Retrofit: Using Straw Bales to Insulate a House

Written By Andrew Morrison
November 2, 2007

tall stack of straw balesI recently received the following question from a visitor to my blog named Pat. I get questions on using straw bales to insulate a house quite often. Here is what Pat wrote:

I have a house built in 1911, the wood frame is 2×10 and 2×4 pine construction the wood is now so strong that a nail must have a pilot hole drilled first. Is it possible to add additions to this house using straw bale construction and tie the two together somehow? I live in Minnesota so I love the high R value of bale construction. I had also thought about completely encompassing the whole house and slowly removing the exterior of the existing house. The purpose would be to allow us to live here while we are building the additions. If you have done this how did or would you do it?

Here’s my answer
The idea of wrapping the house in bales is a common one and sounds like it would be a good idea in Pat’s climate. Here’s a video that discusses three of the major areas of concern when working with wrapping an existing house with bales.

When attaching a straw bale addition to an existing house, the biggest concern is to make sure that the two structures are tied together well. The easiest way to do this is with expanded metal lath at each course. Lay a swath of lath on top of the bales and pin it to the top of the bale surface with dowels or landscape pins, the latter being easier. Then bend the lath up at a 90-degree angle and staple it to the framing of the existing house.

The two structures are now tied together. Be sure to use lath spanning the face of the joint created between the bales and the existing structure before you plaster because the two building materials will move at different rates under different weather conditions.

Finally, make sure the face of the bales and the face of the existing walls are lined up properly “in plane” so that the finish plaster will not have a bump in the transition. The exact line up depends on the thickness of the plaster on the two substrates and the transition used. Just be aware that some thought will need to go into this before you even form the new foundation. Happy Baling!

Want to learn more about straw bale houses and how to build one? Want to do so for FREE? Sign up for our totally free 16 Day Straw Bale eCourse! Find out more HERE.

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Straw bale workshop participants applying cob plaster

Post Comments

101 Responses

  1. Hi Billy. The honest answer about the relative humidity is that I don’t know. I have not seen any conclusive data that relates relative humidity to moisture content in bales by volume. How many days a year would you say it feels humid? Does it ever dry out or is the overall climate a humid one?

    In terms of the design idea, that should be fine. If you plan for your exit strategy, i.e. how you plan to tie them together, in advance, you will be much happier. I suggest installing doors where you plan to have them later on. You can always change the door style or even remove it and make a simple opening later on. It is much easier than cutting through the wall.

  2. Hi there! What a great site.

    I am wondering what you would think of wrapping an earthbag or thin cob wall structure with strawbales, all built on a gravel filled tires (two wide) over a rubble trench? I live in Nova Scotia where temperatures go to minus thirty celcius for weeks at a time and equally hot – plus thirty in the summer. Any and all comments are much appreciated!

    1. Hi Gena. I think that this would provide a couple of key elements. You would have lots of thermal mass inside a very efficient thermal insulator. That’s a great combination for even temperatures. Be sure to place the bales tight to the cob or earthbag construction and you may want to skim the back of the bales with a clay slip before you stack them, or as you stack them actually. You’ll need to attach the bales to the interior wall so they can’t “peel away” from each other. I’d suggest laying diamond plaster lath in the walls between bale courses. You can then build the cob interior walls (or earthbag) so that they encompass the lath. Let me know if you move forward with this as I’d love to see pictures.

  3. Hi, so much useful info on your site I hope I haven’t missed the answer to my question!

    I want to build up against an existing high stone wall and I am wondering how best to do it. I guess you can’t bales straight up against the stone wall and at the very least would require some kind of moisture barrier? Or is it best to set the building back from the wall and to leave an air gap (although this is not ideal)?

