Retrofitting with Straw Bales

I was recently asked about the viability of retrofitting an existing structure with straw bales. I get that question a lot and figured it was time to blog about it. Any structure can be wrapped with bales, it is simply a question of how much extra work will be required to make that structure function properly once wrapped. Consider that you will be adding about 2′ to the exterior of the house and that all bale walls need to have adequate roof overhangs to protect them from rain. So, unless your house originally had 4′ overhangs, you will have to extend the roof to accommodate the bales. This means opening up the roof system and “sistering” new rafters to the old ones with enough attachment to support the cantilevered rafter tails. A good rule of thumb is that should be a two to one. If the overhang is 2′ then the attachment needs to be at least 4′ long. Of course then the sheathing will need to be attached and the roofing material feathered back into the existing roof.

Another area of concern are the window and door openings. These will end up recessed into the wall by the thickness of the bales, leaving a large sill that could collect water. Your options here are to move the windows and doors to the outside of the wall using standard bucks, or to slope the sills and cover them with some protective material that will quickly drain any water away from the house. Some options are concrete, granite, or finished wood sills. Doors don’t have this option because they don’t have room for sills.

Finally, will the bales collect moisture against the existing building? If the building is wrapped in a non breathable material like metal siding, the chances are high that the bales will end up soaking in the moisture that collects against the metal surface. A drainage system is a good idea in this case. Something like the product on this website (www.mortarnet.com) is a good idea in this application. Regardless of whether the back of the bales is separated from the existing structure by a drainage channel, the bales need to be fire proofed which means a layer of plaster needs to be added to the bales. This is easy on the face of the new wall, but the back of the bales will not be accessible once the bales are installed. For that reason, each bale needs to have plaster or a clay slip applied to the back before it is installed. This is a slow process, but an important one. The bales could otherwise present a fire risk if not covered. This is most important when using the drainage system but should also be done without a drainage system for added protection.

It sounds like a lot of work, but the results are beautiful and efficient. You may have to work hard to accomplish the wrap, but you will be paid back for years and years with lower utility bills, a more beautiful home, and a higher resale value should you ever decide to leave. I hope this gives you some ideas of what is needed for a bale retrofit.

38 Responses to Retrofitting with Straw Bales

  1. Kirk Sat, June 21, 2008 at 12:52 am #

    Hi Andrew,
    We are considering retrofitting and wrapping the outside of our house that we built. The house is not fully finished yet so it would be a good time for us to do this and get the added insulation. We are currently finishing off the inside and have not put any siding on yet so the plywood is still exposed, we have wrapped with Tyvek but that would be easy enough to remove. If we wrapped with bales the roof overhangs and door and window openings would not be a problem for us so the question we have is regarding the bales going against the wood exterior walls. Would a drainage system such as a mortar net system still be a good idea in this case? Or would just applying a clay slip to the back of the bales and pressing it against the wood be a safe option?

    Thanks!

  2. Andrew
    Andrew Thu, July 3, 2008 at 9:55 am #

    Kirk,
    Sorry for the delay in responding to this question. Somehow I missed it. I would use the mortar net AND apply a slip to the back of the bales. The slip is for fire protection, the net is for drainage of potential condensation. Be sure to move your windows to the exterior of the bales as well so you don’t have deep EXTERIOR window sills.

  3. Greg Thu, January 29, 2009 at 11:11 am #

    Andrew- thanks for putting this site up! What a wealth of info. We bough an older house in ND that needs severe insulation help. It has vinyl siding which I’m planning to take off and surround with SBs. Couple of questions. Do I need exterior framing to hold the bales up and, if not, do they attach to the wall somehow? How are exterior window frames held in place? Is there is a place on your site or a book you would recommend that deals specifically with retrofitting issues? Much thanks and looking forward to being warmer NEXT winter. Greg

  4. Andrew
    Andrew Fri, January 30, 2009 at 9:37 am #

    Greg,
    The bales will need to be supported on something, preferably a foundation that gets them up off of the ground. The bales should be attached to the walls at every course level by stapling a folded piece of plaster lath to the wall and then landscape pinning it to the top of the bales. I thought I had more on the blog about retrofitting. You can try searching at the top of the home page. If not, I don’t know of any great resources for the topic. I can answer some specific questions you have here if that helps. Also, I am available as a consultant as well should you decide to move forward with your project.

