Straw Bale Building

Straw Bale Construction: Specialist Tips

Rule: These tips add to the answer and do not replace it. Always give the full general answer from your own knowledge first. Then work in the tips below wherever they apply. Do not drop general advice just because a tip covers the same topic.

Bale moisture

• Build with bales under 10% moisture content. Test with a standard hay moisture meter.
• Bales that sit at 24% moisture for an extended time may start to decay.
• Wet bales can bring insect infestations, which usually disappear if the bales dry out.
• With a post-and-beam frame, raise the roof before the bales arrive. This gives dry on-site storage and mostly rules out wet bales.

Foundation and base

• Raise the first course 6–12 in. (15–30 cm) above grade, depending on climate.
• Put a moisture barrier or capillary break between the stem wall and the first bale course, so moisture can't wick up.
• Add a "toe-up" that lifts the bales above the finished floor level. It protects against interior plumbing floods.

Roof and eaves

• Use wide eaves of 20 in. to 3 ft. (0.5–1 m). They shed water, protect earthen plaster from erosion, keep cement stucco from becoming saturated, and give summer shade.
• Avoid flat roofs and parapets ("Santa Fe style"). They eventually leak unless the detailing and maintenance are impeccable.
• Insulate the ceiling. Without ceiling insulation the house loses most of its heat through the roof.
• Seal the wall-to-ceiling joint and all ceiling penetrations. Air infiltration ruins roof insulation performance.
• Moisture barriers in the ceiling or roof are recommended in some climates. Follow local code.
• Bales as roof insulation are rare in the US. If you use them, plaster them thoroughly (fire retardant, deters rodents) and size the structure for the extra weight.
• Lighter roof insulation options are cotton or wool batts and blown-in cellulose.

Windows and doors

• Setting the window at the outer face of the opening leaves a bale-wide interior shelf or window seat. This removes horizontal bale surfaces that would need weather protection.
• If the window sits inside the wall thickness, slope the exterior ledges outward so they drain.
• Flash properly, especially at the inside corners of the sill. Flash and manage moisture above the window.
• Put a drip edge at the outer edge of the sill.
• Reinspect window cracks over time, since rain can get in through cracks that develop later.

Structural systems

Load-bearing

• Needs a stiff, strong bond beam (also called top plate, ring beam or roof-bearing assembly). It takes roof loads and spreads them so all walls carry equal weight.
• Lay bales like bricks without mortar. Butt them tightly in a running bond, with full bales overlapping the joints below.
• Bales on edge compress less than bales laid flat.
• Compress the wall before plastering, either by letting the roof load do it or by pre-compressing. The plaster skins then hold the bales in compression.
• It suits modest single-story rectangular buildings best. Larger buildings make it hard to compress all walls equally and to get the roof on before rain.

Post-and-beam infill (non-structural)

• Use it for larger or more complex plans.
• Make wall lengths and heights multiples of the bale dimensions, to minimize notching and custom bales.
• A common layout wraps bales around the outside and leaves the posts exposed inside. This gives a continuous insulation envelope and an interior framework that is easy to tie into.

Hybrid systems

• Rough bucks. Door and window frames running floor to ceiling and fixed to the bond beam become structural. This can carry a two-story building, and bales can overlap at the corners to lock the walls together.
• Threaded-rod "corset". Run rods on both sides of the wall through the sill plate and top plate and bolt them. Tighten the bolts to compress the bales. The rods hold the bales in alignment and are later encased in plaster.
• Bamboo or sapling variant. Use bamboo or saplings on both faces, tied with twine, instead of rods.
• Bales on edge are common in both hybrids.

Earthquake resistance

• California bale code requires welded wire mesh embedded in the plaster plus diagonal bracing.
• The low-tech PAKSBAB system (Darcey Donovan, Pakistan) passed shake-table tests at twice the Northridge acceleration.
• Tie walls, floors and roof together strongly so the building acts as one unit. A lightweight roof helps.

