5 Tips for Timber Framing in Straw Bale Home Construction
Can you use exposed timber framing in straw bale home construction? In short, yes! AND, to ensure a long lasting, high performance build, extra attention should be given to certain technical details during the design and building phases.
Exposed timber framing is that beautiful craftsman-style, revealed timber look that can pair so well with the organic, soft-curve feel of straw bale houses. Large post and beam timbers are connected with wooden pegs or other decorative joinery to comprise the building’s structure and weight-bearing systems.
To achieve the striking visual effect of the exposed timbers within the home's interior spaces, the straw bale walls are almost always positioned on the exterior of the timber frame. In this arrangement, the timber frame bears the weight or load of everything above it, including the roof.
In turn, the straw bale wall system provides highly effective insulation, air and moisture barriers, interior and exterior wall finished surfaces, custom niches, and attachments for the window and door openings.
When considering combining an exposed timber frame structure with a straw bale wall system, here are five key areas to address during your design process and to be prepared for during construction:
1. Foundation Design
Both the timber frame structural load requirements and the straw bale wall system need consideration during the engineering phase of your foundation.
The timber frame distributes all of the structural weight it carries to point loads in the foundation. This distribution effect makes it vital for the foundation to be correctly sized to receive those loads in those particular locations.
The foundation design must also provide a positive and secure connection between the foundation and posts. This connection keeps the structure in place and eliminates any significant movement, such as what uplift forces may cause. A solid, secure base and connection helps maintain the straw bale wall system integrity and prevent plaster cracking.
2. Green Timber, Curing, & Settling
One of the best practices (and one we strongly recommend!) is to use kiln-dried or air-dried timber. This reduces moisture content to minimize the risk of warping, twisting, and shrinkage by ensuring the timber has been properly dried.
If the timbers used as posts or beams are inadequately prepared, overtime movement within the timbers as they progressively dry can lead to structural and cosmetic problems. Similar to wall instability due to a poor foundation connection, this movement can make it difficult to achieve (and maintain!) a tight, weather-resistant building envelope.
Utilizing kiln-dried or air-dried timber will help ensure that your timber frame structure is a stable and long-lasting installation.
3. Windows, Doors, & Roofs oh my!
When pairing exposed timber and straw bale walls, it is typical to have the building framing run outside of the bale walls rather than being embedded within those walls. The bales need not be notched or shaped around the frame in this style of pairing. This reduces the labor associated with the bale work; however, it brings up some challenges regarding how to attach standard building components to the house. So how does one finish windows and doors or securely attach a roof in this style of construction?
The easiest solution is to use bucks, or a wood frame set into the wall. By using bucks, you can securely place your openings virtually anywhere in the wall. A buck assembly can be as simple as a “floating” box within the wall or a more stationary floor-to-ceiling frame.
Regarding the roof, it is always a good idea to have a large overhang (2 feet minimum!) over straw bale walls. This protects the bale wall from direct moisture, melting snow, and splashback. These large overhangs must be accounted for during the design and engineering phase. In this building method, the measurable structural overhang is actually MUCH more than the distance visible from the finished exterior of the bale wall to the edge of the roof line.
Let us explain more…
The true width of the overhang needed originates from the structural frame and includes the depth of the bale wall system and the roof framing projection beyond it.
To meet this structural demand, consider using a simple roof truss design or plan for the timber frame’s bent design to handle this extended overhang load. As you can imagine, it is essential to know this design need BEFORE completing the home design and construction documents!
4. Prep for Long-Lasting Plaster Finishes
Over time as plaster dries, it naturally recedes or shrinks. And, at the junctures between the wooden posts/beams and plaster walls, a significant gap can develop if you are not intentional with plaster prep and detailing. These gaps aren’t merely an asethic consideration, they can break the air barrier envelope of the straw bale wall system. This can expose the bales directly behind the gaps to moisture-carrying air, which could lead to moisture issues such as mold growth.
To avoid gapping at these junctures, backer boards can be applied to the bale-facing side of the posts/beams. Backer boards are strips of plywood, OSB (oriented strand board), or even plasterboard sized to extend beyond the posts/beams by about 6-8″.
When assembling the backer boards, apply a moisture barrier such as roofing felt over the wood to serve as a means to decouple the wood and plaster. This simple layer of separation is hugely effective at minimizing cracking.
Be sure to attach expanded metal lath with construction staples over the moisture barrier to help the plaster “key in” or attach securely to your backer board. These backing boards are installed before the bales are set into the wall.
Once the finish plaster coat has been completed, you can further enhance the clean aesthetic detail by adding a color-matched caulk line along the joint between any plaster and wood at each post and beam connection.
5. Straw Bale Wall Stabilization
Since the posts of the building frame are physically outside of the bale walls, verses a more infill approach where the posts are included within the bales themselves, it is a good idea to add a custom stabilization feature. One simple approach is to attach each course of bales to the backside or bale-facing side of each post or beam in contact with the bale wall.
This can be done by bending a small rectangular piece of lath at a 90-degree angle so it may be stapled to the post and then secured to the top of the bale with landscape staples. This leaves the lath hidden within the bale wall to preserve the clean lines of the exposed timber look.
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