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.

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I like the way you always like to shoot for the best installation possible, every time.

I figure it’s like camping gear. You can buy the cheap stuff, but when it really matters, do you want to be out in the wilderness in the cheap stuff that might work, or the more expensive stuff that will work? I buy nice camping gear. 🙂

Andrew, you’re quite right about the mesh! I tested my straw bale home ( strawbalecitadel.com ) that I built from the ground up…you HAVE to put on the mesh IF you want it to really last!

what about plastic mesh? I’ve read a couple books give compelling evidence that it is easier to work with, less expensive and ultimately a better way to build. The house I helped build last month was done without mesh and seemed to work fine, but I wonder if the plastering might have gone faster with mesh.

Plastic mesh can work for some of the details. For example, it does help reinforce the plaster and make it easier to apply; however, it only offers that. Metal mesh, on the other hand, offers the same improvements plus it gives you shear strength, out of plane resistance, the ability to secure cabinetry and more, and many other improvements. Anytime I can find one material that can provide several uses, I consider a good idea.

Andrew,

After doing some strawbale work with you, I must add “labor costs” as the factor which makes no mesh the more expensive alternative. It only takes a few walls to redo, or a few plaster problems down the road to completely negate the saving from not buying and using mesh. After all, the point of a strawbale home is to last generations, is it not? As most of your readers know, there are no 200 year old stick built homes, but there are a number of 1000 year old timber frame and earth plaster homes.

I still dream of building a load bearing straw bale structure on our acreage, but right now I’m trying to figure out a way to use plastered straw “flakes” as insulation for a north-eastern exposure room.

Has anyone else tried this approach, using small mudded flakes as insulation, rather than fiberglass or the chemical foam stuff?

Diana, you won’t get very good insulation value in this way because the straw has a relatively low insulation per inch rating. If you only have 3.5 or 5.5 inches to work with, you will only get R-12 at best (for the 5.5″ wall). Compare this with fiberglass insulation which would give you R-21 for the same thickness. You really need the full bale width in order to get the improved insulation values.

Here in Mexico, we use a very watery plaster to get in the cracks and to begin the covering process of the bales, throwing it at them fairly hard. The moisture seems to be the key and we shoot for a very thin coat to begin adhesion,sort of painting them with mud. The following application can then be much thicker once a base is formed. I’ve never felt it necessary to use mesh on bales and our plaster is holding very well.

Hi Randy. I can see how that would improve adhesion. One thing to keep in mind though is that the more water you add to a plaster, the weaker it becomes. This is because as the plaster dries, the water molecules evaporate which leaves “holes” in the plaster. This is obviously on the molecular level, not holes that you will see with your eyes. This is less of an issue with earthen plasters than it is with lime or cement based plaster.

Thank you for the info, Andrew. I built my first straw bale structure last summer here in the wet Willamette Valley of Oregon – a 30ft x 16ft load bearing greenhouse (yes, one 30ft continuous wall) and used an earthen plaster directly onto the bales. I broke quite a few ‘rules’ with the structure, but thought of it as a great learning experience and it is doing a fantastic job, I love it!!! However, on the very front, where the weather really hits it (south facing), I didn’t have rain gutters up yet, didn’t have time to do the finish… Read more »

Andrew, you do realize that wire mesh rusts inside the walls anyhow in moist climates….right? And your results in Canada were just because that was what you wanted to see (experimenter bias…very common). I would be happy to share pictures of my house (no wire mesh) built by complete novices (my Amish friends) with perfect walls and NO damage after 8 years. There is NO need for the extra expense and labor to put wire (which is not eco-friendly anyhow) in your strawbale house. You just don’t know how to do it correctly. I suggest that anyone interested work with… Read more »

Hi Barbara. Thanks for your message. I don’t agree though. Your first point that the mesh will rust is true if you use a mesh that is not galvanized. Using standard metal mesh would indeed rust, however, homes that we built over 15 years ago with galvanized mesh are still in great shape and have not issues at all. Like I said, you can build without mesh and have success, but I don’t think it’s the best way to build for the best quality home overall. Again, it’s more than just the plaster reinforcement. It does so much more to… Read more »

Hey Randy, perhaps you’ve covered this before, but is there an issue with the metal mesh interfering with either wireless signals or cell phone signals? What do the EMF/healthy house folks say about metal mesh?