    Thanks in advance,

    Mike

  4. Hey Andrew,
    My wife and I purchased land in Ohio and there is a metal pole barn on the property. Our intention is to install bale walls on the west and north walls (south and east will be conventionally insulated and finished, mostly because of plumbing, but for other reasons as well). We will remove the metal siding on sides that will be baled. My question is how to best make the connection at the corners where the metal walls will meet the stuccoed bales. Could I extend wire mesh across the metal siding and plaster on that? It seems that because metal expands and contracts, stucco would eventually (and perhaps quickly) crack and fall off. Thanks for your help. Last year’s workshop in Trinity Cty, CA was fantastic!
    Andrew

    1. Hi Andrew. I would need some more detail on this to make a solid and useful answer for you. Can you email me some sketches and more details to my regular email address?

  5. Hi Andrew:
    We are purchasing a flat roof stucco wood framed house which has a 6 ft crawl space underneath it. It has a walk out basement apartment at the back and the front of the house is ground level as it sits on a slope. I’d like to convert this area which has a wood frame door entrance into a usable basement addition by closing of the outside access and adding a staircase from inside the main house. After moving pipes and ductwork I’m hoping to have enough space to add a small 2nd bedroom, bathroom and laundry area. I will need to dig down a bit further for head clearance of the floor joyce’s. I’ve been told that if I do that I will need to add more supports. Is it possible to use straw bale’s dipped in slip as you described above (to prevent moisture) without adding additional supports? Could the bales be load bearing?

    1. Hi Renee. Not sure I fully understand your situation. Would the bales be underground, even partially? I would not recommend that. In terms of acting as load bearing, that can happen but it depends a lot on the loads it will be supporting. Load bearing is hard to do as a retrofit and is also difficult when paired with wood framing (partial load bearing capacities).

  6. Hi Andrew, I am drawing up plans for a house and have access to a lot of cedar boards and much more experience with wood than plaster. I was wondering if I could use the cedar on the inside as siding? I have read your comments about condensation getting in between and wonder if it makes a difference having them only on the inside. I live outside of Seattle, so it is fairly humid here. If I could fit the boards tightly to the bales and reduce fire risk, would this idea work? Ideally, I would use as many as possible and with a large enough overhang, could I do this on the outside as well? Thank you for providing so much information for this process. It has been so helpful already.

    1. That is possible to do. I strongly suggest you do a scratch coat of lime plaster under the cedar. It won’t matter what it looks like, but it will seal the wall for fire risk. On paper, the wall looks flat and the boards would have contact everywhere; however, in reality that is not likely to be the case. The scratch coat will keep the bales safe.

  7. Hi Andrew,
    We’re considering buying and renovating an old weatherboard house. To improve insulation (& because I LOVE everything about straw bale houses), I was wondering if we could retrofit straw bales, by removing the weatherboards and the internal plasterboard leaving just the wooden frame and windows, then put the straw bales in, like you would if building a wood-framed straw-bale house from scratch? I imagine using the existing frame would save a bit of money – unless it makes it too hard to fit the bales.
    Also I’d want to turn this single-storied building into 2 story. So it would get a new roof, but I understand straw bale houses need a decent roof overhang to reduce rainfall on the walls. If it’s 2-story do you need huge overhangs? Or does it need some sort of eaves sticking out between the 1st & 2nd level?
    The floor is wood a few feet above ground on brick pilons. Is this a suitable floor/foundation for strawbales?
    Thank you and sorry it’s so long!

    1. Hi Kylie. In theory, it is a great option to do just what you are suggesting. In reality, I would need much more information to say yes or no in your specific case. For example, how strong is the foundation? Is it to code for a single or 2 story structure by today’s standards? What climate are you in (this affects whether you need a mid level roof assembly as well as a top roof)? Will the electrical and plumbing be replaced as well? That is all in the walls and in the way of the bales when sliding them into place. What is the stud spacing? Is there room to place the bales inside the frame or will you need to place them outside and thus need additional foundation and roof extensions?

      Sorry to throw so much at you, but you will need to know the answers to these and other questions before you commit to the addition. I think it is possible for sure, I just don’t know if it is cost effective and/or the best way to go. It could be…

      I can help you with the planning if you decide to move forward. Let me know.

