Replacing Rotten Straw in an Existing Straw Bale House

Written by Andrew Morrison

Water Damaged straw bale houseA consulting client recently asked me what the best practice is for replacing rotten straw in an existing straw bale house. I realize that this is a topic that concerns a lot of people when they first learn about straw bale construction and I want to make clear that this is extremely unusual and not something that most people will ever have to deal with. The most important thing when designing and building a straw bale house is to ensure that you do so with the understanding that water must be kept away from the walls.

The client I am working with (in the rainy Pacific Northwest) has a home in which the architect did not include any roof overhangs on two walls. That is a recipe for trouble in any house, not just a straw bale home. So although I believe it is important to share with you the proper steps for replacing straw, the overwhelming data shows that you will never need to.

Below are the steps to replacing rotten straw in an existing house. Although each specific location may have subtle differences, the basic steps are still the same. The level of difficulty you will experience in replacing straw in an existing structure will increase with the amount of straw you need to replace and its location. For this reason, it’s best to only replace the straw that absolutely needs to be replaced, and no more.

Whether you need to replace a small amount of straw or an entire section of bales, the process is pretty much the same. The biggest difference is that when fixing a small patch, the surrounding bales don’t need to be supported at all once the material is removed. If, however, you need to replace entire bales, you will need to provide support for the rest of the wall so that things don’t sag into the gap you create when the bales are removed. It is also more difficult to remove entire sections of wall because the plaster in both sides of the wall needs to be removed or, at the very least, loosened from the bales.

Remove the plaster in the identified area of damage with a hammer drill and a chisel bit. Over excavate the area of concern so that you expose the wire mesh about six to eight inches away from the damaged area in all directions. This will allow you to replace the rotten straw and properly tie-in the new mesh.

Cut the mesh away from the damaged area. Be sure to leave at least six inches of undisturbed mesh around the damaged bales. This will meet the requirement for mesh overlap later, which is vital to eliminating potential plaster cracks in the patch.

Scoop out damaged straw. You may want to use a small gardening rake (hand-held with 4 sharp tongs) to more aggressively remove the material.

Use a moisture meter to determine where the damaged straw ends and the clean straw begins. Make sure that you are at least one inch into the clean straw when you stop digging. You don’t want to go through all this trouble and leave rotten straw, in any amount, in the wall. Check for high moisture readings in every direction: up, down, left, right, and back into the wall. Anything above 18% should be removed. Ideally, the moisture readings should be no higher than 10-12%.

Slide a piece of burlap underneath the mesh that you left exposed and intact. Leave the top side of the burlap loose and long enough to be tucked under the mesh later. Use landscape pins to secure the other edges of the burlap.

Stuff fresh and clean straw behind the burlap so that the patch is tight. It may bulge away from the wall a little, which is fine; however, make sure that when you press against the patch, it flattens out properly so you don’t end up with a bump in your finish wall.

While placing pressure against the patch, pull the top portion of burlap tight behind the mesh and secure it with landscape pins into the bales.

Cut a piece of mesh large enough to cover the entire exposed area and place it against the wall. Tie it to the existing mesh with tie wire or cable ties. Be sure to pull it as tight as possible to help flatten out the patch.

Apply plaster to fill the patch. You will need to do a scratch, brown and finish coat. The finish coat will cover the entire wall to clear stopping points. If you just do the patch, you will have a permanent “burn mark” that will not look good. If you are using 100% earthen plaster, you can simply replaster the patched area and feather the new plaster into the old by properly wetting the existing plaster. This only works with 100%, unsealed, earthen plaster.

The reality is that you will likely never need to use the information in this blog post; however, if you do find yourself with water damaged straw, the sooner you dry out the wall and replace the non-salvageable straw, the better. Remember that bales are like giant sponges and the longer they are exposed to water, the more they will absorb and the farther the water damage will extend.

One thing that goes without saying (but I’ll say it anyway) is that you MUST identify and repair the source of the water damage before you start replacing straw. There is no point in doing all of this work without knowing what caused the problem in the first place as it will simply return and damage the new straw you have installed.