I’ve heard both sides of it: it’s not good for such signals and it has not effect. I have not had problems with cell signals getting though the homes we have built this way. I am not an expert on EMFs so I can’t really comment on that. I have heard some things, like I say, on both sides of the discussion, but I don’t have enough expertise to give an informed answer. Perhaps some else here does.

Hello Andrew, have you seen/used earthen plaster applied with compressed air? The same way Gunite is applied.. Not quite the low-tech, friends & family approach, but this method is supposed to really drive the first layer into the bales for better adhesion without mesh. I have a small SB playhouse I built 8 yrs ago, the clay plaster is still holding on – I kneaded a thin pure clay mixture into the bales as far as possible with my fingers (using rubber gloves), and let dry before applying the next layer of local clay, sand, and chopped straw. Thanks for… Read more »

Hi John. I have indeed and I have used one myself. In fact, I own a small sprayer that will shoot both earth and lime plasters with ease. It does do a good job of injecting the plaster into the bales, for sure. I also find that a crew of people with good plastering skills can get just as good adhesion without the sprayer and simply applying the plaster with hawk and trowel. Both ways work well if you know what you’re doing. They are fun though! 🙂

Do codes REQUIRE

Where codes for straw bale construction exist, they typically require mesh for plaster that is cement based but not for earthen or lime based plasters.

Do codes REQUIRE mesh, is the question, maybe addressed already, but I didn’t read…If not, its a hugh expense OFF of the overall costs of building. I think as long as you ‘shave’ (weed-wack) the cut sides of the bales (stacked so the cut sides face the inside and outside of the building) then plaster should adhere very well. There is always going to be maintenance with any plaster application. I think the size of the building should be considered: Large spaces would need more support and strenth, small cozy ‘cabins’ likely would be fine with plaster directly on the… Read more »

Hi Kelly. See my answer below about codes and mesh/plaster. You bring up a good point about the size of the structure. I agree that small cabins are more easily handled without mesh (to some extent) and that larger buildings with 12′ tall walls need more help. I do the vast majority of my work in full scale homes often with tall (10′ or higher) walls. In terms of facing the cut sides of the bales inside and out, that’s impossible. You can only have them face one way or the other and the side without the cut edges will… Read more »

Kelly (#12) could have meant that bales get stacked alternately-first bale has the cut-side in, next bale has the cut-side out, the row above would be opposite and the entire wall could have a checker board pattern for better adhesion of the plaster. Just a thought – would that work?

Hi Christa. The biggest issues with doing it this way are that the cut side behaves differently than the folded side when weed whacking and you increase your chances of overly cutting the walls when moving back and forth from cut to folded sides. Secondly, the strings are often off center on the tops of the bales and so it’s a good idea to put the side that has more room between the edge of the bale and the strings facing towards the outside (assuming an exterior frame that needs to be notched around). That extra space between bale edge… Read more »

About the watery plaster I spoke of, that’s only to get a bare minimum coating on the bales(1/4″ or so), then we go to normal mix, to not have the bounce off phenomenon when tossing the mud on that very first coat. And that’s using a soil-base plaster. I think it should be discussed what type of mesh you use, also. Thin poultry netting can’t give the strength that thicker mesh brings to the building. I like to use the 8’x 20′ ridgid panels of 1/4″ steel rebar(like livestock panels)sold here in Mexico. They are quite strong and make a… Read more »

That sounds totally reasonable Randy. Especially with an earth plaster, but even with lime, s thin coat like that would be okay to water down a bit to achieve some better penetration. I use a 2″ x 2″ 14 gauge welded wire mesh. It is galvanized and structurally very strong. It adds all kinds of benefits to the structure. Like you say, the poultry netting or even thin gauge wire does not accomplish much of anything in my opinion. As for the rusting of the metal, what came to mind as I read your response was the rebar in every… Read more »

Andrew, love a good strawbale blog. Keep up the good work and we will keep reading. We own some of your videos and when the time is right we will use your methods to build or strawbale home (northern AZ). Although everyone has questions that come to mind in regards to strawbale building, it seems that sooner or later they are addressed in one of your articles or blogs. Thanks Again

Thanks Bruce and Denise for the kind feedback.