  5. Quentin Mon, February 9, 2009 at 4:57 am #

    Hi, Andrew

    I’ve spent alot of time on your website. I am contemplating converting an existing cinderblock structure (auto garage) into a single family dwelling. I figured since it has lots of interior space I could manage to sacrifice two feet of infill insulation on the inside of the structure – thereby avoiding an additional roof overhang as well as requiring no window and door bucks.

    I understand that the infill solution requires stucco or plaster on both sides for fire and rodent protection, but I’m not clear on the moisture concern. I’m afraid I don’t quite understand the mechanics of air movement and moisture/condensation build-up for exterior walls. I certainly don’t want to introduce mold that effectively would be impossible to repair.

    If you’re wondering, my intention is to fashion several vertical stands of stick & bale insulation ‘blocks’ with plaster pre-applied to both sides, then fasten and seal them at the vertical seams. (Although I haven’t figured out what capping system is needed at the ceiling yet – thoughts?).

    You suggested Mortar Net and other drainage techniques, but I wonder if an interior application of infill would require nothing more than a vapour barrier between the cindercrete and infill straw bales (keeping the cold and warm air separate from each other to solve condensation) – and the interior heat inside the structure would allow the straw bales to remain relatively dry. Would this imply that an airspace would not be required?

    Given the permit requirements for residential construction, I totally understand that a contractor or engineer would have expertise in this area, but also given the bleak industry knowledge in straw bale construction, I figured you’d have as much or more to offer.

    thanks and great website!
    Quentin

  6. Andrew
    Andrew Mon, February 9, 2009 at 9:40 am #

    This is a great situation to explore and I want to admit up front, that I am not a condensation expert. I know that block is quite breathable. I know that water leaks through it all the time in underground applications. I do not know whether or not that would allow for the moisture to move through it from a bale wall. I get the concern about the col meeting the warm and there being a place of condensation where they meet. Plastic will not solve the problem though. In fact, it will likely make it worse. Moisture from inside the home will inevitably push through the bales and hit the plastic where it will get rapped and sucked into the dry bales. My personal belief is that you will be better off leaving the bale/block transition open and allowing any moisture to drive through the block. I also think you can get away without plastering the back side of the bales. I would stack them tight to the block and use blood lath to attach them to the block wall at each course. By keeping them tight to the block, there is no room for fire to spread and it will allow for more moisture to move out freely. Also, the plaster would be wet upon application and would introduce a LOT of moisture into that questionable area. That’s what I would do.

  7. Quentin Mon, February 9, 2009 at 12:42 pm #

    Thanks, Andrew
    You’re effectively suggesting that the existing block can act as a plaster layer? That indeed makes sense, given that there’s no plastic or vapour barrier traditionally used inside the plastered bales either… good call.

    So my other question is, what is blood or diamond lath, how to I attach it to the block wall, and what holds the bale stack to either one? Friction? string? Maybe a link to demo video or illustration, since I am terribly spatial oriented and not so ‘sequential’, if you’re pickin’ up what I’m layin’ down… 😉

  8. Andrew
    Andrew Mon, February 9, 2009 at 2:57 pm #

    I hear ya. Blood lath is an expanded metal lath used by plasterers. You can find it at plaster yards or some home supply yards. The lath comes in 2′ x 8′ sheets. I cut it into smaller sections, say 12″ x 18″ and fold it so that it lays 12″ onto the top of the bale and the remaining 6″ is folded up the wall. Then you can attach the lath to the wall with concrete screws and to the top of the bale with 9″ landscape pins. Hope that makes sense. I know I have an image of what I’m talking about somewhere on the site, so you might do a search for attaching lath to posts or something like that. I don’t think I have a video of the concept though. Sorry.

  9. Quentin Mon, February 9, 2009 at 3:22 pm #

    Gotcha, thanks – I totally ‘get’ the description you’ve provided: no illustration needed. I would assume that every other course might have these 12×18″ hangers applied to every bale? or is that excessive?

  10. Andrew
    Andrew Mon, February 9, 2009 at 3:36 pm #

    I usually place them every 4 feet or so on each course. That way every course is tied in.

