Straw Bale and Steel Framing

Written by Andrew Morrison

Steel Framing for straw bale houseIf you have ever wondered why you don’t see a lot of steel framed straw bale homes, the answer can be summed up in one word: condensation. That does not mean that a bale house cannot be built with steel framing; however, it does mean that special care must be taken to protect the bales from condensation. I have outlined the easiest way to accomplish this below.

First, let’s discuss why someone might choose to use steel in the first place. One major reason that people opt for steel is the overall strength of the material. If the structure you are building requires large open spaces, and thus large spans for the framing material, steel may be a good choice for you. Take a look at the photo and try to get a sense of scale. When you compare the overall space to the size of the people in the photo, you can start to see how large of a span the rafters are actually dealing with. Spanning that same distance with wood would require either much larger timbers or more of them. Either way, the material use factor of the structure would have to be considered.

Another reason for using steel framing also comes from a sense of scale; however, this time it is a small scale reason: termites. If you live in an area rich in termites, you probably don’t want to build with wood. If you do, you will have to work with treated wood and who knows what the health implications of that are (I mean really…who knows?). By using steel, you can eliminate the risk of termites ruining the structural elements of your building. Don’t worry, termites don’t seem to be terribly interested in straw as a food source. That said, I would still recommend that you build with the intention of keeping the termites out nonetheless.

As is the case with any material, there are ups and downs to using steel. One negative detail that often surprises people is how poorly steel performs in fires. Sure, it doesn’t burn, but it does melt. What’s more, it melts at a lower temperature than thick wood beams will burn through. This means that steel buildings are at risk of collapse from heat even when the structure itself is flame resistant.

Steel comes from a finite material and so the question of sustainability must be addressed. Once the raw materials are all mined and the natural landscapes are all devastated by that process, we will have none left. This does not take recycling into account, which is a very important detail to consider; however, even with the best recycling programs, steel is still resource heavy on the planet. Although not always farmed in renewable ways, wood is a living, growing entity and can be replanted and harvested. In fact, many engineered lumber materials are made from small diameter, young trees that can be planted and harvested in a short time period.

If you have decided to work with a steel frame, you must have a plan for isolating the frame from the bales otherwise you risk damage to the straw from condensation that will form on the cold, steel frame. The easiest and best way I have discovered to accomplish this is to box out the frame so that it is completely isolated from the straw. As you can see in the picture above, the steel posts have been completely encapsulated in a wooden box. The inside of the box is insulated so that very little or no cold joint (thermal bridge) is created in the process. The wood box also provides additional nailing surface for the welded wire mesh. This is a simple way to create the boundary you need to provide a safe transition from bales to steel frame and one you can do by yourself with no special tools or materials.

If your plan to use steel is the result of termite considerations, then boxing out your steel frame in wooden panels is probably not your first choice. In this case, you can step up the cost of your insulation package and have the entire frame sprayed with an expansive foam insulation like Icynene. Make sure that you apply enough insulation to the frame to eliminate any cold steel from coming in contact with the straw. This won’t give you any additional nailing surfaces, and sprayed on insulation in large quantities is typically something you have to hire out. Nonetheless, it works well and when considered in the overall scheme of things, may be the perfect solution for you.

Whatever manner you choose to isolate the straw from the steel, choose it with one major concept in mind: eliminating the risk of the bales coming in contact with condensation formed on the surface of the cold steel frame. With that one goal in mind, I trust that you will create the perfect approach. In fact, perhaps some of you have already accomplished this on your own buildings. If so, I’d love to hear what solutions you came up with. Please join the conversation and share your experience in the comments section below.

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29 Responses

  1. I am planning on a steel frame mainly because buildimg regulations demand it, I was hoping wrapping a hesian type sacking around the beams would solve the problem but now I will look into this more thank you.

  2. I’m curious to know what you base the statements above about steel melting before timber would burn. Can you please state some fact and give temperatures to go along with that? Did you know that steel needs greater than 900 degrees to melt?? Some timbers such as those used in roof trusses will ignite in an oven at 280 degrees.