Hi Andrew Love this new site for us all to have our say! You and Gabriella have done such a great job! I have been living in my Straw Bale house for four years now. We used tenax mesh (very strong, plastic mesh, imported from Germany, for those who are not familiar). I can honestly say that when I saw the house baled, and then saw it with the mesh applied and secured, prior to plastering, it made me feel better about the structural integrity of the house from the angle of the mesh doing exactly what you have said… Read more »

Hi, in regards to points 12, 24 and 26. Firstly let me say, that I have not built my house yet, (I am still hoping to get Andrew back to the land of Oz (Aus), I have however purchased nearly all of the materials, I have a habit of getting 10% more than needed (just incase) and for my rather large retrofit shed (75ft X 25ft) I only needed $1200aud this seems a small price to pay for ensuring I have the best chance of success. In regards to reo in concrete, only if exposed to the elements OR poor… Read more »

Hi Graham. Thanks for your feedback. In terms of the decomposed granite and plaster options, it’s hard to say what to do there. Part of the art of plaster is actually science/chemistry so making sure things respond to other aspects within the plaster is important. I am a big fan of lime plaster because of the research I have done and the experts I have directly spoken with or worked with. I am somewhat of a purest when it comes to plaster, but I am open to new ideas and recipes. That’s about all I can say about that from… Read more »

I was a participant to a great Ontario workshop last year. The only thing that gets me worried about mesh is that some parts of the mesh were not flush with the bales, even after patching with loose straw. This meant that when plastering, you could not even reach the bales with plaster and pushing on theses areas after plastering was like pushing on a spring. I am guessing there is a lot of air between the bales and plaster and that the mesh alone holds the plaster. I can’t believe this can be good so I think that unless… Read more »

Hi Pierre. Your point is a good one based on what yeas at that build; however, keep in mind that the toes ups were placed in the wrong spot for the size bales that the host had and so several sections, the ones you refer to, had to be stuffed with extra straw to fill in for the gap. I whole heatedly agree with you that this was not a great situation and that it held the mesh away from the bales, in some cases so much so that the plaster was not adhered to the bales. This is NOT… Read more »

One way non-mesh plasterers work is:
Thick Earthen Plaster base coat. Rough chopped straw as fiber.

Lime plaster finish coat if you like. 3/4″ thick I think.

If it’s not working – the system isn’t the problem, it’s your technique, skill or attitude. Change is not always easy, and you have convinced yourself not to change!

Keep trying! Save metal mesh for the chicken housing!

Hi Brian. Again, you are looking at this topic, from what I can tell, only from the point of view of plaster strength and nothing else. There are so many other factors to consider as I have continually said. Furthermore, the use of clay plaster under lime plaster brings in a whle new topic which can and I imagine will be expanded into here. As always, there are many schools of thought on this topic. The one I have heard most and from people I know and trust who have years of experience in plastering is that using clay/earth plaster… Read more »

Ha you have me reassured Andrew. You can be sure I will focus on the quality of my walls either way.
Pierre

Hi Andrew, I’ve been building straw bale and other natural buildings professionally for the past 13 years. One of your clients directed me to your site. I would love to invite you to “inspect” some lovely un-meshed clay plastered straw bale homes we’ve built here in the Kootenay region of BC. I firmly believe that properly designed, high fiber, clay based plasters applied directly to properly prepared straw bale walls are the strongest and healthiest choice for straw homes. There are many reasons why I would NEVER use metal mesh under an earthen plaster. Here’s some: As you know, heat,… Read more »

Hi Peggy. Thanks for the detailed response. I appreciate the input. I don’t know that I’ll have time to view homes when I’m up in Castlegar, but I do appreciate the invitation. I still have questions about how all this works as many people have said the same to me and I don’t fully grasp how a plaster without reinforcement can be stronger than one with it. I do hear your points and they make sense; however, in the overall picture, I still don’t get it. Like you say, perhaps seeing is believing. I plan to get some hands on… Read more »

I am in Australia and we rendered lime mix and it was pumped ,sprayed on and penatrated into the bales and gaps and saved a lot of gap filling so the scratch coat adhered well.We also pumped the second coat approx 40mm ,the color coat was done by hand.As far as time was cocerned.2 days for scatch coat inside and out,3 days for second coat.This house is 33mtres by 11mtrs 30+ sqares I would not do it any other way.