  8. I live in a tiny concrete house, and would like to add a room to it. I love straw bale, cob, and other natural homes, but have no idea how to go about this. The main problem is the roof – the house is in an L-shape at the point we want to add on (to fill in the L and make it more square), so the existing roof at the back slopes down toward the South, and the roof on the right side slopes down toward the West. A flat roof isn’t feasible because we do get some snow here. The more “bends” in a roof, the more leaks, and I don’t want to deal with any more leaks. How do I approach this, besides just continuing with the slopes of the existing roof sections? That would make the roof height only 4 feet off the ground, and oddly shaped, too. Someone suggested digging down and setting the room halfway underground, but that seems like a lot of work to do by hand (the only option I have). I need a cheap solution. It doesn’t have to be fast, but it does have to be cheap, and leak-free. Thank you!

  9. Hi Jennifer. The best thing may be to build another roof that slopes towards the existing one (if I’m picturing this right) and then build what is called a cricket on top of the intersection of the two roofs. The cricket basically directs the water away from the joint and to the drainage point at the edge of the rook valley. This is not the best looking example, but if you scroll down the page to the building with the red roof, you will see a photo called “metal roof cricket” on the right side of the page (http://harwigheritagecarpentry.ca/photoalbum.html). This is not my work, so credit goes to the builder and the photographer. It will at least give you a sense of what I’m suggesting.

  10. I just found out about straw bale home. I think it is really awesome. I live in Wyoming where it is really cold in the winter time and tons of snow. The question I have is that I have an older mobile home on a concrete basement. I was wondering if I were to take off the old aluminum siding off to retrofit the straw bales, what about the concrete wall will they create condensation in the straw bales? Will this work? or do I need to worry about the concrete walls? We have strong winds during the winter. The snow blows in every direction. How can I protect the stucco/plaster on the bales from getting wet?

    1. Hi Tod. The bales and plaster can handle getting wet seasonally (to some extent) so you don’t need to worry too much about that. As long as you build the structure well (good roof / foundation and proper detailing at openings, etc.) you will be fine. You will need to consider the interface between bales and other materials (concrete, framing, etc.) but all of that is definitely possible.

  11. ok I have a couple of questions. I am very interested in the straw bale concept and so far am liking what I am reading. my question is this. I want to add on to my house in 2 different sections. I have a small mudroom off the back of my house. I want to totally redo that. make it bigger, add a different door and 2 small windows, on each side. I would do the frame work with wood and then fill in with the bales. this will be done on the outside walls. I would like to use recycled wood on the inside, to finish the walls in some places would that be ok. if I do a plaster finish then do the wood over the top of it?

  12. I would like to know if there is any way I can use straw bale to break a four foot frost line around my greenhouse. My thought is to bury bales wrapped in black plastic sheeting to seal them from moisture. Getting concrete out here is very expensive, so is foam board insolation at $40.00/sheet. Any ideas?

    1. That might work; however, I don’t know if it is a long term solution as the bags may leak and the bales rot. It may be an inexpensive option and something worth trying. Even if it only lasts 2-3 years, it would be reasonable to replace in that time period considering the cost.

  13. I will potentially be building a 2 story straw bale in-fill home for a neighbor this fall, post and beam framing. I have a question about tying in a porch roof (the ledger) to the straw ball wall. The ledger for the rafters would not fall where there’s any framing. How do I provide something for the ledger to tie into?
    Incidentally, regarding humidity, I live in the piedmont of North Carolina which is pretty humid. I have helped with two other straw bale in-fill homes for another neighbor that are 8 years old. Bales were not dipped or wrapped, clay-lyme stucco and plaster only. The clay-lyme stucco and plaster breathe moisture out of the walls on a continuous basis (it’s a breathing membrane). No mold, no mildew, the walls still look great.