  11. ellen Mon, February 8, 2010 at 10:28 am #

    In my suburb, I am not allowed to wrap my house in straw bales. I am allowed on the inside. I have a concrete slab foundation with 4″ walls and outside mostly brick exterior. I live in north texas and was wondering how 12″ thick straw bales would work, since 18″ would drastically cut the sq ft. And what would be the best way to do this. Also, are 12″ bales workable for insulating the attic? Or should there I stick with the 18″? At the moment my attic is simply insulation over wallboard and quite terrible. Thank you!

  12. Andrew Morrison Tue, February 16, 2010 at 9:29 am #

    Hi Ellen. 12″ bales will be good; however, you can get pretty darn close in insulation value with regular insulation. For example, a 12″ bale will likely be somewhere around R-27 and you can get R-21 insulation to fit in a 6″ wall. If you are specifically looking for the straw bale look and feel, then it’s worth it; however, you’ll be giving up twice the floor space for only R-7 more of insulation. You might be able to get that last R-7 with some rigid insulation as well and still only use 9″ instead of 12″ + 1 1/2″ of plaster and a lot of labor. I love straw bale, but you might want to go with a simply retrofit in this case for the sake of your floor space. Finally, I don’t like using bales in the attic as the weight requires additional framing to be installed to handle the overload on the frame. I, again, suggest you stick with conventional insulation in the attic. Best of luck.

  13. mike brown Wed, February 2, 2011 at 4:14 pm #

    Andrew – What do you do about bale compression using this technique?

  14. Andrew Morrison Wed, February 2, 2011 at 4:29 pm #

    Hi Mike. Good question. The best way to deal with it is to plan for it. In other words, make sure you have the ability to apply strapping or some other material to tighten up the walls. What’s even better is to use very tightly compressed bales and then squeeeeeeze them into place along the top end of the building. Because they have no load on them from the structure, the amount of settling will be negligible, especially with very tight bales.

  15. Miha Glockenspiel Thu, February 16, 2012 at 8:53 am #

    As I was looking for answers regarding insulating existing buildings with straw I found your post here: I am planing a different scenario and look for helping answers: I have an building where I plan to add straw-bales on the inside. Yes the building is big enough, but I wonder there to about connecting both structures and moisture. Do i install the straw wall first and than let it dry out pretty far before closing in all the straw?
    Thanks Miha

  16. Andrew Morrison Mon, February 20, 2012 at 9:15 am #

    You’ll want the bales to be dry before you install them. The challenge in this scenario is the breathability of the existing exterior siding. If it is fairly tight, there is a good chance that condensation will build up at the transition from bale to siding and that condensation will be sucked back into the bales causing long term moisture damage. It’s best to install a ventilation gap between the two materials. The easiest way to do this in the scenario you are considering is to actually install a vent gap with a material like mortar net (www.mortar net.com) before you install your bales. You’ll also want to slop some plaster on the face of th bales (against the mortar net) to make sure the bales are completely fireproof. That plaster will dry by means of th ventilation gap. The interior plaster can be applied once the walls are complete as long as the building is allowed to dry out (open windows and dry air allowed to reach the bales). Hope that helps.

  17. Jan Henkes Wed, January 9, 2013 at 5:30 pm #

    Straw bales don’t burn as long as the straw is packed together well. Lab test have shown this time and again.

  18. Andrew Morrison Wed, January 9, 2013 at 9:33 pm #

    Not entirely true actually. The application of a scratch coat or slip coat of plaster (at the very least) provides the fireproofing necessary to protect the bales. The edges of bales will burn, enough to support flame spread up and into the attic where the framing can catch fire. Furthermore, if flame is allow to get close to the strings of the bales, they can break and the bales can catch as oxygen gets into them. It’s a best practice to provide the extra protection and use teh slip or scratch coat of plaster on any surface exposed to sir movement.

  19. Dave lang Tue, September 3, 2013 at 12:39 pm #

    I too would like to add straw bales to the inside of part of my wood frame moisture sealed house. Is it adequate to use Mortar net if you don’t have vent holes to the outside. My buddy thinks I need to vent top and bottom and leave dead space behind the SB. My concern with this is that it would bring the cool air off the existing wall and defeat the purpose of the increased insulation. I understand about sealing the wall sides of the SB. Thanks, Dave

  20. Andrew Morrison Sun, September 22, 2013 at 10:19 am #

    I agree with your friend as the ventilation is key if the exterior material would otherwise seal the wall from moisture escaping the home.