Still plastering … weekend warriors, Jan, Michael and Tony, the Trinity plasters..LOL In regards to the wire being away from the bales Andrew’s suggestion at our workshop was very helpful. we used landscape pins to snug it up. After everyone left and only an outside wall and a half was scratched coated, we went around and detailed before we continued. re stuffing, re securing loose wires, snugging up where bale and wire didn’t cozy up, Took us a long time but now as we are doing the inside we really see the difference as we are plastering. And our plastering… Read more »

Here in South Africa it is quite common to dip the bales in a slurry before stacking them. This overcomes the “getting plaster to stick” issue. Is this done at all in USA or Canada etc?
I found a letter on your site by Andy Horn, an eco architect who developed the idea . . “I have developed my own system that involves a dipping / pre-coating method that greatly sppeds overall construction and makes for a very solid construction.See under projects on our web site http://www.ecodesignarchitects.co.za

Thank you

Bruce

This has been done and it does help the plaster stick; however, it is slow and messy and creates some other issues along the way which are otherwise avoidable when not using the dip method. If you have issues with the plaster sticking, it may be your recipe. Another thought is to use a sprayed on slip after the walls are ready to go and before the plastering begins.

Would the use of a Galvanised flat strap brace that is tensioned and fastened to the outside of the post accomplish the structural support you look for in the wire mesh to prevent racking/side way movement of the wall. I have health concerns about the EM Fields within a house – to be reduced where possible. Maybe replace with a plastic or equivalent in-lue of wire. I see the benefits to using some wire in some areas e.g. corner

Yes, the strapping would provide for the shear strength values. Some other mesh could be used for the other details like cabinet hanging, shaping corners, etc… I have used a plastic mesh in the past which is okay. Check out the mesh provided by Tenax.

I am looking for someone in the northen AL region who has built a strawbale house/structure. My boyfriend and I have discussed building one for over a year now and it would be great to have someone locally to discuss the basics with (foundation, best place to get bales from, dealing with the humid climate, etc.).

Dear Andrew,
I’m buildinng a brewery with straw bales in the south of Chile (a lot of rain). On the ouside I will build a wood wall over the straw bales to avoid water come into the bales. So considering this, my question is the following: is it necessary or recomendable to plaster the straw bales outside, between the straw and the wood? On the inside we will plaster it with a mix of lime and cement because it has to resist moisture and water.

Tks. and rgds.,

Andreas

Hi Andreas. Yes, plaster the bales behind the wood to protect them from flame spread. I would suggest using lime plaster and not lime/cement. Moisture will find it’s way in one way or another. The key is to provide for it to get back out. Cement will trap moisture where as lime will help remove it from the wall through its breathability and its hygroscopic properties.

“As for the rusting of the metal, what came to mind as I read your response was the rebar in every concrete pour done in the US and the world over. Rebar is required. It only rusts if it is exposed to the elements and it stays strong inside the concrete.” Hate to burst your bubble but here in TX, #9 welded mesh is often used in sidewalks and driveways. And even in Austin which is relatively dry, it rots out in about 20 years. 1/2″ rebar takes 50 or so – if the metal is of good quality and… Read more »

Hi Walter. I’ve never used the aluminum mesh. No worries about the bubble bursting. I would venture to guess that the mesh (or rebar) has been installed too close to the surface of the concrete. Another potential is in the fact that these are sidewalks and chances are there is no capillary break between the concrete and the ground. This means that the concrete can wick up as much moisture as it wants to, leading to the accelerated degradation of the mesh/rebar. In a house, a vapor barrier is required to break the capillary transfer of moisture. In a wall,… Read more »

Just completed the outside rendering on my spiral. You can can find pictures on facebook. “spiral straw bale build in bekesbourne” We complete the inside by hand ind two weekends time 8th October. Render was blown in using a mix of NHL3.5 and sharp sand, a hand fed render gun and a 2.5 bar diesel site compressor. The joins between the window boxes and straw were bridged using stainless EML (expanded metal lath) in fact all boundaries were bridged using mesh. penetration into unmeshed bales is > 50mm in unmeshed areas. mesh actually inhibited penetration to a large extent. It… Read more »

Hi Andrew,
Here in the Netherlands using lime mortar on an external wall is asking for trouble. Our climate is simply too wet.
Therefore we are applying hydraulic mortar on the outside and lime mortar on the inside of our eco building.
What will happen with galvanized mesh on the outside in combination with hydraulic mortar. Will the mesh rust more rapidly ? Is this a good and future proof combination ? Mesh will be fixed to the bales and then three layers of hydraulkic mortar wil be applied. Total thickness of 3-4 cm ( 1,5 inch).
Regards

Ronald

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… Read more »

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… Read more »

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… Read more »

How to use mechanical supports for plastering

What kind of mesh do I need to use if I want to plaster a straw bale house with lime, or cement? Thanks, Nolan

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 🙂