    1. Thanks for sharing about the homes you have worked on. It’s always good to get feedback from the field. I would install sub-ledger in the bales, behind the plaster, and under the wire mesh if you have no framing to otherwise attach it to. If you have posts nearby, it would be a good idea to extend the ledger beyond where it NEEDS to be in order to tie into those posts as well.

  14. I have an existing strawbale home. I would like to add and attached heated greenhouse. I am concerned about the moisture of the green house affecting the exterior walls. I would only be opening up an existing window and making it a door to the green house, Therefore the existing stuccoed exterior wall would now be inside the greenhouse. Any suggestions?

    1. Hi Kat. I have seen this done before with success. The key is managing your moisture and humidity so that the levels don’t get too high for the walls. Another option is to remove the bales in that area so that you can also experience the benefits of the greenhouse for the interior space. The heating capacity for a greenhouse is high so insulating your house from that heat source may not be the best idea, depending on your climate. Being that homes are pressurized from the interior, you are not likely to have any issues if you leave the walls as bale and regulate the humidity well. Good luck.

  15. Thanks for posting this, it really caught my attention because the question was asked by someone in Minnesota. I’m just across the river in North Dakota. I inherited the house my grandparents had built in 1980 when they moved off the farm and into town. I would love to reduce our heating and cooling costs. We get some really bitter cold days that are sub-sub zero in the winters and I prefer to be on the warmer side….. But in the same breath would love to add on to give us more space. (By that I mean an actual dining room, a really big crafting room, I need some serious space. But revamping the kitchen living room area because the front door is in a crazy stupid spot with the windows. It’s not balanced at all. But I love the look of cob houses and that with some time spent learning it can be a DIY project. Since we don’t have a hundred grand a year income to work with, more the average American income it needs to be budget friendly. Did I mention that I have access to bales? I grew up overseas as an expat (oil in SEA) so when we came back in 1990) my parents lived their dream of farming where my mom grew up. So now my dad and brother farm and my brother has a bailer as he has horses. So it would be an easy material to obtain plus being green. I’m going to show this to my hubby…. as we both want to learn more. Oh and before we moved overseas, my dad built a big house himself. Back then he went to the public library and checked out a book on how to build a house. The only thing he didn’t do was pour the cement for the basement wall and texture the walls and ceilings. The master election said to add 1 outlet to the living room as they needed to be every so many feet and he was like 6″ off for one of them. I don’t think there is much my dad can’t do and at 75 now he is more active than those half his age and looks at least 20 years younger. (Seriously) So he is an inspiration to me, knowing I can accomplish what I want to by learning how first and be proud of my accomplishments.

  16. Hi, we are building an apartment inside a tin agricultural building. The apartment will sit above existing stables. We want to line the walls of the tin barn with straw bales but are concerned about condensation- would we need to line the walls with plastic sheeting first or leave a gap- if so, space is of a minimum, so how little a gap would we be able to get away with. Thanks in advance 🙂

    1. Hi Liane. I would suggest a minimum 1″ vented air gap between the bales and the metal. Be sure to plaster the face of the bales that will be against the metal siding before you install them to minimize any fire risk. Simply dipping the face of the bales in a lime slip will be sufficient. Good luck!

  17. Hello Andrew,
    Thank You! For sharing your wealth of knowledge on your wonderful web site.
    I have been reading your posts here but I feel I am in a little different situation than others who have sought your input.
    Ok, so we are in the planning stages of our off the grid retirement homestead. So keep that in mind. I am looking at ISO Shipping Containers to make up parts of the Superstructure of our 6000 sf homestead. 7-53’ and 2-32’. Our plan, at retirement is to have 2 dropped off here in Wisconsin, so we can place all our belongings into them and then go to Missouri. This is part of the reason for containers. We will be starting from scratch there so we will need shelter for ourselves, temporary workshop, storage for materials, rain water harvesting, water treatment system, solar heated water for radiant heat and hot water, water for thermo mass and solar battery bank storage. The containers meet our needs in many ways. I have worked with steel all my life, this is part of my wanting some steel in the design. My question: Can I place the Containers and wrap the exterior and roof with Straw bales to get the R-Value and Sound proofing?
    I plan on putting the container on a short crawl space, concrete frost wall, then placing a gravel trench to the outside, next to that frost wall with the treated lumber ladder embedded in gravel to begin my Straw bale wall. All exterior Straw bale face would be the recommended 3 coats. What actions do I need to take for the straw bale against the steel container? Should this be done at all?
    ALSO