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13 Responses

  1. I have a large shower with an interior tiled wall covering , and window in an exterior strawbale wall. I’d like to remodel the entire bathroom, would you suggest ripping the tile off the interior wall and moving the shower to existing interior walls made of plaster? There is no plumbing in the strawbale wall of shower. Also, can I do this anytime of year- it’s about 25degreesF now with snow- lots of sun in the days. Could you also recommend a good ceiling fan for humidity in the bathroom. Thank you! Jane Evanson

  2. Hi Jane. Nice to hear from you. I would see no problem in completing an interior remodel during the winter. As long as the exterior walls stay in tact, you would be fine. I prefer to keep my showers off of straw bale walls so that splash does not compromise the bales. If you can accomplish this within the design without difficulty, it is worth it. In terms of fans, most are adequate these days. The biggest difference is in how loud they are when on. If you can find something that moves 90CFM, that would be fine in most cases. I would strongly recommend that you add a special switch though. Choose either a timer or a sensor for better results. This way, you don’t have to remember to turn the fan off (timer) when you leave the room. The sensor is even better because it will turn on anytime it senses moisture in the room and will turn off when those levels drop.

  3. Great Article. My parents have strawbale addition in Northern Wisconsin that is now having moisture issues. The rot is wide spread and we have been considering taking all of the straw out and replacing it with spray insulation. The outer stucco wall would become a traditional stick framed wall/siding as water splashing off the roof was the main cause of moisture . Have you ever heard of this being done? What do you think the best way to do this would be? Any insulation products you would recommend for this application?

    Thanks, Peter

  4. Hi Peter. Sorry to hear of the failure. I have not heard of this situation happening before. I would suggest that the roof overhang, gutter design, and the design of the base course of bales be reevaluated, but if the decision to move away from bales has already been made, then that is a different story. My biggest concern with the spray foam would be two fold: 1) the health side of it. I am not a fan of the material as there are for too many stories of health related issues with the material. 2) the expansion of the material within any plastered wall could blow out the plaster (assuming you remove straw from behind existing plaster due to total failure of said straw. If it were me, I would look to solve the source of the problem and go back to bales. At worst, I would add an exterior detail to the wall to help reduce the potential for water damage (a skirting of metal, stone, or wood below a water table to manage splash back). I would also address the falling water by adding gutters, a sloped mote around the home and/or landscaping to breakup the falling water. Best of luck to you whatever you decide.

  5. Hello I was wondering after reading other expert straw bale builders saying never to use steel pins , or rebar in straw bale houses because in high humidity places and places where you need supplemental heat inside t can cause condensation and therefore rotting of the bales from inside out …what is your take on this ?

    Thank you, Lori

  6. Hi Lori. I completely agree. I have not used rebar pins in years. I use welded wire mesh on both sides of the wall and tie it together with baling twine on a specific pattern. It is INCREDIBLY strong and improves the entire building process in many ways (simplifying everything from electrical installations to increased shear strength for the building).

  7. Hello! We purchased our Strawbale house from the previous owner who had built it himself. One of the bathrooms has a shower against the main wall to the outside and there is a window in the wall as well. The tile that was covering the window sill begin to have a leak in the grout about a year ago. We patched it here and there, but didn’t get it completely fixed. We tore into it today and found a bunch of black mold throughout a lot of the bales down a Single line where the wall met the window sill. I’ve been removing a lot of the damaged straw myself, and there is not a big enough gap to put a new bale in there. We were thinking about putting blown insulation and where the gaps are. Thoughts?

  8. Sorry to hear of your situation. That’s a bummer for sure (you’d probably use a different word, I’m guessing!). The first thing is to get a moisture meter with a long probe (I use a Delmhorst meter) so that you can make sure you have all of the damaged bales out. You can’t always tell just by looking and it’s vital that all the straw with a moisture content (MC) above 15% be removed. A perfectly dry bale should be closer to 8-10%, but if their is no rot and the bales are able to stay exposed to dry air for a while, then they often dry from 15% or so back down to ideal levels. This also gives you a 5% buffer for the bales should the air be humid and they actually gain MC.

    Once you have that taken care of, you can place stuffed straw into the opening if you can’t fit an actual bale. If it’s big enough (an area over 9″) then you can use tied flakes to build things back up. This is easiest with long straw (stalks over 12″) as you can use 9″ landscape pins to secure the flakes to the surrounding straw. If you can’t get the straw to stay in the gap, you can use burlap, pinned around the gap with landscape pins, to stuff against. You want this to ultimately lay flat, behind wire mesh if you have it in the wall, or directly behind the plaster. You may need to pin the straw THROUGH the burlap in a few places depending on the size of the gap. You want that to be flat, but TIGHT is more important.

    I have not used blown insulation in a situation like this before, but it could be done for sure. You would need to provide something to blow against, similar to the burlap idea above. The blown insulation I have seen has a specific product for this use, albeit usually attached to studs in a conventionally framed home.