  3. I have a lot of 2 3/8 and 2 7/8 drill steel I thought about using as posts and beams. What if the steel frame was framed in on the interior side of the bale? would you still get lots of condensation? It would be easy to cut in with a saw.

  4. Andrew,
    Your good advice will save most folks the trouble of having to deal with the thorny subject of how to avoid condensation (a subject which few understand) and probably keep them out of trouble. Of course, it is possible to design all building types (including straw bale) so water does not condense inside the building assembly where it could cause damage. For those who cann’t resist asking “why”, there is some good information at on condensation issues.

  5. I would think you could box the steel with hard foam and glue. It is only there to stop a thermal bridge.

  6. This is based on my conversations with firefighters. They talk about seeing metal studded buildings collapse while the larger wood beams have only charred on the exterior. The melting point and burning point of any material will change somewhat with thickness. For example, it may be easy to melt a steel stud but likely is much harder to melt a large steel beam. The same would be true for wood studs versus large, structural beams.

  7. Andrew, How big a problem would condensation actually be? With most of the steel work wrapped in a strawbale wall, and straw wicking out the moisture? I would have thought that the temperature fluctuation of the metal and the relative humidy variation at that point would be regulated by the wall?

  8. Hi Andrew, this subject interests me, as stated by Dave regarding “Drill Steel”, we have alot of this material available here in SW Saskatchewan and it is very cheap or next to nothing for costs to acquire. I was thinking of wrapping the steel in tar paper say 6 mil. Would that solve the condensation concerns issues? Thanks for posting these topics! Cheers! =0)

  9. Steel beams can actually be cheaper too!
    I changed the spec on my garage during construction to get rid of a central post and the engineer required a steel beam instead of the pair of laminated wood beams.
    By the time I did all the accounting, a single steel beam turned out to be a little cheaper and quicker to install (we had a crane on site) and stronger than the laminated wood beams.
    We also saved on the footing and post required for the lam beams.
    There’s a cool factor too.
    Any garage lover who walks into the open, concrete floored, 25×25 work space with an I-beam down the centre of a 12 foot high ceiling immediately thinks of rebuilding a classic car or butchering their own game.
    I was surprised at how affordable steel was, considering that the rest of the house is built of timbers and I live in lumber-land.
    Another note is that ‘froth packs’ are an option for the DIY spray foam application and see if you can find ‘new wood’ instead of treated wood (my local dealer said it’s no longer on the market -too bad, it’s awesome).

  10. Hi Dave aka #4
    I had the same idea of using used oil drilling pipe Talked to 3 counties and each said i had to have a structural engineer stamp $$$ The structural engineer said since I did not have a steel certificate from the mgf or the mgf stamp on the steel. I would have to have the pipe stress tested by another type engineer $$$ So I asked “what if I weld 3 pieces together in the form of a triangle ( the wall of the pipe is app 1/4 inch) Same answer, stress test it and i would have to have a certified welder weld them and a certified steel mgf build connectors or buy new ones out of a catalog
    From the talks with the bld depts the steel consortium will not give the bld depts a code to make informed decisions unlike the wood industry Life goes on but not building
    I am still looking for counties that have no or minimal bld codes There is 1 in eastern NV that only requires a state acceptable septic system I am now looking at prefab hay storage roof systems with a residential roof certification and build under it