    At the top of the container I would like to sheet it with ply wood and build a steel sheet roof support structure so I can place Straw bales on top of the container and then place my steel roofing sheets. Straw insulation is what I am after here. Any thoughts? Thank you in advance!

    1. Hi Edmund. It is possible to build the structure you are considering. I would suggest a 1″ (minimum) ventilation gap between the bales and the shipping containers. The plane of the bales that is facing the metal container should have a scratch coat of plaster on it. That can be applied by dipping the bales before placing them. This reduces the risk of flame spread in the ventilation space. I would want the bales up off of the metal in the roof structure as well, so perhaps a ladder frame to raise them a few inches. That space could be insulated with Roxul so it is still fairly benign and will resist rot. As for the bales sitting in a gravel ditch with the wood kickers, I would personally want something a bit stronger. You would have zero resistance for uplift in the scenario you suggest. Concrete or at least cement reinforced earthen foundations, would be better. Hope that helps!

  18. Hello Andrew,

    Thank you for all the inspiring information! I have an upstate New York house with vinyl siding and concrete-plastered OSB board around the post and beam foundation. We are on a sloping grade. I would like to insulate around the foundation with straw bales and poured earth (perhaps a 50/50 mix of quikrete and gravely earth). Next to that will be a 4″ drainage pipe wrapped in landscape fabric and gravel, with plastic and concrete partly beside it /under it sloping away from the house. I am currently putting a 6mil plastic vapor barrier with a 2″ concrete rat slab in my crawl space. Do I also put the 6 mil outside under the concrete to support the bales? How thick does it have to be, since the bales will be only 2-4 stacked high? Can I use the poured earth to plaster them? Can I stack them directly against the OSB and use door/window flashing to seal the bottom part of the vinyl siding to the top of the plastered bales, then use large thin stones (ample on our land) angled as a cap/roof over the bales and the sides? And finally, for this short of a wall, do I need to tie it to the foundation? Thank you again, and many blessings on all you endeavors!

    1. Hi Tiffany. This is a tough one to answer with the information you gave me. It sounds to me like a temporary solution. If you want it to be permanent, then you may need to build a foundation for the bales and also provide better plaster and cover for it. I don’t see how the rain won’t get in easily with the picture you have painted. Sorry to not be a happier camper on this one.

  19. Hi, love the site. My question is this. We are building a shed roofed house, all wood outside, we want to spice up that square ness on the inside by using strawbales and cob. We will place studs accordingly to accommodate the bales. Do we need to add a lime slip to the outside next to the wrapped wooden walls? The wall are plywood with felt paper like in regular stick construction. And can we paint the interior cobbed walls with regular paint or stains or do we need to do something different? Thanks for any input.

    1. There is a lot to respond to here and more than I can address in a short response. What I can say is that you will need to provide a ventilation gap between the siding and the bales to allow potential condensation to escape. Further, I would not use felt paper over the plywood. Instead, I suggest housewrap. The bales do need a plaster slip on the exterior surface as well to lower potential flame spread risk. Do not use regular paint on the bales. Stick instead to a lime wash or other natural pigment solution. At the worst, you COULD use a regular paint on the INTERIOR wall surface only over a properly cured lime plaster, but it’s not the best option.

