What Is Straw Bale Insulation?

Straw bale insulation typically describes house construction using straw bales as exterior walls. The R-value of straw bale insulation is usually well over R-30. In many houses, the bales also provide the structural component of the exterior walls–eliminating the need for most wood framing and fiberglass insulation.

Straw bale construction has increased steadily since the 1980s. Straw bale houses have been constructed in every state in the US. Over 1000 in California–including both residential and commercial buildings. They are also found in over 50 countries around the world.

Straw bale insulation

History of Straw Bale Insulation

Straw houses have been built in Africa for millennia. Straw bale houses were built in Germany over 400 years ago and straw has been used to thatch roofs for centuries. Native North Americans used straw between the inner and outer layers of teepees for insulation.

Straw bale insulation and construction became easier with the invention of the mechanical hay baler in the 1850s. One of the first straw bale buildings was a schoolhouse in Nebraska. It was eaten by cows which made builders use concrete-based stucco on the exteriors to provide protection.

Straw bale construction has been enjoying a revival since the 1980s. Straw bale insulation is:

  • Environmentally Sound.
  • Low Embodied Energy.
  • Relatively Affordable.

Straw bale insulation is now used in some Structural Insulated Panel (SIP) construction. Instead of sandwiching polystyrene foam between oriented strand board (OSB), they sandwich straw between 1” thick concrete panels. The walls are 16” thick and boast R-values of R-35 – R-40. Eight foot by eight foot wall sections weigh approximately one ton.

How Straw Bales Provide Insulation

Straw is the left-over stalks after the grain is extracted. It is usually baled and used for animal bedding, or chopped and incorporated back into the land to add nutrients and tilth. The most popular small bale is a 2-twine (string) bale 18” wide x 14” high x about 36” long. Length can be adjusted at the time of baling.

Straw bale R-values can vary from R-30 to R-40 or sometimes higher. The R-value is dependent on how tightly the bale is packed when it is made. Tighter provides better insulation. Straw bales are cellulose with trapped air pockets. Both provide poor heat conductivity making good insulation.

The sheer mass of the bales combined with dead air spaces incorporated in the straw also make straw bales very good at attenuating sound. The thick layers of plaster inside and outside along with the straw bales provide a huge amount of thermal mass that can be incorporated into a passive solar designed home.

Advantages of Straw Bale Insulation

Straw bale insulation/construction has many advantages.

  • Environment. Straw can present disposal problems for farmers. If it is baled and used as house insulation, some of it will be put to good use. At the end of its lifespan–usually in excess of 100 years–the bales can still be recycled into gardens and flower beds. (Farmers in India and China burn untold tons of straw as waste each year.)
  • Low Embodied Energy. Straw uses very little energy to grow. It is often considered a waste product because the value of the crop is in the grain–usually wheat, oats, rice, or rye. Any added energy consumption comes from baling and hauling.
  • Cost. The cost of a small square bale ranges between $2.00 and $6.00.
  • High Insulation R-value. Straw has roughly the same insulation value per inch as fiberglass. Most straw bale insulated houses have R-values in excess of R-30. The added R-value is achieved because the walls are typically 14” or 18” thick.
  • DIY Project. With one knowledgeable straw bale house builder to provide guidance erecting a straw bale insulated house is straightforward for someone with framing experience.
  • Window & Door Placement. Extra wall thickness allows for–and encourages–creative window placement. From window seats to recessed windows to beveled or rounded corners, thicker walls can make real design statements.
  • Easy Design Modifications. Straw bales can be modified and molded using a variety of saws–chainsaws, reciprocating saws, and even sharp knives. Create window seats, niches, or creative substrates before applying a stucco finish. Ensure that the bales are solidly placed. Cutting through twine will release the straw. It is virtually impossible to get the straw back in place without mechanical help.
  • Flame Retardant. Straw bales are less flammable than conventional wood stick-frame construction. It is packed so tightly that the lack of oxygen makes it smolder instead of burn.

Disadvantages of Straw Bale Insulation

The disadvantages of straw-bale insulated buildings should be considered before beginning a major construction project. They can be very impactful.

  • Contractor or DIY. It may be difficult to find a contractor with straw bale construction experience. Building a house without a contractor may feel too daunting for the average DIYer.
  • Building Codes. Most inspectors are not familiar with straw bale insulation of straw bale construction. Getting plans approved can be difficult or impossible. The International Building Code Appendix S addressing straw bale construction was added to the code in 2018 making approvals somewhat easier to get.
  • Keep Dry. Straw bales must be kept dry. Wet straw loses its insulation value and invites mold, mildew, insects, and odor.
  • Floor Space. Straw bales consume a lot of floor space–usually 20” per wall compared to 6 ½” for conventional construction. Floor plan designs should take this into account.
  • Consistency. Straw bale lengths can vary from baler to baler. Purchase bales from the same farm. Baled at the same time from the same field. Even a few bales an inch or two longer or shorter interspersed in the walls will make construction more difficult and could affect R-values.

Straw Bale Insulation Costs

Straw bales cost between $2.00 and $6.00 each. When used in house construction they provide approximately 3.5 square feet of exterior wall. Which replaces studs, sheathing, and insulation with up to twice the R-value.

Straw bales have been stacked around mobile homes and the base of houses to keep the heat in. They have been opened up and spread over gardens to protect perennials. A very versatile and inexpensive product–often considered waste material.

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Written by Murat

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