Plastering Without Mesh

Written by Andrew Morrison

Over the years through conversations and books I have heard that it is fine to plaster directly onto straw bales without the use of any type of mesh reinforcement as long as the mix is lime or earth based. I recently did somewhat of a test on this by plastering a house that had both: bales covered with welded wire mesh and bales left without mesh. I know I will catch some flack for this, but to those of us involved in the “test” the answer was clear. Plastering bale walls that are covered with welded wire mesh is way better than an unmeshed wall.

straw bale house with one wall plasteredCan it be done without mesh? Yes. Is it better than walls covered with mesh? No. In fact, the walls that were left without mesh had some major problems that needed to be dealt with, and sometimes the only fix was…you guessed it, to add mesh! I understand that some people don’t like the idea of using metal mesh on their house. I know that some want to keep with a more environmentally friendly material like jute (see recent blog post about why this is a bad idea!). Others don’t like being encased in a metal mesh “cage.”

I can’t change your mind on those things (perhaps), but I can at least point out that a house built to last is more environmentally friendly, safer, and more economical than one built on the cheap.

I’ve listed the biggest problems with plastering directly on the bales here.

  1. Here’s mud in your eye! When plastering on walls that have no mesh, the straw is likely to flick the plaster back at you as you apply it to the wall. Not a big deal if it is “mud” in your eye, but it is more of a big deal if you are using lime plaster (which in my mind is almost always the best choice) as lime can and will burn your eyes. In fact, of all the warnings on bags of lime, the most prominent are those warning of direct eye contact. Yes, glass are a cheap fix to this problem, but it is something that is easily fixed by using mesh.
  2. Stuffing. It’s pretty common to need to stuff areas of a wall with loose straw between bales or against a post. Not everyone is perfect with a chainsaw or with measuring and retying a bale. As a result, loose stuffing is often used to pack those gaps. To begin with, it is very important that these gaps be filled with long straw and packed very tightly in place. In fact, a clay slip applied to the packing can be a good idea too. Even further, some folks choose to use cob in the gaps (if they are big enough to warrant doing so) but this brings up other issues with bonding between materials like cob and lime plaster. That’s for another blog post another day. Anyway, back to the stuffing. With no mesh to hold the looser material in place, it can fall out of the wall under the weight of the plaster. Obviously, not good.
  3. Loose straw doesn’t hold plaster. Just like the stuffing issue, the uncut edges of bales can be hard to plaster. In other words, if there is a part of the bale that has long straw hanging out of it, the plaster will stick to the straw and then potentially fall off the wall. It also tends to hold the straw away from the solid surface of the bale meaning that the connection between the plaster and the straw is connected to the loose ends only, not the main bale.
  4. Smooth bales are hard to plaster. On each bale, there are two edges: a cut edge and a folded edge. The folded edge can be very slick, depending on the type of straw used, and as such, it is hard to key the plaster into it. This means that even plaster that appears to be sticking well can ultimately fail if it is not well anchored to the bales.
  5. No reinforcement. Plaster without reinforcement, is not as good as plaster with reinforcement, it’s just a physical reality. Yes, the plaster can get that reinforcement from the bales if properly keyed in; however, as mentioned above, that keying is not as easy to accomplish without mesh. By adding mesh, you are adding tensile strength to the plaster. Like concrete, plaster is very strong under compressive loads (pushing it together) but weak under tensile stress (pulling it apart). The mesh give you extra strength in the tensile stress situations meaning less cracks and overall better plaster.

So I don’t suggest that everyone who says you can plaster directly on the bale is wrong. In fact, I agree that it can be done; however, I believe that it is not as good of an installation as can be accomplished with mesh. I prefer to shoot for the best installation possible. To me, that means mesh is best.

If you’re interested in really learning how to build with bales and you want to have perhaps one of the best weeks of your life in the process, then come to one of our workshops. We ALWAYS have a good time and you will gain the confidence to build your own house too. CLICK HERE to see what workshop locations and dates we have available this year!

59 Responses

  1. Hi Ronald. I have to admit I don’t know what you mean by hydraulic mortar. Is this a cement based material? I would imagine that the galvanized mesh would be fine within most materials, but it would be good to ask the manufacturer as they will know better than me. I have not seen problems with it used in lime nor in cement based plasters here in the US.

    Keep in mind that if you use a material on the exterior of a house that has a slower rate of moisture transfer than the material on the inside, you will end up trapping moisture in your walls. Consider that if the exterior material can allow 6 units of moisture to pass through it per hour and the interior material can pass 10 through it in the same timeframe, the remaining 4 units will be stuck in the wall. In order to counter this, I suggest you thicken the interior material to slow the rate of transfer.

  2. I could be wrong but I assume Ronald Salters means Hydraulic lime Render not Mortar, Given your obvious expertise and passionate advocacy for lime Andrew, I’m surprised you’re not sure what hydraulic lime is…. If the base limestone has above a certain proportion of silicate/alumina. ” impurities”, when fired at the necessary high temperatures, the resulting quick lime will set using water or “hydraulic” as opposed to non -hydraulic lime setting through carbonation. Hydraulic Lime is much less water permeable. Also your cautionary example regarding the rates of moisture transfer only works if you assume both sides of the wall are subject to the same amounts of water, which is rarely the case, 10 units moving in to out , 4 will be trapped? So if it is raining on your external wall you want to be able to transfer that through ? You must have heard of adding “pozzolaic” materials (brick dust, pumice, pulverised fly ash etc..) to non hydraulic lime to improve it’s water-shedding properties? Severe problems can occur when cement is used as a pozzolan.. Anyway good luck Stan, unfortunately here in Oz we don’t have native hydraulic lime so brick dust or some linseed oil in the final lime wash on the exterior may help me balance my interior/exterior water equation!

  3. Thanks Kamal. I have not re read this post, so I don’t remember what I said in it that would have you say I don’t know what hydraulic lime is. I say this because hydraulic lime is pretty much all I use. This must be an older post or something. Anyway, I agree with everything you said; however, I think we are missing the communication on the water transfer through the walls. No, I don’t want rain water to enter the building. What I am talking about is the many cases in which I have helped people fix their plaster in which they have used an earthen plaster on the interior and a lime on the exterior. The moisture that moves through the wall cannot get out as easily as it gets in (from the inside moving outward) and thus gets trapped between the straw and the lime causing plaster delamination from the wall. That’s where I want the plasters to perform in a similar way and by this I mean bringing up the moisture resistance of the interior plaster, not depleting that of the exterior. Cheers and thanks for the input. Much appreciated.

  4. Hi Andrew
    wondering if you’ve used the 2″x4″ welded wire mesh and how it compares to the 2×2 welded wire mesh?
    i would assume that 2×2 is a little better as far as plaster adhesion goes but it seems that one could easily get by with the 2×4 mesh. any insight you have on this would be much appreciated 🙂

  5. Hi James. I have used the 2×4 mesh and although it can work in a pinch, I much prefer the 2×2. The reason being that the 4″ gap allows the mesh to stretch out of shape more easily than the equally divided 2×2 material. Further, the larger gap allows for loose straw to fall out, or at least not be as easily contained. This is especially true when working with stuffing around windows and doors. Hope that helps.

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