  20. Hi Andrew.
    We recently bought a great straw bale home in the Texas hill country. It’s perfect – except for the interior color, which we’d like to change. The interior of the bale walls is currently properly coated with a cementitious lime plaster. I’d just like to know the easiest and the preferred options for adding another layer of something (paint, plaster, wash) to make it our desired color, without negatively affecting the important properties of the bale walls.
    Regards,
    Jo

    1. Hi Jo. The interior is easier because you can use paint without much risk. Painting the exterior is an issue because it locks up moisture in the walls. The interior paint doesn’t have the same effect. Be sure to use a primer that is designed to work with lime plaster as the pH becomes a major player in the adhesion. You’ll also need to make sure the walls are completely free of dust and any “chalking” that the plaster might have created. There is a product out of Italy (I don’t know the name) that is designed to incorporate any excess chalking into the primer so that it anchors it to the wall and eliminates the risk of peeling paint. Again, the right primer, with the right pH range, and perhaps the special product, and you’ll be good to go with repainting the interior!

  21. Hi Andrew,
    In your retrofitting video you talk about fireproofing the bales by dipping their back side in a thick slurry. Do you then set the bales in place wet, or do you have to wait until they dry out?
    Thank you,
    Kay

    1. Hi Kay. You can set them in with the material still wet. Keep in mind, you’re just dipping the edge and ever so slightly. You’re not saturating the bales in a way that would cause long term moisture problems.

  22. Hola a tod@s! Mi nombre es Alfred y me gustaría saber si alguien conoce de alguna pagina o lugar que haga casas contenedores con aislamiento exterior con balas, pacas , o fardos de paja.

    Hi everbody!

    My name is Alfred and I Looking for information about shipping containers homes whit straw bale insulation . If you have information please helpme and if you speak spanish is better for me,

    Thanks

    1. Hola Alfred. Gracias por tu mensaje. No tengo mucho información sobre las casas de paja y casas contenedores. Yo sé que es muy importante tener ventilación entre el metal y la paja. Esto es porque no quieres agua en las balas. Espero que esto ayude.

  23. Hi Andrew!
    I from Bishkek, Kyrgyzstan, Central Asia (ex USSR country).

    My have unfinished house, double brick masonry walls (no cavities), 500-510mm thick (I guess, 1 and 2/3 feet, concrete slab ceilings between 1&2 floor and on the 2nd floor as well. concrete foundation, pitched roof with metal roofing shingles. Thermal mass is huge.
    coordinates: 42’52”/72’34”; arid continental climate, average precipitation: 450mm/year. average max temperature in hot summer months: 32C (35-40C is quite normal in summer), average minimum temperature in winter monthes: -7C (quite rarely gets down to -15-20C).

    I am considering to use strawbale as an exterior insulation (as opposed to rockwool insulator) for walls and roof.
    Walls: If I understood your correctly, I need to put some mud/earth/clay between brick wall and strawbales?
    Roof: do I need to have mud/dirt on both sides as well? any other roofing materials needed to protect from possible dew moist from metal shingles?

    If I install wooden frame windows that are inferior to PVC windows in air tightness, does it make any sense to bother with insulation at all?

    Appreciate your help. Unfortunately here I can barely find info about sustainable building techniques.

    1. Hi Sanjar. I am not an expert in this type of construction as we do not build with concrete in the same way you do. As long as the dew point does not occur in the wall assembly, you will be fine to place the bales tightly to the outside face of the concrete walls. Any moisture that moves through the concrete from the interior will be pulled into the bales and moved to the exterior plane due to the natural pressure of the house. You would not need to apply plaster to the face of the bales against the concrete walls. If you want to be extra safe, you can provide a small ventilation gap between the two wall assemblies. In this scenario, you would want to apply a thin plaster coating (dipping the bales prior to stacking is adequate) to the side of the bales facing the ventilation space. The same is true for bales used in a roof assembly, assuming that the structure can handle the extra weight of the bales.

      Either way, you will have an amazing home when done due to the high levels of thermal mass within the super insulated envelope of the bales.