    You may want to consider changing the way the shower meets the wall as well. If things are pulled apart right now, it may be worth framing a flat studded wall against the current wall and over the interior window sill area so that you can completely waterproof that area. You can use plywood and a product like Grace Ice and Water Shield or even a liner typically use to create the pan under a tile shower. The idea is to make sure there is NO WAY for water to get back in there again. You’ll lose a few inches (maybe 2 1/2″) to the room, but you’ll not have to deal with this issue again.

    Hope that helps and best of success to you!

  9. Andrew,

    I cannot tell you how elated I am that you took the time to write me back! I have been feeling very down about the prospect of what is going on with our home. We found this to be the case in the second bathroom, as well, at the base of the shower and close to the toilet. The damage is not nearly as extensive, but you get the idea that we have quite a problem on our hands. Thankfully, we have a Jacuzzi tub on the opposite side that we can still use. We have four small children, so it just compounds the problem. My concern is exactly what to do with some of the rotten Straw. If there is kind of a dried whitish to light gray powder on it, do those need to be replaced, as well? Some of it is not black mold. We are going to leave the one wall exposed, as well as the other wall and use the Jacuzzi tub for a while. This will allow the bales to dry out completely for a while. Is there another moisture meter that you could suggest? The meter that you suggested is very expensive. We are somewhat limited on our funds given the extensive work we’re going to have to be doing. Further, all of this is taking place from the inside out, as you can imagine. Do you suggest completely getting rid of that window area in that one bathroom? I’m just completely overwhelmed right now. Thank God my husband has a construction background. Thanks again for all of your time!

  10. I can imagine you are very stressed about it. If it helps to know that you WILL come out the other side of this and upon looking back, it won’t be a major event in your life. Sometimes it’s hard to remember that in the moment. Sometimes even the dmsllest hiccups feel like life changing events until we get some perspective.

    Anyway, it’s great that you’ll be able to let the walls dry out for some time. That will be very helpful. In terms of less expensive moisture meters, I don’t know if those exist or not as I don’t shop them. What you might do is see if there are any farms local to you (or even within a drive) from which you can borrow a meter. You might ask at a farm supply co-op or store if they know of anyone or perhaps if they rent them.

    If the moisture content of the straw is within range, then I wouldn’t worry about the white/grey material in terms of removing straw. Instead, I would spray the area with a bleach solution to make sure there are no living spores in the exposed straw. Once everything has sufficiently dried back out, you can move forward with closing up the wall. Good luck! Enjoy that Jacuzzi tub!!!

  11. Thank you for all of your help! Unfortunately, as we continued to do demolition, we found mold and rot right along the bottom of the straw bales as the water traveled to the floor and ran across the floor. Our current dilemma is trying to figure out how to support the bales that are up high and undamaged in order to replace the lower bales which were damaged. Also, we are unsure what exactly constitutes a “rotten” bale. The straw bales are currently still open and exposed, but this is an interior exposure and not an exterior exposure. Would the moisture content fluctuate with the humidity in the house and not dry as well? Would running a dehumidifier be beneficial? We are now going to have all of the windows replaced in all of the house, as the windows were not the most energy efficient, and tended to have some air leakage. They were not very well insulated in comparison to the insulation of the bales. Is there anything different that we should tell the window installer, as we are probably the only straw bale residence in a 50 mile radius? I am learning a lot about the whole concept of straw bale construction. I am very intrigued, and impressed with the theories and concepts behind it. I would desperately like to continue with this sort of structure here at this house, but unfortunately, feel very ignorant, as I have mostly a medical brain. Further, in our demolition, we discovered that rebar was put through some of the bales. From my research, I understand, that this is not the most desired way to construct straw bale houses. Thank you for all of your help! I’m continuing to learn and taking the 16 day course to try to inform myself.

  12. Sorry to hear about the water damage. You should be able to support the bales within the wall as is. In other words, if the walls are plastered and that plaster is well attached to the bales, that should be enough to pull the lower bales a section at a time. You will obviously need to leave some in place to create support while others are removed.

    A rotten bale is one that is over 20% and will not come down below that level. It may not be rotten right now, but if you can’t get the MC below 20%, it’s just a matter of time.

    Nothing special to consider about the windows as long as they’re flashed really well. If they’re inset from the face of the wall, you want to be sure to install a sloping sill that extends over the face of the bale wall and has an integral drip edge in it.

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