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!

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101 Responses

  1. I am about to purchase a single story, stick framed and stuccoed building. I am planning to retrofit the existing walls with straw bales to improve the insulation. I am also planning to change the current roof for a gambrel roof thus giving me an extra floor and the perfect opportunity to create larger overhangs. My question is regarding the finish of the walls. The existing stucco is in excellent shape despite Canadian winters and is very easy on the eye. I was originally planning to cover the bales with 2 coats of plaster and then clad them with wood siding in areas where large overhangs may not be possible but with the original stucco performing so well would it be feasible to use a commercial stucco product to finish the bales? If not, what is the best way to attach the siding?

  2. Andrew,
    You can use conventional stucco only if the climate is relatively dry (in terms of humidity, not rain). The stucco does not breathe very well and so you may need to install a de-humidifier in the house to help regulate the humidity levels. If you decide to install siding, you will need to “let-in” wood studs on the exterior of the bales or frame the bale portion with 2x material 2 feet on center. That is necessary to support the siding on a maximum of 2′ intervals. Be sure to check out the video in the above post if you have not already. It discusses another potential need: ventilation behind the siding. This may not be necessary, but it will depend on how tight your siding is. Good luck!

  3. What about building a straw bale house with a cob wall or other thermal wall? How would you attach the bale walls to the cob wall or, even harder, to a cement block wall? Thanks.

  4. Cindy,
    Attaching bales to cement block is not that different from attaching bales to wood. The big difference is in the anchors used. On wood, nails or staples can be used; however, for the cement block, specific concrete screws or anchors must be used. It may be best to use two anchors or screws and a small piece of metal between them to fully hold down the lath. You can find the anchors or screws that are needed at any quality hardware store. I don’t work with cob, so I can’t answer that part of the question.

    Erik

  5. I work with cob a fair amount and I thin the way to go would be to anchor the bales to the cob with mesh or dowels laid in between each coarse of bales. That mesh or dowels would then be incorporated into the cob as it is built. It would depend on how the walls are built as to when the material is laid. The best option would be to build the bales and th e cob at the same time. The problem here is that the bales will go up considerably faster than the cob, so you will have to plan for the timing to match up.

    Chris

  6. I like this idea. Can I wrap my house which is a single wide mobile home with bales? Would I need to use the mortar net stuff you mention or could I put the bales right up to the existing outside of the house? Thanks.

  7. Adding Bales to an existing Home

    Andrew,

    In 1998 I have been asked by a Customer if it where possible to fit Strawbales to an existing Home.
    I came up with two options:
    1) To adapt to the extra width of the bales, build a new roof over he existing one, leaving a space of 100mm between the roofs for insulation purpose (called a tropical roof)
    2) Build the wall externally and extending over the existing roof level (Hacienda Stile)

    Option 2 has been selected.

    We poured a new footing (300x500mm) around the existing dwelling to accommodate the size of the bales.
    As a dampcourse we used bitumen on the footings and laid the bales on the sticky tar.
    Bitumen is also a white-ant deterrent!
    There is no need to render the backside of the bales, since they lay tight to the existing wall.
    As reinforcement we used steel mesh (8mmx 200×200) on every 3rd course and fixed to the existing wall with tek-screws (3 per bale) and steel wire. A thin layer of mortar poured onto the mesh adds strength to the wall.
    The bales are trimmed on a flat floor on both sides prior to installing. There is also no need of stucco netting since the trimmed bales are a perfect base for the mortar render. Once I had to cut a window into an existing Strawbale dwelling I built years ago. The render was just about impossible to remove from the straw! Straw is hollow and the mortar penetrated up to 15mm into the stalk. The only time we use netting is around openings.
    We used an acrylic sealer in the mix to seal the top of the wall.

    Rudy Stoffel, Strawbale Construction, Western Ausstralia

  8. I would think that ventilation behind the bales would be needed if there is a non permeable surface that is in contact with the bales. I would be afraid that the bales would suck up any moisture that was collecting against the surface of the walls, like the steel siding of a mobile home for example. If the walls are wood, I don’t imagine it would be a big deal to place the bales tight the walls. I also agree with Rudy that the bales should not need to be “back plastered” if they are installed tight to the existing walls. I don’t see how they would be a fire risk if they are compact against the existing walls, especially if they have been shaved of the loose straw prior to installation.

