I know that the hosts of my upcoming straw bale workshop in Idaho are not the only people to love the idea of combining timber framing and straw bale construction. As such, I thought it was a good idea to share with you some details about pulling off a beautiful and durable home using both techniques. In general, the detailing of a timber frame straw bale home is very much like that of a regular post and beam straw bale home; however, the differences are important to discuss. How the bales attach to the frame and how to best seal the transition from plaster to frame are among the important details to consider.
Let’s start with how to attach the bales to the frame. Because the exposed timber frame is not buried in the bale wall, the point of attachment is very different than regular post and beam homes. You will need to attach the bales as often as possible because the framing members are far apart from each other so each attachment is key. I suggest that you attach at every bale course and every post intersection. I use plaster lath to make L shaped points of attachment as shown in the drawing to the left. Those pieces are attached to the bale with 10″ landscape pins and stapled to the framing with structural staples (1-3/4″ long, 7/16″ crown, 16g). Be sure to staple as close to the corner of the lath bend so that there is very little room for the lath to stretch should the wall be stressed.
You will notice that directly behind the frame there is a piece of 1/2″ plywood which the bales need to be notched around. This serves three main purposes. First, it provides a place to nail the field mesh (2″x2″ welded wire mesh) to provide a strong and tight bale wall. Second, it also provides nailing for plaster lath to be attached tight to the exposed frame. This will minimize cracks at the plaster to frame transition. Finally, the roofing felt that is applied to the plywood before it is attached to the back of the post will provide a vapor seal at the plaster to frame transition. I still recommend caulking this joint, but if that seal ever fails, there is a secondary vapor seal in place. You can better see how the plywood and roofing felt work in the picture to the right.
Another place where the detailing is slightly different in an exposed timber frame structure is at the eave lines of the roof. Because the roof line comes down into the space occupied by the bales, it can be difficult to get a tight fit without providing additional framing. As you can see in the picture to the left, the additional framing can be used to create an enclosed soffit on the outside of the bale wall. The inclusion of a bale stop at the top of the bales helps to tighten the bale walls and stop any loose straw from piling up into the rafter space. This void is best filled with insulation, making sure to provide for tight air seals at the transitions. Spray foam, although not environmentally friendly (in general) nor specifically healthy, does a great job and providing that seal. This is something you will have to consider the pros and cons of (foam seal).
If you want to complete your home with the look of exposed rafter tails, then additional framing is still needed; however, it is slightly different than what I show above. The image to the right shows how to complete an exposed tail look on a timber frame home. Other than the obvious difference in framing details, the rest remains about the same. Be sure to fill the rafter void with insulation and provide adequate air sealing as well.
The last thing I will say about exposed timber frame, straw bale homes is that they are a bit harder to plaster. Because there are exposed framing members to avoid damaging and, perhaps even harder, there are angled knee braces to avoid, the overall plastering process is more difficult. The look you are left with is, no doubt, beautiful, but you WILL work for it. Also, keeping those exposed timbers clean and free of dust and cobwebs will likely be a full time job as well. That said, it sure is a beautiful look!