Cob as a replacement of wooden beams?

My goal is to build smart, eco-friendly and with no more cost than necessary for a good home. I am still in an early fase. No decisions have been made yet.

I did some initial ‘research’ to cob building. There are many advantages, but also a few properties to be taken into account:
- almost no insulation value (there are not enough sun hours for a passive solar solution in our winters);
- labor intensive;
- needs time to dry/settle.

First I was looking for a way to insulate the outer shell of the cob construction. To me that makes the most sense. I am not convinced so far of a good technique. I want it to be a natural material, and it must breathe. For example: straw bales add significant thickness to an already thick wall.

An alternative is a wooden construction, fill in straw bales for insulation and cob only for the interior (thermal mass). However, that adds costs (wood).

A construction where the straw bales bear the load of the roof will likely not get a building permit in my country.

So then I thougth about how to use cob as a replacement for wooden beams. For example: would it be possible/wise to make vertical cob-beams that can handle the roof load? Maybe by making a casing of thin wood around it? Could this make a strong building?

I would love to hear some thoughts about this! Any input is welcome and much appreciated. Thanks!

Cob cannot replace wood under tensile load.
Try pulling a piece of wood (even just a tiny branch) apart and compare it with cob.

However you are on the right track with surrounding cob/straw bales with wood.


http://www.habitatvegetal.com/fr/construire-en-paille/theorie

He has build several straw-bale houses in the Netherlands, so that should definitely work.

both bale and cob can adequately support a roof if built correctly, though if you have strict building codes I doubt you will get either approved as load bearing structures. Cob is much more rigid than bales and can support quite a bit of weight when built organically--round/curved walls. Bales and cob together is called balecob and is great for cold climate housing as it provides strong thermal mass that will support your roof and huge insulation without the bales compressing over time. Balecob requires a huge amount of cob to the inside of the walls and a thick plaster on the outside each filling the cracks between the bales almost all the way through to provide strength and rigidity. I've attached a few photos below.

When you say to replace a beam, I think of a beam as spanning a horizontal distance across a gap, say left to right. Cob won't do that for you. But a beam, that is vertical, relies on compressive strength and cob can do that, but I don't know that it would work as well or perhaps as conveniently as wood.

Wood is certainly a green/eco option, you can grow your own of course. So if you have suitable trees and it's allowed I think that would be the way to go. Bale-cob is one option, as is an earth-bermed building along the wofati/Oehler lines. If John Hait's PAHS system is used to keep the bermed earth dry it would likely insulate even better (something a wofati includes).

Thanks all.

@Sebastian.
A clear argument. I was expecting it not to work as a horizontal solution, but thought maybe a vertical beam (maybe the wrong word) could.

The Cell Under Tension method is appealing, since the construction bears the load while using less wood than conventional (I also liked his tire-foundation!).

I have a meeting with the local authorities in two weeks. I will mention this method to see if they are familiar with it.

@Daniel.
So the bale cob is a method to incapsulate the bale in cob. How thick will the wall become?
My concern is that during the building process the straw bale will be exposed to rain, since adding cob takes time.

@Mark.
Ah yes. Beam indicates a horizontal line; I wasn’t aware of that :). I was thinking vertical.
Wood is the safe way to go, but I am afraid of the cost.
I will look into the earth-bermed option, but it doesn’t appeal to me directly.

Any alternative suggestions of insulating cob?

Light straw Clay is another option, you mix loose straw with clay slip and pack it into a form, then move the form once it sets up. Similar insulation as bales, around R2 per inch, and you make the walls as thick as you want. I would just go with full bales though and do bale cob personally. Just put up the roof with timber support first, and then infill with the bales. Keeps the rain off them that way, and it's up to you how much cob you add for thermal mass. Do you have trees on the property that you can use?

My suggestion is to really study some building terms before talking with your building code team. I say that because a vertical structural member is called a Column and is a fairly basic structural term. I realize english may be a 2nd language here, and may be part of the problem, but I fear that if you start discussing structural terms outside of normal building methods without knowing the basics, it will be a huge red flag to the very people you are trying to convince.

This is like me being a sheep farmer and someone comes up to me and starts talking about how I should do this or that on my farm and refers to my flock of sheep as a herd. Yes it is a small misnomer, BUT sheep are called flocks and instantly (right or wrong) I dismiss them as someone's advice I should take.

If you make the mistake of asking for a code variance for a cob column by calling it a cob beam, you might be denied immediately no matter how convincing your argument is.

To that end, I would think one hurdle would be that cob can be very inconsistent in construction. You might want toprepare arguments on how you are going to ensure yours will be very consistent and that it can handle x-amounts of loads and why.

@Mark.
A hybrid between Cob and Light Straw Clay doesn'’t make sense to me. LSC as an insulate infill seems more logical, but than I would prefer straw bales (as you mentioned as well).

@Travis.
You are absolutely right and thanks for the heads-up. I am still inexperienced and with only a very basic understanding of building, so I hope this first meeting doesn'’t come too soon.
To be any kind persuasive, I will discuss a normal wood-skeleton with straw as insulation. However I will also ask for the possibility to file in a plan where the traditional wood-skeleton is replaced by the CUT-method and the tire-foundation. Just to see how much room I have.

Protecting bales and/or cob from rain has been mentioned a few times here. In a damp climate it could be a huge consideration. Putting up a basic wood frame to support a roof first seems like an essential step, as well as likely needed to satisfy building officials. Most of the wood would be needed anyway in the roof itself and interior supports, and posts need not be huge timbers.

Light straw-clay is supposed to have a maximum thickness of about 12"/30cm, in order to allow it to dry in a reasonable time.

Not sure what you mean by 'posts'.

Indeed. However if the straw is needed for the roof support, that poses a problem if the objective is to use as little wood as necessary. Maybe a preliminary roof will suffice.

hau Jasper.  Perhaps a little nomenclature for you for clarity think of timber framing since there is quite a lot of that in the Netherlands.
A post is equivalent to a column = vertical element of the structure.
A beam is a main Horizontal supporting element
A Girt is a minor horizontal supporting element
A Purlon is a connecting element that joins one rafter to other rafters of the roof

you can get an entire glossary of builders terms with a google search (I like to use Timber Framing for the search).


Cob has been used in Ireland for centuries and there are many of the old houses that it is the cob wall that supports the roof structure without any posts or columns being used, these walls are generally 31 cm thick at the top that supports the roof structure and 62 to 80 cm thick at the base (foot) of the wall.
Doors and windows are framed in with 3 cm planks for fastening the door or window in place.