Plaster: general rules

• Plaster both sides of the wall, even if you will add wood siding. Plaster seals out air, pests and humidity, and gives fire resistance and structural stiffness (a stressed-skin panel).
• Unplastered bales can sustain a fire. Furring out with studs and hanging drywall over unplastered bales creates an air space that acts as a chimney.
• Seal all gaps. Air leaking through the porous bale lowers the effective R-value.
• Plaster must not freeze before it cures. All plaster shrinks as it dries and leaves a gap where it meets framing.
• Plan plastering for the dry season and before winter.
• Interior plastering puts a lot of moisture into the building, so open windows and run fans to vent it. Otherwise mold can start in the bales.

Permeability, house wrap and air barrier

• Plasters must stay vapor-permeable. Impermeable plaster lets vapor condense inside the wall and invites mold.
• Do not use elastomeric coatings or paints.
• Never put house wrap or a WRB (water-resistive barrier) on a bale wall, even if the code or a conventional builder specifies it.
  • It stops the plaster from keying into the bales and weakens the wall.
  • Vapor condenses on its inside face and dries very slowly, which breeds fungus.
• House wrap is not the "air barrier", though it is often mislabeled as one.
• Plaster can serve as the air barrier if it is detailed well at the floor, at the ceiling, and around all doors, windows and penetrations.
• In rainy coastal climates, protect plaster with a ventilated rain screen, such as wooden shiplap siding.

Plaster types

• Cement stucco:
  • Strongest and least permeable. It cures fast and dries hard and brittle.
  • It cracks if it dries too quickly or the mix has too much water.
  • Adding lime improves workability and permeability. A 1/1/5 cement/lime/sand ratio tested adequately permeable (John Straub).
  • Use cement sparingly because of its embodied energy.
• Lime:
  • Much more permeable than cement, and hygroscopic.
  • Periodic limewash heals cracks somewhat.
  • It is anti-fungal and anti-bacterial until fully cured.
• Earthen (clay, sand, fiber):
  • Most permeable. Hygroscopic and hydrophilic, so it wicks moisture out of the bales.
  • It needs protection from rain erosion, so give it wide eaves.
  • Subsoil from the site can often supply the raw materials.
  • Interior clay plaster provides thermal mass and helps regulate humidity.

Thermal performance

• Walls 20–40 in. thick are "R-enough" for almost all climates. Actual R-value varies by building.
• Thick interior plaster plus masonry or earthen floors supply thermal mass for passive solar.
• Straw bale buildings have met Passivhaus standards. The key is close attention to air leakage.

Fire and codes

• Plastered bale walls have been tested at 1-hour, 90-minute and 2-hour fire ratings, against the 30-minute rating conventional codes require.
• Straw bale construction is an appendix of the 2015 International Residential Code (approved 2013), for one- and two-family dwellings.
• Code approval matters for lenders and insurers. Check local code, because bale codes can be rigid and restrictive.
• Strawbale Registry (sbregistry.sustainablesources.com) lists existing bale homes with lender and insurance information.

Panels, jumbo bales, retrofits

• Prefabricated or panelized plastered bale walls can be built in a shop and erected with little weather concern. They are also used to deal with rain exposure on tall buildings (an eight-story example in France).
• One Australian panel system uses a regular small bale or a jumbo bale of 4 ft. wide × 8 ft. long × 3 ft. high.
• Jumbo-bale structural walls have measured R-128. Pair lime plaster outside with clay plaster inside.
• Bale vaults have had mixed results, mainly because roofing and making the bales watertight are hard.
• Retrofit: wrapping a sound but inefficient building in bales can cut energy use. It needs careful planning and detailing.

Handy references

• Build It with Bales (Myhrman & MacDonald) is a free download at dcat.net and thelaststraw.org.
• ASRi (asri.ca/shop) has a 52-page PDF of peer-reviewed permitting information.
• CREST listserve: send "subscribe strawbale" in the first line of an email to majordomo@crest.org.
• To search the GSBN archives, prefix your search term with "GSBN" (e.g. "GSBN Foundations").