  9. Frank Monk

    I’ve tested the moisture content of the straw after one year (13%) to check the durability of the acrylic sealer on top of the wall. The existing wall of the dwelling has an 80mm cavity and is lined inside with plaster boards, eliminating the chance of condensation entering the Strawbale wall.

  10. I appreciate all the comments and the video tip. Very informative. I am wondering though as I have never gotten a straight forward answer on if straw bails work good in all climat zones? I live in a rather humid part of the USA but would like to build a Stawbale home. Is this possible? Also, my wife isn’t particulary fond of the stucco. Can we side the straw bales with any kind of siding? We were looking at the log cabin look with real wood siding, but will vynal siding work as well? Or stone?
    Lots of questions, I know but can’t seem to find answers anywhere. Please help. Thanks

  11. Straw bale construction can work in many climates; however, high humidity areas are not great for bales. That is not to say that you cannot build with bales in a high humidity area. You will need to consider the use of a whole house de-humidifier and moisture sensors. In terms of siding, you can use other siding applications like wood, but it is a lot more work to do that. In general, it is best to stick with plaster or use a stone veneer that is applied directly to the bales. I would not use vinyl at all as it is an environmental disaster and it would likely trap moisture behind the surface which would harm the bales.

  12. On the topic of rock as a siding(I’ve done no masonary work so pour out all the knowledge you see fit): How would a person go about putting rock on strawbale? Would you go with the original clay/lime/sand mix and stick the rock to it? Is there more that needs to be done to hold the rock in place?

  13. The first step is to place a scratch coat on the bales just as if you planned to plaster the whole house. After the scratch coat, you would use a setting compound, which could be the lime plaster, to back butter (put mud on the back of the stone) and butter the wall. Then you would place the stones starting from a straight edge that is screwed to the wall. Be sure to screw the straight edge in a place where a final layer of stone can later be added to the building, once the majority of the wall is complete.

    Another option is to start at the top and work down. This is good because you don’t end up spilling mortar on the face of the veneer; however, you have to be sure the mix is right so the veneer does not sag under its own weight.

  14. I have sort of the opposite question to ask. I have an old garage at the back of my property that is basically unused. Just the other day I was looking at it and wondering if I could add a bale structure INSIDE of the existing structure. I like the look of the old garage and wouldn’t mind losing whatever inside square footage might be lost.

    Ever since we bought this property I’ve wanted to do something with that old building (like a dedicated home theater building or a studio/office). I live in Minnesota and think the bales would be great in the winter to keep warm.

    Would this work?

    Looks like a great site, I hope to learn more about straw bale building.

  15. Phil,
    You most definitely can install bales inside an existing structure. The hardest part is getting the bales to attach to the framing since most structures are built with studs 16″ or 24″ on center. That makes for a lot of notching. You can simply stack the bales against the studs and use plaster lath at each course to attach the bales to the frame. This is labor intensive, but less so than all the notching. You will also need to apply some mud (clay slip or thin plaster) to the back of the bales so that fire cannot spread up the face of the wall against the framing. You can then pack light straw clay behind the bales and into the stud bays. Depending on the climate, you may need to add a ventilation area between the bales and the exterior wall to stop moisture from getting trapped there and sucked into the bales. I think that for the structure you are considering, you can simply place the slip directly against the bales and the exterior wall or even leave whatever insulation is currently there (if any) in place.

  16. Thanks for the answer to my question. I have one more question concerning the floor, right now in the structure there isn’t a floor or foundation of any kind.

    Would it be recommended to put some sort of foundation down prior to adding any bales?

  17. Phil,
    You’re welcome. You do not have to pour a new foundation for the whole place; however, you will need to be able to loft the bales off of the ground so they are safe from water leaks, etc… You will also need to have a toe ups system that can be used for an attachment point for the mesh or lath, depending on what you choose. I hope that helps.

  18. Dear Phil,
    My partner and I have been trying to figure out how to fix the ugly cement block house we live in, built very badly in the 1950s. We live in the Redwood Rain Forests of northern california and thought since the cement is way too porous, well we already have mold issues. We have the need for some way to insulate the concrete blocks, without creaing more of a moisture trap. Yes, cement blocks really retain the damp…bad choice by builder, be we are trying to remedy this.

