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Cob Internal Walls

By October 16, 2015May 9th, 2022No. 68, Fall 2015

Building natural homes for a job in the modern world involves compromises, like all things in life. The one compromise that upsets me the most is the use of sheetrock or plasterboard on interior walls. I get the privilege of building natural homes for a job, yet I still oversee or personally install an industrialised product using suspect mud into the same homes. Worse still the materials are a really great medium for the growth of mould. Oh but wait a minute, we can prime it with zero VOC primer that has some sand mixed into it and plaster it all with clay and sand, over the sheetrock, wow I’m getting dizzy; perhaps it’s the fumes.

First step in using cob is excavating clay and mixing it with straw

First step in using cob is excavating clay and mixing it with straw

So where do we go from here? Wattle & Daub, lath & plaster, paperless sheetrock, light straw clay, straw wattle walls, tamped dry straw, straw clay blocks; wow I’m getting dizzy again from the range of choices. In an attempt to start sharing details of successful alternatives to sheetrock I thought we could look at one of the methods that I have used and really like, which is internal cob walls in a conventional framework. It is so easy to over complicate everything, even the name of this quite simple and elegant wall system, so let’s stick with “internal cob walls.”

Internal cob walls start with whatever the conventional norm is for stick frame construction in your area. This may have been framed before the decision was made to do internal cob, it does not really matter what the framing is, as long as it is code approved in your area. To install the cob, one side of the wall is covered with plywood or similar. Once again the type of plywood or thickness is not too important. I tend to use ½ inch plywood or OSB, other particle board or even wood could be used. The plywood simply stops the cob from falling out the backside of the wall while it is being installed. Next a whole lot of nails or screws are put into the frame for the cob to stick to. Electric cables are best put into conduits for ease of modification later. Then fill the bays with cob from the open side. The cob mix has to be stiff enough to not slump, but wet enough to screed off. I use a scrap piece of 2 by 4 to screed the cob flush with the studs as each bay is getting filled. Leave for 24 to 48 hours and strip the forms. Once dry the walls get plastered.

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The use of the term cob here is fairly loose, pretty much any mix of clay, sand and straw that can be mixed quickly, thrown into the wall and screeded off works. It is of little concern if the mix cracks as we are letting it dry out before plastering. The main thing is that it stays well bonded to the nails or screws in the frame. I like to try not to process the subsoil much at all if I can help it to save time and labour when making this mix. Often soil dug from the foundations can be mixed with water and straw, with no other processing. A machine can be used to mix the cob quickly if available, bobcats and front end loaders work great. The key is to design a mix that can be made quickly, installed quickly and provide a solid base for plastering. Trial and error will be your guide.

Depending on the weather the formwork comes off 24 to 48 hours after filling. This allows good air movement to each side of the wall, to facilitate drying. Fans can be positioned either side of the wall to speed up the drying if needed. Once dry the plastering can start. It is a good idea to take a core sample of the wall in a few spots to make sure it is dry. An old auger bit on a bit and brace works well to take core samples. The walls may have cracked up pretty bad on drying, this is not a concern if they are solid. The plaster will fill in the voids and create a flat surface. Plaster stops should be installed at the base, top and any openings of the walls to define the final size and shape of the wall. I typically do 1 inch plaster stops as this gives me ¾ of an inch of base coat plaster and ¼ of an inch of top coat plaster. These plasters can be hand or machine applied.

The main issue to try to overcome with plastering these internal cob walls is the amount of wood studs in the wall. Typically plaster will crack at the junction between mud and wood, this is as they are dissimilar materials. Dissimilar materials expand and contract at different amounts when exposed to humidity changes. Wood also typically shrinks over its life, where mud does not. Some type of bridging is needed over the wood to create crack free plaster. A high amount of long straw in the base coat plaster can be effective in bridging over all the wood, but to be sure some type of mesh or netting is the go.

As metal mesh should never be avoided with clay plaster, a burlap, jute, plastic or fibreglass netting should be used. This netting is embedded into the base coat of plaster, all over the whole wall while it is still wet. Some people push it in with their hands, while others find a wood float is the go. The netting should not be visible once pushed in to ensure a strong bond. If the plaster has dried out a little too much it may be necessary to trowel a little more fresh mud over the top of the netting, or if it is too dry underneath the netting. A sand clay or straw clay plaster will work with any of the types of netting, as long as the netting is well embedded.   Once dry any clay based top coat can be used.

Finished Internal cob wall

Internal cob walls work well because they are simple to install and totally bomb proof once dry. Are they the fastest or cheapest option available? Who knows? One thing I know is that you cannot really go too wrong, so have a go.

James Henderson has built straw and earth homes for clients in Australia and the USA.  Working as a full time General Contractor in Washington State USA, James can be contacted at (360) 460-3484 or naturalbuildingsupplies@zoho.com.

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