  11. Hi there,
    I’m a lecturer in two subjects here at my local University: (1) Civil Engineering Materials and (2) Structural Steel Design. Which, when combined with my desire to build a straw bale house soon, means I feel that I should (finally drag myself into the e-age and) address the issues posed above.
    Basically, steel doesn’t “melt” until the temperature gets to around 1500 degrees Celsius (or, 2730 degrees F, for my US friends). What does happen though, is that at around 720 degrees Celsius (or, 1320 degrees F), the “Young’s Modulus (E)” of structural-steel can drop away fast. “So-What?”, I hear you say? Well, with smaller values of E, for given stresses within the steel (where these internal stresses come from the Loads applied), the amount of displacement (or movement) of your steel increases, often dramatically. So, whilst your steel will certainly not melt, it becomes way more “plastic” when heated above 720 degrees Celsius; which is a really easy temperature to reach in a “standard” fire. Whereas large cross-sections of structural timber – so long as they’re not burnt through – will not lose elasticity (anywhere near as near as much) when heated to the same relative temperature as their steel cousins. Of course, with enough heat over enough time, all timber will burn, but, experience and experiments have shown that charring on the surface of timbers can actually provide some insulation (for a little while). The upshot is that, in many situations, a steel structure can be far more dangerous (in terms of instability) after a fire, than a timber structure (under the same loads); hence, why fireman are always very interested in the materials used in the structure. Which is why fire protection of structural steel is a big part of engineering design & construction.

    With regard to insulating the steel to prevent condensation, the big issue is all about the temperature differential between the steel and the straw. With no differential, there’s most likely not a lot of condensation. One solution I’m thinking of is this: how to get the steel up to the temperature of the inside of the house/bale, and then stop/minimise any heat loss from out of the ends of the steel segment. That way, the steel’s temp and the straw’s temp is much the same, thus, no condensation perhaps. I guess this is an area that is worth doing some experiments on, and see what the actual data is.
    Dr Michael N, PhD.

  12. The problem with a steel structure in a fire is not so much melting as a reduction in load bearing strength. Structural steel maintains its rated strength to about 600-700 degrees F after which it drops very fast. At 900 degrees its strength goes down to about 50 percent and at 1100 degrees, the temperature of an average house fire, it drops to 30 percent. The melting point of steel depends on the alloy, but no alloys of structural steel will melt at under 2,000 degrees F.

    My source is the link below that shows various metals including structural steel and their strength as temperature changes.

  13. Regarding steel melting in a conventional house fire.

    This is from Wiki pedia “structural steel” The lowest temperature at which a plain carbon steel can begin to melt, its solidus, is 1,130 °C (2,070 °F). Steel never turns into a liquid below this temperature. Pure Iron (‘Steel’ with 0% Carbon) starts to melt at 1,492 °C (2,718 °F), and is completely liquid upon reaching 1,539 °C (2,802 °F). Steel with 2.1% Carbon by weight begins melting at 1,130 °C (2,070 °F), and is completely molten upon reaching 1,315 °C (2,399 °F). ‘Steel’ with more than 2.1% Carbon is no longer Steel, but is known as Cast iron.

    The article goes on saying that melting is not what you need to think about as it is the softening and deformation of the structural members that will allow the collapse of a building.

    At least steel can’t give off toxic smoke which is what kills people in fires anyway.

  14. I kept thinking about the five elements and simple. And I think it would be possible to coat the steel with a mixture of woolfibre and clay or plaster and woolfibre. Wool doesn’t burn and clay and plaster only crack. Wool as well as clay and plaster absorb moisture and release it slowly. Still, wood is the quickest option, it’s compatible with water. What do you know about mondholz, do termites eat that? I know it,s fireretardent, in Switzerland, Austria and Bavaria they use it in chimneys. Chars on the contactside but doesn’t burn.

  15. It’s not the fact that the steel will melt, like in red hot dripping metal. It is about the fact that steel looses his structural stength way before the wood does. Also the fact that once the outher layer of the wood is chared, a oxygen-poor area in the chared cracks slow down the further burning of the core. Steel conducts the heat of the fire directly into the core of the beam.



  16. Condensation is a relatively big deal if it happens inside the bale wall because the straw does not move that moisture to the exterior very quickly. Houses built with rebar pins experienced a lot of rot around each piece of steel rebar in houses that I have seen “opened up” for remodels, etc. I would take it seriously myself. I think there is some great information in the comments about how to manage the impacts. The big marker is the dew point and the different material temperatures. If you can insulate the steel in a way to minimize the temp gradient, then you should be fine. If nothing else, consider placing the steel frame on the interior of the wall (just inside the bales) so that the steel is very close to the temperature of the bales and the ambient room temperature.

