If 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.
About the Author
Andrew Morison is a specialist in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs, as well as his hands on workshops. You can check these out at www.StrawBale.com/store.
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