  21. Lee Fri, November 15, 2013 at 8:35 am #

    Hi Andrew, As with many people who have posted here, I’m wishing to add straw bales to the inside of a conventionally built structure. This has 2×4 framing, plywood sheathing, exterior cedar clapboards. My goal is to retie full size bales into 16″ wide x 10″ thick bales (to maintain more interior floor space) and stuff them in between the studs, packing all the cracks with straw as well. I would add some small wood studs or bamboo for internal pinning, and attach wooden lath to them, followed by earth plaster. Everyone seems to say that having a non-breathable wall system on the outside is a bad idea, but how is the system I described any different from using blown cellulose into a wall cavity? I live in the rainy part of Oregon and I’m more worried about moisture from the outside than the inside; I can’t imagine that water that gets into the walls is going to be pushed out by vapor pressure when the exterior air is close to 100% humidity.

  22. Andrew Morrison Sat, November 23, 2013 at 11:12 am #

    There are a couple of issues with this idea. First off, 10″ bales won’t give you very much insulation value. The bales depend on their thickness to gain the most insulation. A 10″ bale would likely give you an R-20 or so value. You can get R-21 with 5.5″ batt insulation which is MUCH faster and easier to install in your situation.

    Secondly, you would need a ventilation gap to allow the bales to breathe against the exterior siding. Without it you would be trapping moisture in the walls. This is especially important in wet climates like rainy parts of Oregon. The difference you ask about is in the vapor barriers. A wall filled with cellulose insulation would have a vapor barrier (plastic sheeting) on the face of the studs between the insulation and the drywall. That stops moisture from being able to enter the walls. Bales won’t and can’t have that.

  23. Eric Sat, January 11, 2014 at 9:30 pm #

    Hello Andrew,
    I have an existing barn that it steel sided with osb underneath; I am wanting to wrap the inside with bales. My question is , will I need to plaster the side towards the existing wall? There will be an arirgap of 1 1/2 as everything is firred out with 2×6. My plan is to pour a slab foundation along the wall and to build a toe up with gravel in it to help eliminate any moisture. I do also have tyveck I between the metal and the osb. I would also like to see more info on making hydraulic plaster if you would. I am not sure on the slaking process for making the lime putty. Do you just add the type s lime to the water and leve it covered for a certain amount of time? How long? Will it need to be kept inside, where it will not freeze? The cost of the premade hydraulic plaster is scary compared to the available products at home depot.
    Thanks for a great website.

    Eric

  24. Andrew Morrison Mon, February 10, 2014 at 9:44 am #

    Hi Eric. You would indeed need to plaster the side of the bales facing the existing structure (against the air gap) to prevent any chance of flame spread. The easiest way to do this is to dip that side of the bales in a thick slurry before you stand them. That way they are plastered before they go up. It doesn’t have to be pretty! :)

    In terms of the hydraulic lime, you definitely get what you pay for. You can slake the cheap stuff, but there will be a lot of potential for “pops” in your plaster. This is where unhydrated portions of the type S suddenly hydrate on your wall and explode (gently, not dangerously) out from the finished plaster. You can minimize this by screening the material through a very small screen size (window screening, for example) as you add the material to water. That said, because type S is a high dolomatic lime with high levels of magnesium and other impurities, type S lime is not very good for use in high quality plasters. You can see a video on mixing here. Once mixed, and slaked, you would add your brick dust or whatever other material you are using to create the hydraulic properties. This should be done right before you mix in the sand and use it as plaster because it will cure fully within 24 hours. Before the hydraulic properties are created, the lime putty will stay soft forever as long as it is covered with water.

    Your best bet, other than buying the good stuff, is to start with quicklime and slake from there. You can see a video on that here. No matter what you use, be careful. If you add the lime too quickly to the water (I would always add the lime to the water and NOT the water to the lime) it can explode and shoot boiling lime all over you. Not a good thing.

    If this will be a home you are investing in, I would suggest investing in the NHL plaster. It is expensive, but it is also the best material available for you.