    We had seen some staw bale construction shows with some applications that we thought might work, but our big concern othet than money, is mold. We live in a warmish moist rain forest and would like to be able to put in central heating, but doesn’t make sense uinless we fix the other problems.

    So, if the staw or hay bale construction is a possibility, would you recommend actual straw/hay construction? Or there are some newer synthetics made out of recycled that we saw in a show that kinda made sense given our walls. What are they called? Are they all consumer waste or at least partial, formaldehyde free?

    Thanks for an help on this

  19. Janet,
    The moisture is a concern; however, if you insulate the concrete block with the bales, including a ventilation or drainage cavity between the bales and the block wall you would be fine, especially if you install a heating system that can help drive moisture out of the house. If you place the bales inside the block, you can stucco the exterior of the house to help minimize the moisture movement. If you place the bales on the outside, the insulative value and the plaster will all be eliminate the moisture problem in side the house, but you may have a harder time reducing the moisture levels in the straw. In either case, be sure you use straw and not hay. They are very different. You can see my post about the differences in this blog. Good luck.

    Andrew

  20. Dear Andrew,

    My husband and I are considering the prospect of purchasing a straw bale home. We are very excited about the potential purchase but are concerned as it is smaller than we’d like for our family. Can we plan to construct straw bale additions on the main floor and the second story without compromising the energy efficiency of the home? One room would be a completely new addition and 3 rooms would be expanded in the renovation. The main floor and second floor addition will be stacked in a rectangular formation about 25’x 12’on both floors. Your input would be invaluable as we are novices in the straw bale building process.
    Thanks so much,
    Stacy

  21. Stacy,
    You can indeed add an addition to the house without weakening your efficiency. Adding to straw bale homes is a bit harder than conventional homes; however, with the right knowledge and skill, it is possible to accomplish the addition(s) with great success. Let me know if I can help should you decide to move forward. Good luck.

  22. Dear Andrew:

    I have read on your site and others that building with bales may not be the best idea in humid climates, unless certain alterations are made. (Not rainy, but high relative humidity.) So my question is this: what is meant by a humid climate? Most parts of the United States, except for the desert southwest, see relative humidity of 80% in the morning in the Summer, and many parts see that level of humidity year-round in the morning. This is true even in the midwest where bale building originated. So I am confused a little and would love to have some guidance on how to determine whether an area would be considered “humid” for purposes of building with straw bales.

  23. Great differentiation on the humidity question. Bales can handle the humidity as long as they have the ability to dry out. If it is a morning thing, the bales can handle that. If you live in an area like the Southeast, that is harder to deal with because the air never dries out enough to give the bales a break. I hope that makes sense. Let me know if you want more details on this.

  24. Thanks much, Andrew. I do currently live in the Southeast, but I do not plan to build here and live in a part of the Southeast where compared to other areas, it’s not “that” humid. (Atlanta, rather than New Orleans.) I am actually looking at building in either Texas or Missouri.

    Major cities in the vicinity would be San Antonio and Kansas City. I have provided links to the monthly averages for the morning and afternoon levels for each city. (And I’ve added Houston and Phoenix for comparison.)

    Thanks again.

    http://www.cityrating.com/cityhumidity.asp?City=San+Antonio
    http://www.cityrating.com/cityhumidity.asp?City=Kansas+City
    http://www.cityrating.com/cityhumidity.asp?City=Houston
    http://www.cityrating.com/cityhumidity.asp?City=Phoenix

  25. Thanks for this Anne. You might want to check out my latest blog entry about Urban development. It just so happens that he picture I chose to represent quality urban design is from a firm that is interested in straw bale design in Texas. I did not know this before I posted the blog, but you may want to connect with the firm to see if they could help you with your dream design!

  26. Andrew..I live in Mid Michigan and have been considering adding on to my home. I love the straw bale idea. What about a basement under the straw bale built addition? Also, is there anything special we need to do with the bales because of the weather in Michigan. I would love to find more information about straw bale houses in Michigan.

