Log home with cob interior walls

My husband and I are planning on building on our land and considering different building options. One idea we had was round log exterior walls with cob interior walls. Our thought is the log exterior walls provide durability and insulation, and are less labor intensive to construct with, while cob interior brings the benefits of thermal mass and temperature, humidity, and air quality regulation. If we design with passive solar in mind, it seems like this could work. However I haven’t seen anyone else doing this so wondering if I’m missing something here.

It's sounds like an interesting concept, but this way you will have to build a house and a half, because of the second set of different material walls. What is enticing here is the fact that after building the log home you would already have a shelter that you could furnish with cob walls, wall at a time. Personally I would try to leave no cavities between the logs and the cob wall. They would be like a 20" plaster, but erected from the bottom. You would have to make wide footings to accommodate both walls. I hope that logs completely covered with cob would be ok - as the clay (and pure lime mortar) is supposedly compatible with wood, but I'm not completely sure in this case, because it's something I never encountered plus in Oklahoma I think the summers are humid.

I did a similar thing in the following roof covering of my home:
-rafters
-diagonal sheathing from 2x8 lumber nailed to rafters with stainless nails
-clay/straw mortar
-ceramic pan and barrel rooftiles
So the mortar is tightly covering the sheathing, but the sheathing has gaps and it helps with ventilation, plus I'm located in a very dry climate.

If you left some cavities then eventually they will be a home to vermin and the cob wall would be a separate entity. On the other hand, the wood works - a lot and I assume you will use green logs for construction that will work even more, and they can easily break the cob wall attached to them. So as usually in case of using various materials, the greatest challenge is making them work together.

Just because nobody has built what you have in mind, does not mean you cannot do it.
I have been involved in many earth homes, but never a log home.
There area few questions;
- Are log homes built with green timber usually?
- I take it you have plenty of clay based soil and suitable trees?
-What size are you looking at?
- Have you thought of compressed earth blocks, called mud bricks in Australia and abode in other places?
- will it be a a self build?

I assume you are not building the log home from a kit.  The one log home I have been familiar with that was built from freshly cut trees did not adequately allow for shrinkage.  While some allowance was made, at 6’3” my hair would brush the bottom of the ceiling joists a few years after it was built.   I would not discount the value of the mass of the logs.  Depending on the thickness of the logs and how well they fit together, the cob may not be needed for mass … but it may be something you desire anyway.

Thinking more about the log/cob. The problem of wood around the cob is the major issue here that may simply not work or may work but will be too complex. If gap would be left between the logs and the cob then, like I said, it would be home to rodents eventually. If the gap was big enough to walk - it would be really a house within a house - not too efficient approach and it would also create problems with windows - the house would be too dark.

How about building all 3 walls with logs and the southern wall with cob - I recommended it in some other discussion for shed renovation. You would have solar gain and nice hybrid solution. On top of that ALL the interior walls (not the interior part of the log walls, but walls that divide rooms) could be made of cob.

Why do you not want a log interior?  I think those interiors are beautiful.  I love the look of wood.

Thanks all. To clarify a little more, we would possibly be using a log home kit that comes with 8-inch dried southern yellow pine logs for exterior walls, and blueprints. So, we would have to do the foundation, roof, and interior dividing walls ourselves. This is where we are considering using cob for the dividing walls only (not cob on log outer walls). Our desire is to not need any kind of heating/air unit but make the home as temperature stable as possible.

The labor and time involved in building only with cob is a concern for us as we both work full time, and my husband works for the railroad, so he's only home a few days a year. So, our thought is to have someone build the foundation, log exterior walls, and roof, then we can pick up and start on the interior dividing walls at our own pace. We do have ample and quality cob budling materials on our land, and stone for fireplaces, it's the time/labor to construct, insulation, and cost balance we're trying to account for.

The home would be probably 2000 sf. We are hoping to move there permanently once built, but right now we live 3 hours away from the site.

That makes a whole lot of sense. Cob for the interior walls will increase your thermal mass and you won't have to worry about the interface between it and the logs except at junctions.

Another technique you might want to consider would be timber framing with light straw infill. You can then finish the light straw with earth or lime plaster and the process would cut your labour and materials input by a fair bit.

R,

That clears a lot and I like your idea. I don't know if with 8" logs and cob walls inside you will have stable temperature without need of heat. Thick wall does not give the same results as a deep underground building or a cave. The former will keep an average daily temperature and latter average yearly temperature, approximately.