  24. Hi Andrew,

    I have purchased 2 basic 10×20 wooden sheds to convert them into homes. I would like to insulate them with Straw Bales. I would like to cut the bales in half to a place between 2×4 frames to accommodate the limited space of the shed. Questions:

    1. Will 1/2 bales be adequate insulation & high R Value for -10 winters and humid summers?
    2. What will I need between the bales and the wood framed walls?
    3. What would I need outside (if anything) on the bales besides cob and plaster?
    4. Can I also insulate the floor this way?
    5. Will the building be largely fire resistant?

    Thank you so much for your time and consideration. Please contact me by my email as my computer is acting up and I might lose this site.

    – Rae

    1. Hi Rae. Thanks for your questions. Unfortunately, cutting the bales in half will not provide adequate insulation for your climate. The key to bale construction (or at least one of the keys) is the thickness of the walls. The “per-inch R-value” of straw is not that great compared with conventional materials, so the thickness is vital to the overall system. If you plan to plaster the exterior of the bales, you won’t need anything between the studs and the bales. I would recommend using welded wire mesh over both sides of the bales and sewing through the bales to connect the individual layers as I show in my videos. In fact, the entire system that I show in my videos would be effective for your build, assuming plastered walls inside and out. I would NOT recommend using cob on the surface of the bale walls. Cob is a structural material and the extra weight and thickness would not provide anything of value to the structure. My preference is three coats of lime plaster on both the interior and exterior bale surfaces. Using bales in the floor or ceiling is not worth it in my opinion. They are heavy and require a lot of extra work to keep them in place. Further, they present a fire risk if left unplastered, and the bales below the floor are at risk of moisture problems. If you avoid the bales in the floor and ceiling and fully plaster the bales in the walls, the house will be fire resistant (as related to the straw).

  25. Thanks for doing this. What a kindness for people with questions.

    I am thinking about a barn conversion and don’t mind losing interior square footage. If the exterior is properly clad and watertight, will I still need to plaster bales on the exterior side? What other considerations should I keep in mind?

    Thanks in advance.

    1. Hi Erich. The biggest potential snags will be flame spread and condensation (two different potential issues). If the bales are placed tight to the existing siding, there is a chance that moisture will condense along the back side of the siding, against the straw. That moisture would be sucked into the bales and could potentially cause moisture damage in the walls. Common solutions to this are to a) make sure that the exterior siding can allow for moisture to escape (walls are not wrapped in tar paper and/or the cladding is wood rather than metal, for example); b) leave a ventilation gap between the siding and the bales.

      If there is no plaster on the exterior face of the bales AND there is a ventilation gap between the bales and the siding, there is a risk of flame spread in that space. This is where a skim coat of plaster would be needed to defend against the potential flames. You can simply dip the bales in a clay slip before they are stacked. This will give you adequate protection from flame spread and won’t require you to remove the existing siding.

      Cheers.

  26. Lots of great information.
    We live in Texas and are exploring options for exterior insulation. I am definitely interested in using straw bale. We have a 53 ft shipping container. If we were to use this method my question is for the straw bale in contact with the Steele. Is there an application for this area or once you apply Cobb or lime plaster this area would be protected?
    That is the only consideration I have about this method. We live in an area where the hay bales are ample .
    Thanks in advance!

    1. Thanks, Sonja! When it comes to using metal in a straw bale wall assembly, dealing with moisture condensation is really important, especially if you’re wrapping a shipping container with straw bales. The idea is to keep the bales dry so they can provide good insulation. However, if there’s a temperature difference between the outside and inside of the container, the humidity in the air can condense on the metal. This condensation will happen both inside and outside the metal, which can lead to the straw in contact with the metal eventually rotting due to excess moisture that can’t dry out.

      To avoid this issue, it’s recommended to create a gap between the bales and the metal to allow for air circulation. The only problem is that having this air gap conflicts with the insulation properties you want. In this case, finding a solution that balances both insulation and moisture control can be a bit challenging, and I don’t know of a simple answer for it.

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