  27. Utilizing the space under a bale house for a basement is a great idea. The only thing I can think of that may need to be addressed with the bales is the snow levels. If you get snow drifts, you may need to accommodate for those in the design. With a basement underneath, you would be fine. Check out my blog post about snow drifts for more on that subject. You can search using the Google bar above (just make sure it is set to search SB.com) Thanks and good luck.

  28. I live in Reno Nevada and would like to build a second story addition/apartment with straw bales. The existing structure is stick built from the late 1970’s to early 80’s, is this a feasible option to save money or would a small cottage be the way to go? Thanks!

  29. Hello Andrew,
    I recently purchased an old single-wide mobile home. I would like to insulate under the house, (INSIDE the skirting/underpinning) with straw bales. My concerns are:
    1) Mold/Mildew;
    2) Fire hazard;
    3) Vermin

    Should the bales be stucco’d or sealed in some way before placement under the house? I live in central Kentucky which is fairly humid, especially during the summer months. There is heavy plastic and gravel underneath the house, but I still expect humidity to be a problem. What do you suggest? THANKS!
    Larry

  30. The bales should be dipped in some kind of slip material (clay slip is best) before they are placed. Your concerns are reasonable, especially the mildew if you have high humidity. If the bales are plastered or covered in slip ( a think coverage will be needed) then they will not be a fire risk. The key is to keep as much ventilation around the bales as possible. Vermin should not be an issue, but if the bales are not plastered tight, they could get in there.

    Andrew

  31. Charmaine,
    You can build with bales on the second floor as long as the original building has the capacity to handle the extra load. This is a question for an engineer who understands the load requirements of straw bales. I would not talk to just anyone on this as an unfamiliar engineer will likely overestimate the load of the walls and your sticks will likely not be able to hold the additional weight. If cost is the only concern, you may consider the cottage idea as it will be a more complete building experience for you than an addition.

  32. Hi, great information! We have a single story, slump-block house in the Phoenix, AZ area, and I’d like to add straw bales to the house for insulation – we have a west facing garage wall and the garage is always 10 degrees hotter than the outside temp – a real oven! The concrete block is painted and we have a 24 inch roof overhang. In addition to fixing the bales to the wall with a slip, how would we prepare the ground to take the bales? Would we need to fasten to the roof overhang or just nestle snugly? Any opinion or advice is greatly appreciated!

  33. Anna-Marie,
    You would need to install some type of foundation system to hold the weight of the bales. The best option would be a concrete bond beam on top of a rubble trench foundation where the bond beam is anchored to the existing block wall. You would also want to attach the bales at the roof overhang as well most likely with some type of mesh that spans from the foundation toe ups to the rafters. Keep in mind that if you have a 2′ overhang right now, that will be cut down to roughly 4″ when the bales are plastered. That is not very good so an extension of the overhang would be a good idea as well. If you think you can handle the rain with only a 4″ overhang then you can avoid the extensions; however, direct rain on the wall is not a great idea. I assume that the rain there is heavy albeit not very often. A gutter and some good landscaping may be all that is needed to fully protect the new bale wall against the rain.

  34. Hi there! I was surprised to find that many of these things I’ve been thinking about have been done before.

    I am considering building a mobile straw bale house that I can move to a new location eventually. I am looking at buying an old double wide school room and retrofitting it with straw bales. is this a crazy idea?

  35. Hi Davis. This can be done; however, you may end up with cracks in the plaster. It is hard to get a rigid material like plaster to give enough to survive a move without cracking. Good luck and let me know if it works for you.

  36. Давно искал такую информацию, Сенкс за Вашу работу.

  37. Great info – we are currently adding on to a 2×6 framed cabin – a log-framed, straw bale and cob infill, passive solar living room, two bedrooms and a play loft (in high part of shed roof)I’m wondering about the best way to connect to the existing cabin’s siding (cheesy particle board crap)since the addition’s logs are at the juncture and bales/cob will attach to the logs. would it be best to use some sort of flashing at these junctures (existing siding-log-bales) in lieu of plastering right up to the siding? Really appreciate your extensive site and experise. Thanks!

  38. great info – we are adding onto a stick-built cabin with log-framed strawbale. How should we handle the junctures of exisiting siding (particle board)to log posts? I understand from above how to attach the bales to the posts, but that last ~4″ gap is stumping me.

