Connecting a Straw Bale Addition to an Existing House

Written by Andrew Morrison

straw bale hallwayIf you are building a straw bale addition as an infill structure, in other words with a frame other than the bales that will support the roof, then all you need do is attach the frame of the addition to the existing home where the two meet. That can be as simple as adding a stud in the corner at the transition and nailing it or lag screwing it into the existing frame.

In order to make the transition in exterior siding work, you will need to either plaster the entire run of the wall (if the addition joins the wall in a straight run), or add a trim piece that creates a clear line of separation for the joint. “If you can’t hide it, make it show.”

If you are building a load bearing straw bale wall as the new addition and there is no structural frame to attach to the existing building, then make the attachment at the toe ups and at the box beam on top of the wall as both of those items are necessary in the load bearing structure. In either case, load bearing or infill, be sure to extend your shear resistance onto the existing wall if the joint occurs in a straight run.

If it is located in the turn of a corner, then you do not need to wrap the shear transfer material as the corner will likely have shear resistance in it already. You may want to consult with an engineer to insure that your shear transfers and load transfers are adequate throughout the addition as well as at the union of the two structures.

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6 Responses

  1. At 7:00 AM, Dale Basinger said…


    I will tell you what I am doing and then ask your advice and opinion. I have a single wide mobile home, and I am putting a roof over it and an addition for two bedrooms. The mobile is 14 x 68, the roof will cover it and 12 feet along the entire length. The addition will only be 12 x 28, the rest of the area under the roof will just be open. I will support the roof with 26 treated 4x4s. I want to use strawbale for the addition, but need the floor to be even with the floor of the mobile, as I will only be heating with a wood stove in the center of the mobile, and a lower floor in the addition would not heat well by convection. If I frame up the floor of the addition, 2×10 band joists with 2×8 floor joists, would this be adequate for strawbale walls? Thank you for your time, Dale Basinger.

  2. At 7:28 AM, Andrew Morrison said…

    Thanks for the question. This sounds like a good idea. Here’s my feedback: If I read your post right, your roof on the new addition will be 12′ from the trailer and the addition is also slated at 12′. The addition will need a roof overhang so the entire roof from trailer to end needs to be at least 14′ yielding a 2′ overhang on the bale addition.
    The floor joists need to carry the weight of the bales. I suggest you look in your code book and see what the requirements are for an additional 40 pounds per square foot of dead load on the floor system. That is a good number to consider for bale construction. Remember, bales are heavy. I think 2×8 floor joists will not be strong enough if spaced at standard intervals. I usually go with 11 7/8 engineered joists 24″ on center at a minimum. If you need to stay at 8″ due to the height considerations you mentioned, you will have to shrink the spacing to say 12″ or double the joists and stick to 16″ centers. Again, the code book for your location will give you better direction on this. I am not an engineer, so you need to confirm whatever you decide to do with an inspector or an engineer to be safe, but at a minimum, I suggest you follow the recommendations above. Good luck and happy baling.

  3. A nice little article,
    I am planning to do just that -add an addition to my mobile home. Thanks for being so specific!
    One word of caution reg’d wood heat in a mobile home -it may not be tolerated by your insurance. I was strongly advised against it by my insurance agent, the boost in premiums (if I could even find and insurer) would have covered half my gas bill!!
    I was also thinking of ‘outsulating’ the existing mobile home with straw bale, but the extra work on the roof and foundation has discouraged me from that. Can you think of an easier way to ‘outsulate’ the existing structure?

    Thanks again, cheers!

  4. It’s hard to wrap a building without running into the “extras” as mentioned here. You might be able to do it with a small rubble trench foundation and “mini-roofs” built to tuck in under the existing roof and supported on the outer edge with posts to small concrete piers. Just a thought. Good luck.

  5. Hi Andrew, I am hoping to use the winter to plan out the straw bale addition to our farm house. Here are some of my concerns… how do you suggest I elevate floor for the strawbale addition to match the height of the floor of the existing house or should I just make it a step down? I am thinking of mixing traditional construction with supports for the roof, the addition is 24’x20’ what are your thoughts on that? Thank you for your time.

  6. Hi Kristina. That will depend on the current height of the house and whether matching that height would make sense for the addition. You should be able to create the foundation for the addition at any height, assuming the topography isn’t too restrictive. When setting the foundation, be sure that you’ve calculated the flooring system and finish materials into your equation. It’s best to either match the existing levels perfectly or make then a full step (6″ to 8″) different. Anything closer than about 6″ will be an awkward transition and may cause you to stumble when going from one room to the next.

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