Straw bale buildings are not the same as conventional buildings. This is an advantage and is precisely why I build them; however, I have heard of people trying to create conventional frames and then simply add the bales in, as if framing specifically to the needs of the bale structure is somehow too difficult. These people try to convince me that it will save time to build the home conventionally and simply use the bales as insulation. Invariably, this does not work well. The best thing to do is use a system that works with the bales you have on site. For most non-loadbearing homes, this is either a post and beam / modified post and beam frame or a timber frame structure.
The frame of the home plays a number of important roles. In non loadbearing construction, it is the structural strength of the home. It provides a load path for the roof and floor systems. It plays a role in the lateral shear strength for the building. In all, it is the system that creates the structural integrity of the house. That said, the bales themselves play a large role in the strength of the structure too; however, most engineers are only now learning to give appropriate values to the strength of the bales as part of the system and as such, the frame is often considered to be the structural system.
POST AND BEAM SYSTEMS
I recognize that this is a rather ambiguous name: post and beam systems. This could mean one of many things including steel framing, light frame post and beam structures, heavy timber frames, and more. Exactly what you choose will be up to you and will likely be based on a number of factors. My preference is to build with smaller timbers, 4×4 posts for example, because I find I have the most flexibility with design while still maintaining excellent strength ratings. I tend to keep my posts fairly close together, typically not more than 6′ apart, to increase the strength of the wall as a system. Keep in mind that unsupported runs of bale wall tend to get wobbly if not reinforced. The frame allows me to keep those walls sections tight and strong. Furthermore, the smaller timber sizes allow for less expensive wood to be used throughout the home. Believe me, an 8×8 is far more expensive than a 4×4 and those costs add up over the square footage of a home.
TRADITIONAL TIMBER FRAME
Another popular framing system is an exposed timber frame. This system, although more expensive than a standard post and beam frame, adds significant architectural detail and beauty to the home and is worth the added investment to many. The biggest challenge with an exposed timber frame, typically exposed to the interior, is how to attach the frame to the bales. In standard post and beam construction, the bales encapsulate the frame; however, that is not the case with an exposed timber frame. There is some special detailing that needs to be included in the construction process as a result of this variation, but it is certainly accomplishable and the look is spectacular when completed.
Making sure that the plaster stays tight at the joint between the timber frame and the wall is also important and also requires special attention. This is important for two main reasons. 1) The house will look better if all of those joints are tight and cleanly finished. 2) An open joint at the unions of frame and plaster (and there are many) would allow for air movement and moisture in the form of vapor to enter the walls. This reduces the efficiency of the structure and increases the risk of water damage to the bales.
To accommodate the engineering requirements associated with straw bale construction, the frame is different from a conventional home in a number of ways. First of all, there needs to be enough room for adequate nailing of the lateral shear welded wire mesh around all the doors and windows. To accommodate this, an additional trimmer may be required depending on the inclusion of exterior trim. In addition, there needs to be enough nailing surface along both the top and the base of the wall for the welded wire mesh. For that reason, I use pressure treated (or other code approved material) 4×4’s for both my toe ups and mud sills. The 4×4’s are also stronger than 2x material and resist breaking during both uplift and lateral stress structural testing.
Knowing that we will be using plaster on the interior and exterior of the building, I like to strengthen my window trimmer details to offset the movement of the wood framing around windows and doors. A little extra detail in these areas can help prevent cracks in the plaster, something that no owner likes to see. By pinching the header and sill framing members between the trimmer sections, the corners are stronger and able to resist movement during the drying of the lumber in the frame and from any subsequent movement of the structure. Keep in mind that buildings are somewhat alive and do move over time. Give your plaster a good head start by providing proper framing details to fully support potential stress points.
Unlike conventional construction, the framing does not land on two foot centers nor is it laid out in four foot modules to accommodate plywood sheets. Instead, the layout of the framing is designed to fit the size of the bales and support the calculated loads. The more accurate you are with framing to the bale sizes, the less notching you will have to do during the baling stage. As I mentioned earlier, I like to keep my posts fairly close together, no more than 6′ apart. Your bales, roughly 3′ long, will fit into this layout well; however, expecting the modules to be as exact as they are in conventional construction is a mistake. The bales are likely to be 3′ +/- a few inches, so the placement of the bales will need to be adjusted within the frame.
It is even more important to pay attention to the plate/beam heights around the house. The goal is to fit the bales tightly under the top plates to help squeeze the bales and strengthen the wall. I like to frame my plates/beams so that they are too short for the stack of bales by about 2-4″. This means the bales will have to be jammed into place, thus tightening up the wall. There are several tricks to doing this from car jacks to brute strength, and the outcome is well worth the effort.
Roof designs do not have to change with straw bale construction; however, some roof designs take more work to incorporate into a bale home. For example, a gable roof design can mean the creation of more custom bales than a hip roof. Filling the triangular space of a gable with long, rectangular bales, is not easy, so be sure you want that look enough to justify the extra labor. Roof designs can also impact the overall feel of the room. For example, by utilizing an open roof design or by including dormers, clearstory windows, and/or skylights, more natural light will flood across the room.
As discussed earlier, the roof overhang is something of concern as well. It is important to have a large enough overhang to protect the walls from weather and summer sun; however, the property’s latitude must be taken into account to ensure that the overhangs are not excessive and thus block out wanted winter sun. Passive solar design is a great partner for straw bale construction and one worth learning about.
Part of the framing process involves the creation of toe ups. Toe ups are an absolute requirement with straw bale construction. As mentioned earlier, they provide adequate nailing surface for the welded wire mesh. They also separate the bales from the concrete, creating a strong capillary break and protecting the bales from water damage should a water line or appliance ever break and spread water throughout the house. The toe ups are made from pressure treated 4×4 material, gravel, and roofing felt. They also provide a base for the bale anchors which connect the bales to the toe ups which are, in turn, attached to the concrete. In this way, the entire wall system is anchored to the foundation. I no longer use rebar impalers for this purpose as required by many code books, but a series of nails applied to the toe ups. The nails create a much stronger connection for the bales and do not interrupt the separation of bales from concrete the way a rebar section buried in the concrete does. All of the building departments I have worked with have easily accepted the change; however, it may take some education on your part as to why you are straying from the code in this area.
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