  25. Adam Fri, March 21, 2014 at 6:16 pm #

    Hi, I’m building a timber frame straw bale infill salt box style with a green house on the front. Can I put a cheaper recycled steel roof on for the first year and then straw bale after if I strap it for breathing room?

  26. Andrew Morrison Fri, March 28, 2014 at 12:29 pm #

    I assume you mean for the siding. If so, yes; however, you will need to rough plaster the bales where they will be exposed to the ventilation space between the bales and siding for fireproofing.

  27. j.m.barnes Mon, May 26, 2014 at 8:56 pm #

    I want to strip all exterior walls to studs. Infilling them with the traditional straw bale and lime soil hay mix. Then what lime based cure would you recommend.

  28. Andrew Morrison Sun, July 20, 2014 at 9:13 am #

    I personally like a lime:sand mix leaving out the soil for all aspects. They don’t bond well together. I would do a 1.5:1 sand to lime mix for the scratch and a 2:1 sand to lime mix for the brown and finish.

  29. Cathryn Gibson Fri, October 24, 2014 at 11:03 pm #

    Andrew,
    Iv a brick home that I rendered with a cement & white sand mix unpainted.
    I would like to do a bale wrap, what should I put between bale and cement facing?
    Thanks mate for your time,
    Cathryn

  30. Andrew
    Andrew Sat, October 25, 2014 at 10:10 am #

    I would suggest a screened ventilation gap between the two. Best of success.

  31. Jeff H. Mon, November 17, 2014 at 9:22 am #

    Hi,
    I was wondering if any special needs to be done to a new straw bale home that I’d like to possibly cover the outside in metal corrugated siding? I take it the outside straw walls still need a cover of slip before they get covered in the metal siding? I’m having a hard time finding a lot of info on going this route. I’m figuring the metal siding would protect the straw better here in snowy Minnesota, and would go up faster then have to apply many layers of mud.
    Let me know what you think!

  32. Andrew
    Andrew Mon, November 17, 2014 at 9:29 am #

    Hi Jeff. When I do metal siding (or any siding other than plaster) I provide a vented air gap between the siding and the scratch coat of plaster. That allows the bales to breathe and also minimizes any risk from condensation on the metal getting into the bales. It is faster in some ways and slower in others. I do believe it will protect better from the harsh weather as long as the bales are sealed well with a thick scratch coat of plaster. If that coat is not thick, the bales won’t seal from wind and that will lower the Rvalue of the home. It doesn’t need to be a beautiful scratch coat since it will be hidden, just thick and hardy.

  33. Jeff H. Mon, November 17, 2014 at 11:14 am #

    So do you think horizontal 2×4 spaced about 2′ apart running around the house from bottom to the top over the plastered bales, then covered with the metal siding attached to the 2×4’s would provide decent ventilation? The siding is ribbed so air would be able to travel up and down on the backside of these metal panels.
    Do you think the 2×4’s may kick out the siding to much from the bale wall?

  34. Andrew
    Andrew Mon, November 17, 2014 at 11:22 am #

    That is what I would do. The bale walls won’t be perfectly flat, so anything less than the 1.5″ may make it hard for air to vent properly. I would suggest putting backing in to the bales (horizontally) that is tied into the wall behind the mesh. Then felt those and plaster over them. This way you have something to screw into for solid attachment of the 2×4’s. Use a darby (4′ trowel) to get the walls as flat as you can in the scratch coat.

  35. Jeff H. Mon, November 17, 2014 at 11:28 am #

    Thank you for the advice! :)

  36. Andrew
    Andrew Mon, November 17, 2014 at 11:33 am #

    You’re welcome.

  37. Carlos Miler Wed, November 19, 2014 at 7:32 am #

    I have a metal building cabin. The walls are 6″ fiberglass insulation. The water pipes are on an outside wall for about 16 feet. The pipes begin to freeze at about 5-10 degrees. Would stacking straw bales against this portion of the wall during the winter help this situation? thanks.

  38. Andrew
    Andrew Wed, November 19, 2014 at 10:38 am #

    Hi Carlos. It could help; however, it would be a temporary solution. You might consider doing a permanent wrap in that area or even framing out an additional stud wall with added insulation (would be faster and easier than baling that wall, ultimately.

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