bed so you don't need to build as high.

I love the balecob house, it is best of both worlds. Insulation with maximum amount of thermal mass. This February, my wife and I walked away from the house for 9 days and the temperature dropped from 20C to 14C. The average temperature during that time was about 0C.

Daniel Ray wrote:No problem!

Took a solid year to build, but I was working full time on the house digging the foundation by hand and mixing cob by foot. A loft is a good idea for a bed, maybe a crawl in that is just a bed so you don't need to build as high.

I love the balecob house, it is best of both worlds. Insulation with maximum amount of thermal mass. This February, my wife and I walked away from the house for 9 days and the temperature dropped from 20C to 14C. The average temperature during that time was about 0C.

It’s incredible eh? We have never gone below 10c in the depths of winter. I love straw bale. Someday I’ll work up the courage/energy to plan and get a permit for a Bale workshop/garage!

When you use rocks in drainage trenches, they need to drain to a low point that empties the water away from the project.
Is that possible with a frost line deep in the ground?
If you need space inside, why not move the posts out the thickness of the proposed wall structure.
Thus maintaining the interior dimensions required?

John C Daley wrote:When you use rocks in drainage trenches, they need to drain to a low point that empties the water away from the project.
Is that possible with a frost line deep in the ground?

Good question! I have chosen a spot that’s fairly “high and dry” compared to the rest of the property. Generally speaking, the rain doesn’t hit heavily until late April and into May, well past the ground thaw. That said, I am not well versed in drainage. Can you suggest any remedies?

John C Daley wrote: If you need space inside, why not move the posts out the thickness of the proposed wall structure.
Thus maintaining the interior dimensions required?

I’m quite limited in my wall thickness, no matter where I put my posts, as I am building to the “shed” limits in my bylaw and want to maintain a reasonable amount of interior space.

DRAINAGE
Just make sure any hole you dig for the floor has trench that drains away lower than the floor.
Walla
Ok, exterior limits can be an issue.
But is going higher within those wall a way to improve the viability of the structure?