How to calculate wood/mortar need in a cordwood building

Hello everyone,

I am in the very early stages on planning our next home. A mortgage free cordwood home in the country! I know this will be a long and exhausting process, but we're looking forward to it.

Anyway, we are working on coming up with a design to use. We have pretty much decided upon a round (or octagonal, hexagonal, etc) design. I am wondering which would be easier and use less materials, One big structure or several smaller ones linked together.

How do you determine how many cords of wood and how much lime/mortar you will need for a given wall size? I'd like to be able to "crunch the numbers" for each design idea we have so we can see the difference it would make in materials needed.

Any other suggestions for one structure vs several smaller ones?

Thank you all!
David

A large circular structure is going to have the smallest amount of wall for a given floor area. That said, I personally wouldn't want that design, as I never had the desire to run away and join the circus to live in the big top.

If you build smaller structure and link them together, it may take more materials, but it is also more suitable for an owner builder, since you don't have to complete one big project before you move in. You can take your time building more modules and adding them on.

If you haven't already been to this website cordwood construction by all means, go check it out. The amount of mortar will vary building to building how much separation of the logs are you planning to use? diameter of the logs is consistent? Length of walls? Also are you going to add some sawdust to the mortar mix? Remember you are using two 3" wide runs of mortar separated by saw dust or other insulation (I like spray foam but there is a definite learning curve to using it effectively). Day creek has lots of good information and a list of wonderful books on this type of building. Rob Roy and Richard Flatau are the leading experts and have a few good books on the market. All that being said, the type of construction, multiple build or single build depends on what you want in the end. Like John Elliot says, the mono structure takes longer to move into than a multi build structure. Round houses tend to be more efficient at being heated by wood stoves or furnaces since the outer wall will normally be equidistant from the heat source, un like a rectangle or square building. If you build multi-structures with connecting halls, each build would need its own heat source for effective heating. To me it is a matter of doing what you want in the end. Your the builder, make the dwelling the way you want it.

I will use my current journey as an example here. I am in the process of building a 28' x 40' round timber home, currently we live in an RV at one location and we have a 20' antique travel trailer up on our land to live in on the weekends while we get the spaces we need cleared finished. We will be moving onto our land by the spring of 2015. There was a home on the site years ago and the footing of that structure is in good enough shape for re-use but I will have to build piers for the timber frame to stand on and for floor joist support. I am cutting and debarking proper sized trees (white oaks and hickories) for the bents. These will by necessity dry for a year before I can put them up. Oak can twist when going from green to dry so to use them in the normal green wood method of building, they could turn out badly, hence I will let them air dry for a year before working them. The hickories will be worked more green than dry because they won't have the propensity to twist nearly as much as the white oaks. I am using these woods because that is what I have for the cutting. I do have some cedars stands but these trees are sacred to my people and the would not be substantial enough even if I desired to utilize them for anything other than fence posts, so better to honor them by making them grow well.

Hi everyone, sorry if some of these views are counter to others thoughts or experiences...

D. Brown, I wish you all the good luck there is for your project! Building is fun, but can be very challenging as well.

If one has not, "lived in the round"....before, I would strongly suggest thinking twice about building a round structure like this to live in for the first time. They can be very compelling to "look at" but living in them is another matter and what I tell students/clients I teach and consult with...round is for tents...squares and rectangles are for more permanent architecture. This is a "rough rule" and as much tangible logistics of building, as it is personal preference. I have been in some lovely "free flow" architecture that I could easily have enjoyed living in; most...not so much.

Having known Rob (Roy) for some time, and debated the history and dynamics of "stack would" or "infill" methods of building in length over the years, I think I have a pretty good handle on it modality. I really like Rob's enthusiasm and willingness to help, yet even he will confess this has been as much a "living experiment" as much as or more so than following and understanding traditional methods of building this way. He has roll modeled much successes in his many books, and also quite a bit of "concept" as well...Many books on what is being called "cord wood" building are more "guides" to it than actual "how to manuals." You can also find much here at Permies by doing search for "cord wood" in general or under specific members.

