What Is Straw Bale Insulation?

Straw bale insulation is a building method where straw bales act as exterior walls. The R-value of straw bale insulation is over R-30. In many houses, the bales also provide the structural component of the exterior walls–eliminating the need for wood framing and fiberglass insulation.

Since the 1980s, straw bale construction has increased steadily. There are now straw bale houses in over 50 countries and every US state, with over 1000 in California–including residential and commercial buildings. 

Straw bale insulation

History of Straw Bale Insulation

Straw houses originated over a millennia ago in Africa, and builders began using them in Germany over 400 years ago. Aside from house construction, builders have used straw 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 insulation is:

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

Today, some manufacturers use straw bale insulation in 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, with eight-foot by eight-foot wall sections weighing approximately one ton.

How Straw Bales Provide Insulation

When farmers extract grain, like wheat, rye, oats, or rice, straw is the byproduct – left-over dried stalks.  In a typical setting, farmers bale straw and use it for animal bedding or incorporate it 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, but farmers can adjust the length at the time of baling.

Straw bales are cellulose with trapped air pockets, which provide poor heat conductivity and make good insulation. Average straw bale R-values range from R-30 to R-40. The R-value is dependent on how tightly the bale is packed – tighter bales provide better insulation. 

Since bales provide large mass and dead air spaces, they’re also good at attenuating sound. The thick layers of plaster inside and outside provide builders with a huge amount of thermal mass they can incorporate into a passive solar-designed home.

Advantages of Straw Bale Insulation

Straw bale insulation/construction has many advantages.

  • Environmentally Friendly. 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– users can recycle the bales into gardens and flower beds. (Farmers in India and China burn untold tons of straw as waste each year.)
  • Low Embodied Energy. Straw requires little energy to grow. It’s a waste product since the value of the crop is in the grain–usually wheat, oats, rice, or rye. Any added energy consumption comes from baling and hauling.
  • Low 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 since the walls are typically 14” or 18” thick.
  • Suitable DIY Project. With a knowledgeable straw bale house builder to provide guidance, erecting a straw bale insulated house is straightforward for someone with framing experience.
  • Creative 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 make design statements.
  • Easy Design Modifications. Builders can modify and mold bales of straw using a variety of saws–chainsaws, reciprocating saws, and even sharp knives, which allows them to create window seats, niches, or creative substrates before applying a stucco finish. 
  • Flame Retardant. Straw bales are less flammable than conventional wood stick-frame construction. The lack of oxygen in tight-packed straw makes it smolder instead of burn.

Disadvantages of Straw Bale Insulation

There are some disadvantages to straw bale houses. Consider the following:

  • Difficult to Find a Contractor. It’s difficult to find a contractor with straw bale construction experience. Building a house without a contractor may feel too daunting for the average DIYer.
  • Harder to Meet Building Codes. Most inspectors are not familiar with straw bale insulation or construction, making it harder to get plans approved. The International Building Code Appendix S addressing straw bale construction was added to the code in 2018, making approvals somewhat easier to get.
  • Must Keep Straw Dry. Since wet straw loses its insulation value and invites mold, mildew, insects, and odor, you must keep it dry.
  • Takes Away Floor Space. Straw bales consume a lot of floor space–usually 20” per wall compared to 6 ½” for conventional construction. 
  • Consistency in Bales is Important. Straw bale lengths vary from baler to baler, making it important to purchase bales from the same farm, baled at the same time, and from the same field. Even a few bales an inch or two longer or shorter will make construction more difficult and could affect R-values.

Straw Bale Insulation Costs

Straw bales cost between $2.00 and $6.00 each and provide approximately 3.5 square feet of exterior wall, which replaces studs, sheathing, and insulation, with up to twice the R-value. Expect straw bale insulation to last up to 100 years.