Plaster Mesh
Questions answered by Kelly Hart

Q: Can you tell me if you think I need chicken wire or similar for plastering the inside and outside of the earthbag dome?

A: I would definitely suggest using mesh on the outside to help keep the plaster intact over the years. On the inside it is not as crucial, and it is harder to attach, so it is optional.

Q: How can a slick plastic bag that gets brittle from exposure to the elements promote adequate adhesion to any surface material unless it's solidly mechanically anchored?

A: Because of all the deep crevasses inherent in earthbag walls, I think that plaster has more to hold onto than just the plastic. Also, wire netting and other wire will help a lot in keeping the integrity of the wall system.

Q: How to attach the netting/lathe to the bags? I've read that strawbalers use some kind of "pin" to attach netting to their bales, but I'm thinking you would want something rough or with some bends in it to keep from easily pulling out. Any thoughts or other ideas for 'pinning' the poultry netting to the bags?

A: I have used large wire staples of the sort that strawbalers use, and they work pretty well if you hammer them in at angles so they don't pull out easily.

Q: Could you describe these 'staples' for me?  (length, size of wire, width of the bend, etc.) so I have a better idea what you're talking about?

A: I manufactured the staples myself, using rather heavy duty wire that you can barely bend with your hands. The U shape has ends that are about 6 to 8 inches long and have about 1 to 2 inches between them, and you can usually pound these in with a hammer or large stone.

How do you attach the chicken wire?

I made large wire "staples" out of heavy gauge wire bent in a U shape and pounded them into the bags to hold the wire in critical places. Or, if you are not lining the bags with plastic before plastering, you can periodically lay pieces of twine across the bags when you lay them and use these later to tie up the mesh.

Wouldn't pounding metal staples through your plastic waterproofing defeat the purpose?

If you do it carefully, they are nearly self sealing, and if not you can seal them with a bit of silicon caulk. Also, you use them sparingly.

Q: I've been looking at hay bale net wrap for a while now as an alternative to chicken wire. The problem is that it comes in 48 and 60 inch widths, which is hard to work with. I found some pallet wrap that comes in 20 inch widths, and even has a handheld roller. I know that many people just stucco right to the sandbags, too. Please share your input on this.

A: I have done both, using chicken wire mesh and none at all, and they have both worked. I did place poly twine around three courses of bags and tied it periodically, so that there was some degree of positive grip...and I'm sure this helped in the areas without the mesh. I used the chicken wire on the outside where I knew it would see more weather. And I was using papercrete plaster, so I kept the mesh to nothing less that the 2" wire, since anything smaller tended to gum up too soon. Owen has used old fish net and says that it also works well. I expect that ultimately the mesh will increase the longevity of the plaster, but this is just a guess.

Q: I read somewhere that earthen plasters don't need the chicken wire/lathe cover (because the clay sticks to the bags just fine) but lime plasters still need the support of the wire. Is that correct? Does that include the "cradles" mentioned in the K&H book, like around corners or under arches, or are those not really necessary either?

A: I am not sure about this distinction, but in general I think that a mesh of some sort is usually a good idea because it is positive insurance that the plaster will remain over time. I like the cradle idea under arches because this is a very difficult area to get a mesh to stay put, and the cradle is easy to do during construction.

Q: You mentioned burning off the bag after assembly. Would there be a need for a wire mesh covering after the burning of bags and before plastering?

A: I don't recommend burning away the bags since this is not essential, and only works if the fill material is very solidly consolidated. Wire (or synthetic stucco) mesh will likely make the plaster more durable over time, but is not essential either.

Q: I leave the finish rough and before applying the second coat, I wet the wall down before applying the cement mixture. Do I need to use the mesh in between each coat? I planned on doing 4 coats.

A: It would be unusual to use any more that one layer of plaster mesh, and this is usually embedded in the first coat. Beyond that, as long as the subsequent layers are left fairly rough so that the next coats will have something to "key" into, this is all that is necessary.

Q: We are considering an earthbag home in Tennessee. With regards to insulation; I was curious if I could place wool insulation on the exterior of the earthbags, hold it in place with a wire mesh, and plaster over it? Do you see any concerns in doing this?

A: Usually plaster requires a very firm substrate for it to reliably remain intact. Even with wire mesh, I think the wool would be too soft and squishy.

Q: Would not poly fish net be a better reinforcement than a steel netting? It would seem to me that the poly could provide similar reinforcement to the exterior shotcrete, holding it together when cracking occurs but without the inherent oxidation and failure of steel reinforcement. Has anyone tried this and were the results positive?

A: You are right that steel reinforcement in concrete can be the weakest link over time. Some people have actually used bamboo to reinforce concrete to avoid the corrosion, although I'm not sure if the bamboo can be expected to last much longer. Owen and I have been advocating the use of synthetic netting for plaster reinforcement for some time now, and it has been tried many times with positive results. Sometimes this is fish netting, but also there is now synthetic stucco netting available commercially.

Q: I am almost ready to get fill for my hyperadobe earthship. My local paving outfit has 'road base' that has 3/4" crushed gravel in it, estimated at 10% clay content; do you think this may be suitable for raschel bag fill? Should I order some more higher clay content soil and mix it? Or perhaps mix some Portland into it? My onsite soil is almost completely sand so I'm going to have to truck material in either way, but it seems that clay is fairly hard to come by around here (Nelson, BC).

A: A little bit of gravel probably wouldn't make much difference, but very much gravel will certainly weaken the mix, which is already pretty weak with only 10% clay. I suggest getting enough of a sample to try it out in some test bags and see if it seems to solidify sufficiently.

Q: How important is stainless steel metal in the wall? For example, what about the wire mesh for the stucco? Stainless steel? 

A: I prefer polypropylene bird netting to metal; I actually think it will be more durable over time.

Q: Is it possible to make an earthbag home EMP resistant?

A: It is my understanding that it is primarily electronic equipment that is vulnerable to EMPs, and the way to protect it is wrapping the device in metal. I would not recommend wrapping an earthbag house in metal, as this would inhibit its ability to breath.

That’s my understanding too, but a fine metal mesh wrapping can also work. I was just wondering if anyone had done something like that on an earthbag home. I’m seriously thinking about an earthbag home (and indoor pool) to be built in Nova Scotia. This “technology” intrigues me because of its superior thermal characteristics and its ability to withstand major storms.

You could use a continuous metal stucco mesh that might give you protection. In that case I suppose that a dome or cluster of domes would be more effective. In Nova Scotia, you would need an insulating envelope over thermal mass for comfort.


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