I have so many links and documents on this topic, which I wouldn't know really where to start in offering more "research based" advice. The Kubbhus style of architecture has been around for a very long time and Dhajji Dewari modalities of architecture may well have been around for several thousand years. In either case I seldom, accept for small structures, recommend a full "stack wall" system for building and only use it as an infill.

Traditionally some form of Cobb (clay) is used as the mortar or a lime based mortar...never anything that has any opc in it which is still wrongly recommended by too many. OPC even in small quantities leads to rot when in contact with wood. If a "light straw clay" mortar can be employed with this system of architecture...it may be all the better for it; as it also is more insulating in properties. I also do not recommend "foams" as they sometime (often?) trap interstitial wall moisture against the wood. I have used foams...but generally do not recommend them in most systems...especially for DIYers with little background in traditional architectural systems.

As for drying wood...this can go both directions. When they shrink...you get gaps. When they expand...they can push a building over. I have seen both and know that a timber frame armature greatly mitigates this aspect of the architecture. I would also point out that most (almost all) traditional building is done with "green" (wet) wood. It is not (accept for species like the Conifers) to expect wood that is greater than 100 mm in diameter or square to dry out to any great degree for some time. So a white oak log round, or timber 200 mm (~8") square or round will take quite a few years to actually "dry out." and something in the 200 mm range, depending on environment, could take as long as ten years. This also makes the wood very difficult to joint. As has been suggested...wood can move a great deal, yet this is where understanding wood movement and dynamics among the different species come into play by the Timberwright working the timber. Yellow Birch is probably one of the strongest and hardest of wood species used in old timber frames, and nothing I know of "twists and turns" worse for a species (few even come close to its cantankerous nature.) Know the wood and learning to read its grain, as well as, layout and jointing methods is the goal...not trying to dry it completely. I also do not recommend "hard woods" for any of the infill methods, unless it is a "double wall system and the hardwood cordwood sections are going on the inside of the structure.

That should give you much to consider and read through. Let me know if there is anything else I can specifically try to answer for you.

Regards,

j


some more reading perhaps...Cord Wood Architecture.

Interesting thoughts as always Jay. Whats the idea of avoiding hardwoods except to interior of double wall systems?

Hardwoods just do not have the insulative capacity of softwoods. So if considering U factor, placing them on the inside is much better with some form of traditional or modern "permeable" insulative layer perhaps placed on the outside. If I was to facilitate just a "hardwood stacked wood" infill wall, I would insist on it being a minimum of 300 mm thick and prefer 500 mm, with a plastered finish of some form on at least one side of it. This is a very "romantic" system that has great aesthetics...seldom do I see it done as well as it could (should?) be.

ThThenank you all for you info.

I am very concerned about air leakage.

I am thinking I will build a (8"ish) cordwood wall. Then an inch or so of spray foam to insure everything is properly air sealed, then build my interior cordwood wall. I may even do the exterior cordwood, 3-4" of spray foam then some other interior finish.

A) would there be a problem spray foaming the cordwood? Moisture issue, logs expanding/contracting cracking the spray foam and creating leaks, etc.

B) could I plaster directly over the spray foam? Chicken wire needed?

Thanks again

A) would there be a problem spray foaming the cordwood? Moisture issue, logs expanding/contracting cracking the spray foam and creating leaks, etc.

That is a very BIG YES!!! Foam and wood contact exposed to any type of moisture...interstitial or otherwise ALWAYS leads to decay. I will not allow foam to have direct contact with wood without some form of constant air circulation..and even then there is often issues in time with decay. If you are concerned with "air infiltration" (drafts) use the cord wood in a traditional fashion as I suggested above. Hardwood to the inside, and a soft wood layer to the outside with Cobb mortar and plaster. If this is done properly you have a "draft free" structure that is still "permeable" for moisture. With effort and good design, a "net zero" structure can be facilitate this way.

B) could I plaster directly over the spray foam? Chicken wire needed?

The short answer is NO. The long answer is addressed above and in other areas on plastering modalities on the net and here on Permies. Simply put...there other natural/traditional systems that are far superior to both these concepts suggested.

Good luck, and keep us posted,

j