Strawbale Construction

Structural Associates is proud to have teamed with Doug Graybeal, AIA, to construct a 5,700 s.f. off-grid home at the top of Capital Creek.  The home employs numerous sustainable technologies including a 14.4kW photovoltaic system, post and beam frame, and strawbale construction.

Strawbale construction uses a sustainable resource with many positive attributes, making it a true “green build” method.  This article touches on many of the aspects that make this a viable type of construction to provide a quality home that can be built using almost all natural materials.

Materials

Strawbale construction uses straw from wheat, oats, barley, rice and other grains as a structural and insulating material.  The straw is traditionally a waste product which farmers do not till under the soil. Instead, it is typically used as animal bedding or for landscape supply due to its durable characteristics.  (Note that straw differs from hay in that hay is used to feed livestock and is not suitable for building use.)  Two hundred million tons of straw are burned annually in the US.  By utilizing this renewable resource, not only are we saving the wood typically used in construction we are also reducing the pollution related to the burning of the straw.

Strawbale construction provides high thermal insulation and sound insulation performance.  With factors typically ranging from R-30 to R-45, heating and cooling bills can be significantly reduced when compared to traditional 2×6 stud wall construction.  The attractive “thick” wall assembly also leads to a very quiet home interior.

Plaster is applied over the straw, creating a virtually fireproof assembly.  The plaster used in strawbale construction can be made from soils on the building site with the addition of natural hydraulic lime. Colors can also be added if desired but many owners and designers prefer to use the native soils as this provides a look consistent with the surrounding terrain.

While the floors in the main level of the Capital Creek home were burnished concrete, many straw bale homes also incorporate “mud” floors made from native soils with the addition of lime.  This design further reduces the “carbon footprint” of more typical concrete sub slab floors or wood frame floors.

Techniques

There are two types of strawbale construction: Post and beam infill for the walls or a more traditional structural wall system. While the Structural Associates strawbale home used the post and beam technique, there are benefits to both types, with the choice often depending upon local building codes and personal preference.

Structural wall strawbale construction: This is the original design of many of the older straw bale structures.  It is a “purer” form of this type of construction and there are examples still standing that are over 100 years old.  The structural wall design is more likely to show cracks in the plaster due to settlement than the post and beam structure.  Fortunately, cracks can be repaired with plaster relatively easily.  The structural wall design is also more difficult to receive building department approval for since it is difficult to calculate the load bearing capacities of the assembly.

Post and beam infill: The primary benefit of post and beam design is that the bales do not have to be compressed and held in place prior to the roof framing. Another positive side of the post and beam infill is that building departments are more likely to approve this type of construction due to the engineered values of the post and beam structure. On the negative side, there is often additional expense associated with the post and beam materials. The cost of labor is probably close to equivalent in both methods due to the effort required to pre-compress and hold the bales in the compressed (settled) state for the structural wall application.

The techniques for both types of strawbale construction have been refined in recent years and the designs for door and window openings are now far superior to what they were previously, preventing both air and water infiltration.  Details for door and window openings have changed from a basic “buck” detail around the openings to a detail that can be attached to part of the post and beam framework. A plywood buck supported by a 2X6 is installed from the posts and then heads off the straw bales on both the sides and at the top and bottom. Returns are recessed back into the wall to protect the door/window assemblies from weather elements. Larger than typical roof overhangs are also often utilized to prevent water from splashing onto the plaster walls, which could result in water migration into the straw.

How well is a strawbale assembly sealed? Based on the infrared photos that we took of the straw bale structure, the penetrations sealed well with typical foam insulation between the buck and the window unit and the straw tightly packed against the buck.

What should owners and architects look out for in building such a home? It would be wise to use a contractor and architect familiar with this type of construction. Research and a good review of the details is critical to be sure that water infiltration into the straw is prevented. Moisture infiltration into the straw must be avoided from both the inside and the outside. Good detailing in wet areas (e.g., baths and laundry rooms) is also essential to ensure longevity of the strawbale assembly.   It should also be noted that straw has not shown to be susceptible to any type of termite infestation.

Once plaster has been applied to the strawbale construction, the walls provide sheer resistance to the structure that is consistent with what one might expect from concrete. In fact, straw has many of the attributes of concrete with the exception of the R-values, where the R-value is considerably higher with the straw bale assembly.

In summary, strawbale construction as a sustainable building system is viable due to its reasonable construction costs and high insulation characteristics.  The straw is a renewable resource that will provide an attractive and comfortable environment while the plaster provides a warm and soft feel on both the interior and exterior of the home while creating a structure that can be made virtually fireproof and weather proof.  As this type of construction becomes more common,  it is likely to attract more attention from both architects and owners due to its sustainable nature, structural integrity, and low carbon footprint.