  17. Greetings All,

    I have been fascinated by strawbale construction for along time. I recently saw a steel frame house infilled with Strawbale. They put the posts OUTSIDE and the straw well under the roof. That said there were still the rafters and I am not sure how that was “handled” if at all. I live in TN and we have termites here (but not like Miami, FL), so that and the relatively low cost and easy dry-in of steel framing is good.

    I have thought that you could insulate the roof with insulated roof panels, but did not know how the rafters that would protrude from the structure would be insullated to avoid the thermal bridge (we get cold weather here to some extent). I guess you could sleeve the steel out with insulation a bit. Maybe do some sort of flashing around the rafters too with tar paper or something like that.

  18. The issue of condensation is reduced if the building is not heated or occupied by humidity-generating activity. For example, an agricultural building with lots of ventilation or a storage facility would not have these issues. Most steel framed (steel studed) buildings are isolated from the exterior elements with a layer of rigid insulation as a thermal break. I have not heard of steel studs in houses attracting moisture and condensing on a widespread basis, thereby creating damage to other types of insulation, but it could, in theory, happen.

    The other way to minimize this risk is to install a vapor barrier (required by code) on the interior of the wall. Of course this strategy hinges on the treatment of penetrations and joints in the plaster or drywall. A vapor barrier can be achieved with the right type of paint and/or with plastic under the drywall and at joints, or lots of expanding foam (not desireable).

  19. To Dr. Michael N. PhD: We have a steel building 33’x 48′ that we want to convert into a house. I want to put the straw bales on their own foundation around the house, rather than on the slab and between the beams. Do you think that the R value of the straw being wholly between the beam and outside will insulate the beams enough to prevent condensation?

  20. We are looking at building a free standing metal roof with a straw bale perimeter wall well inside the overhang and away from the uprights. Can any of you recommend an engineer with a CA stamp we could talk to? we are looking at a roof a bit more complicated than a horse arena, but with the same tech.

    BTW, Andrew, The DVD’s ROCK!

  21. Thanks Mike. I would suggest you contact Bill Taha at Precision Structural Engineering, Inc in Medford, Oregon. Tell him I sent you and he will take great care of you. 541.858.8500

  22. @Mike Pulskamp, where in Cali? We are building in spring 2015 in Roseville using a steel building with straw bale in-filled walls. Sounds very similar to your build actually. And if you and Andrew don’t mind, I’ll be making a note of the engineer’s info as well!

  23. A question… If we were to put up a vapor barrier on the outside of the steel frame’s girts (horizontal supports on outside of vertical supports) and butt the straw-bales up against this vapor barrier… would that be sufficient or would we still need to render/seal the inside face of the bales?

    This would fully separate the steel from the straw, but access to render the inside of the bales would be nil.

  24. The issue here is that you would not have any keying into the bales for the plaster. Also, there would be no way to sew the walls tight nor add welded wire mesh which does wonders for the overall strength of the home and for things like hanging cabinets, electrical work, etc. I think you would be better off to isolate the steel with spray insulation or frame boxes around them.

  25. Nick C.

    Please get in touch with me. I have looked all over your site and tried to contact you to no avail.
    I am very interested in your build! I am down in Ione area of Amador County. Maybe an hour to hour and a half South of you.

    I would really like to talk.

    [email protected] will work.

  26. Has anyone tried building a straw bale tiny house using an existing metal carport as a starting point? I am envisioning putting straw bales outside the upright of the carport and SIPs over the top of the metal roof with enough overhang to extend well over the straw.
    These metal carports are ubiquitous in my area, very sturdy, but not very attractive. If this was a viable building model there would be a lot of opportunity to turn these funky structures into functional eco-housing.

  27. Hi Donna. With the right detailing, this could work for sure. There would be things to pay close attention to that are specific to those builds (moisture migration, frame cover, etc.) but can definitely be done.

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