  39. I am not picturing this junction very well. Can you tell me more? Is the particle board crap you are talking about really particle board? Is it on the exterior of the house? If so, I doubt it is particle board. Perhaps it is OSB (chip board type plywood). When you say “log framed” what do you mean? You call it a 2×6 framed cabin (existing) and a log framed SB. I assume you mean post and beam just with poles rather than dimensional lumber. It sounds like there is a 4″ gap between the last post of the new structure and the existing building. Why is that? I imagine that the bales would go right up to the old building. I would prefer that the new post be tied directly to the framing of the old structure (nailed into existing 2×6 studs). This would allow you to connect the new and old together well. I would then run some flashing between the framing and siding or subsiding and siding of the old structure that extends into the new bale portion of the home and then plaster over the junction. A final trim piece would be a good idea to give the junction a clean finish. For the flashing, I suggest a roll of metal flashing that would slide in under the siding. You could also use some adhesive flashing if the application would work. Let me know if this answers your question.

  40. Hi Andrew, Thanks for the awesome site and videos.
    We are about to start our first straw bale mini project. We want to build a composting toilet using a slab with a concrete block chamber built flush with the back wall such that there can be an access door, and of course bale walls. I imagine I attach the bales to the blocks using the lath method you mention. What do I need to do to stop moisture from the blocks getting to my bales?

  41. Hello
    I am considering a straw bale wrap on a mobile home (around existing siding). I’m aware of some of the considerations such as moisture, overhangs, and plastering inside of bales. Since I cannot create any permanent structures (foundation) does that mean I have to extend the floor on the chassis? Does any one have experience with this? What resources are out there specifically for wrapping mobile homes?

    Thanx

  42. You will need to supply support for the bales and will also need to build bucks to move the windows to the exterior and will also need to extend the roof. There is not a lot of specific information out there about this construction, but it is possible to do. Best of luck.

  43. Hello Nathan Venn

    If you could send me some Pics of the mobile home and surroundings,I would draw up all the plans in detail with information how to do it, for free!

    Cheers

    Rudy

  44. Andy I own all of the SB books, purchased by my wife for my last birthday (now 41) and everywhere I look on the net about SB construction there you are.

    My wife and I moved from Ohio 4 years ago with the intent of build a SB house in Marquette, KS. As wheat straw is more abundant in this state than anything else, it made sense to build SB.

    I have design over 100 homes sense I was first smitten with the idea 7 years ago but due to unforeseen circumstances the dream has not come to fruition.

    So setting our sights on build our dream house in the future be bought a stick build with an acre of land for $69k. The 1200sf 2 story house has a very small partial basement and 2 bedrooms. The back porch (roughly 8’x26′) was enclosed about 16-18 years about and serves as a small mud room and laundry room. I would like to tare it off and add a 15’x26′ 1 or if possible 2 story addition with straw bales.

    The roof needs to be replaced anyway as there are 3 layers of shingles, one of which is wood. I would like to use these to enlarge to laundry and if 1 story create a new family room. If 2 story I would make the second floor a new bedroom.

    Any thoughts?

  45. Hi Chandler. That all sounds good. I think you will be happy with the results. There are some details to be clear on before you start like how to attach the new addition to the old structure, but it is all very manageable. Best of luck and remember to have fun in the process!

  46. Andrew, thank you so much for this site!!! It has answered many many of my questions. I do have a couple that I’ve not seen addressed yet (and I apologize if I’ve missed them, there’s a LOT going on here on this site :-).

    1) I live in Central Arkansas. According to the NETSTATE website, we have an average relative humidity of 57%. Is this too high to consider strawbale?

    2) I’m planning to build in stages. I’ve drawn up a set of rough plans that calls for individual “rooms” to be built around a central open area. These individual rooms are self contained. The idea is that I can build each ~15×15 room as a separate project and when all these rooms are complete, enclose the central area and tie them all together. However, this will essentially mean that every individual room will eventually have an “addition” added to it. (I really should put a pic of the plan up somewhere as this is kind of hard to explain). Are there any special concerns I need to be aware of?

    Thanks,
    Billy

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