Strawbale Wall Systems - Your Wall, Your Way
Post and Beam. Infill. Hybrid. Load Bearing. When choosing a strawbale wall system for your build, it can be easy to get confused with the names that are often tossed somewhat loosely about. Selecting a wall system is an early-stage design decision that will influence not only the straw bale home design but the construction process, bale raising experience, the labor involved, plaster work, environmental impact, and even future home maintenance. If you aren’t yet clear on the implications of the different types, we recommend taking time to explore the topic a bit further.
The best strawbale wall system for a particular structure will depend on a variety of factors, including:
Specific site conditions
Intended use of the building
Available materials
Budget considerations
Labor availability
Design and/or aesthetic preferences of the architect, engineer, builder, and owner.
Ultimately, which strawbale wall system you use is a decision of preference and site-specific requirements. No single type of wall system is a best fit for all situations.
One simple way to think about the different wall system types is to identify where the load of the roof assembly, or everything above the topmost bales, is being supported. Ask, ‘How does the load, or weight, of those materials get transferred down to the foundation?’ Typically, it transfers through the framing, the bale wall, or both depending on the wall type.
Below is an overview of common strawbale wall systems
and why you may opt to use one type over another.
When the Structural Load Bears on the Strawbale Wall
The original. Straight out of Nebraska, where US straw bale home construction has its deepest roots. This wall system is commonly referred to as “Load Bearing,” “Nebraska-style,” or “Structural.”
Load-bearing is well suited for small, simple structures and is relatively easy to construct, as there is far less framing to accomplish. In this method, the strawbale walls are the structure and bear the load of the roof assembly.
Windows and doors are handled by bucks or wooden boxes built to provide support for the openings. Because there is no wall framing, there is little to no custom notching or shaping of bales. Put a motivated crew of people on a load-bearing bale raising, and the walls may go up so quickly you’ll forget to install a window!
There are some critical factors to be aware of to maintain structural integrity, such as ensuring the ratio of opening-to-wall is correct, keeping it to a single story, and paying attention to ‘unsupported wall length.’ We cover these ratios and other essential details in our free Load Bearing Report - Download here.
When the Structural Load Bears on the Frame
This might be the most commonly used wall system and is broadly referred to as “Non-Load Bearing” or “Infill.” In addition to home construction, Infill methods are often used for larger structures, such as commercial buildings, schools, or community centers. It breaks down further into two basic categories:
1. The Structural Frame is WITHIN the Bale Wall
Frequently called “Post and Beam Infill”. This method is highly adaptable and widely used. The Post and Beam framing offers mechanical attachment points for doors, windows, and plaster prep.
Plus, as the lumber is hidden within the wall, making off-the-shelf local lumber a material option without the same aesthetic impacts as an exposed frame. Including the posts within the wall provides a contemporary aesthetic with an uninterrupted wall plane for living and furniture.
Bales are often installed flat or with the strings orientated to be on the top and bottom of the bale once in the wall. The width of the bale then determines the wall thickness; for example, an 18” thick wall is typical for 2-string bales. The string orientation is super important! When set in a flat orientation, the strings are protected (mostly!) from cutting used to custom shape the bales around the commonly needed notches that accommodate the typically used 4x4 posts.
Since the strings are usually 5-6” in from the side of the bale, a 4” deep notch will fit nicely into the side of the bale without the added labor to retie the strings.
2. The Structural Frame is EXTERNAL to Bale Wall
Oft mentioned as “Timber Framing Infill” or “Wrap,” this system usually positions the bales to the exterior of the structural frame. There are a few reasons you may use this method, for example, to expose the craftsmanship of the timber framing, to maximize interior space, or as a retrofit.
There are some details to be aware of in the construction process to make for a better-finished product and more straightforward installation when using timber frame infill; see our blog “5 Tips for Timber Framing in Straw Bale Home Construction” for details.
Using a Wrap can be a great way to retrofit an existing structure, making it more energy-efficient. The upgrade is usually applied externally to the framing to save interior space as it is doubtful the original building was designed to accommodate 14” to 18” thick walls! If using this method, pay close attention to how the retrofit will interact with the foundation, roof assembly, and any openings, such as windows or doors.
It is worth noting that while bales can be flat or on edge in a Wrap, the overall thickness of the finished wall will be bale width + posts + plaster applications. For example, a wrap with bales set flat, 4x4 posts, and 1 ½” plaster on both inside/outside; the full wall thickness would be 18” + 3 ½” + 3” for a total of 24 ½”.
For both categories, the bale wall is compressed through the wedging of the top bale course against a framed bale stop. Compression combined with the application of welded wire mesh provides a secure substrate for plaster application.
This also means the roof can be installed before the bale walls are raised, a key benefit for wetter climates, sheltered working conditions, or where temporarily covering the bale walls isn’t a great fit.
To be clear, blends of the two Infill Categories can happen if desired within the same structure, although this is rare in our experience. One of the appealing aspects of using natural materials or building your own home is that it allows for creative expression in the construction process!
Mixing It Up - Hybrid
The “Buck and Beam,” “Wall-Truss”, or “Hybrid” method combines some of the best of both the load-bearing and post-and-beam methods.
It can be an excellent construction method for a small crew, as it can require little to no notching or fewer custom bales. And, while Hybrid can incorporate bales flat or on edge, if it is designed for bales on edge, there will be fewer overall courses to gain the same height. Both of these can significantly reduce the amount of labor needed.
Here’s an example typical steps of the construction process for a hybrid system:
A custom box beam is built, which will be used to compress the bale wall. (Learn more about custom box-beam construction and best practices, such as using the foundation as a layout tool, in our Load Bearing Report.)
The posts are installed as corners or as part of the door/window bucks.
The bales are set in the wall.
Then bale wall is compressed down from the box beam to the foundation.
Finally, the roof or second story is installed.
One item to note is that even though the bales may bear some of the load in this style, it is still engineered so that the structural load is borne upon the frame. The box beam compression, however, can provide an extremely stable substrate for the plaster to adhere to, which contributes to a durable finish.
To highlight the flexible nature of straw bale construction (and why perhaps there is such a looseness to these names!), you can also design a structure as Buck and Beam and then install the roof BEFORE installing the bale wall.
In this case, the bale wall is then a version of Infill, where the bales are fit tightly into the wall space for compression instead of from the box beam cinching to the foundation. To further strengthen the wall, wire mesh is installed, similar to Post and Beam Infill methods.
While we can’t hope to cover all the nuances and variety of wall systems in straw bale construction in a single blog, understanding the three basic straw bale wall systems as they relate to the structural load will get you started in the right direction as you make plans for your build. As well as navigate the often confusing cross-overs between the wall system names!
Any construction project is a process of steps and decisions. Just keep taking the steps and soaking up learning to support making your choices, and you’re on your way to your own straw bale dream home.
Looking to learn more about straw bale design decisions, like wall systems?
Join us for a Straw Bale Home Design Workshop.
We review the start-to-finish of good strawbale home design. Bring your ideas, concept drawings, construction documents, and all the questions!!
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-Dainella & Timbo