Strawbale Overview

Straw has been used as a construction material for as long as there has

been agriculture. Early structures implement straw-clay combinations. The

straw provides tensile strength and insulation, and the clay glues the loose fibers

together. Baled straw was first used in construction in the late 1800's by settlers in

Nebraska. The innovation was a product of newly invented baling machines, as well

as limited availability of conventional building materials in the prairie Midwest.

Strawbales were used to construct many types of buildings, including schools, barns,

houses, corer stores, and more. Some 75+ year-old buildings are still inhabited

today. And historic strawbale structures can be found in a variety of

climates: from hot to cold, and from dry to humid.

Straw is the stalk of any grain plant: oat, wheat, rice, barley, etc. Straw is high

in cellulose, similar to wood, and is therefore not highly nutritious to farm animals.

(This is different from hay, which is dry grass, and is grown as food source.) After

grain harvest, a small percentage (up to 15%) of the stalks can be tilled back in to the

soil to re-supply nitrogen. The remaining stalks, however, are essentially a waste

product. Farmers used to burn much of this waste, releasing fine particles and CO2

into the atmosphere...contributing to pollution and climate change. Burning is now

widely banned, leaving farmers to search for other uses for straw. When

California banned straw burning, grain farmers were instrumental in

helping to pass a building code for strawbale construction.

Strawbale wall construction takes 2 forms: loadbearing (or Nebraska

style) and infill (non-loadbearing walls). Loadbearing construction, as its name

implies, uses strawbales as large building bricks that support all of the building loads.

Infill construction uses an independent structural system with strawbales filling in

between. This overview addresses strawbale infill techniques.

Best Use

Use strawbales to minimize energy for heating or cooling.

Strawbale walls provide very high insulation (R-36 minimum). Insulation

slows the transfer of heat through your walls. In other words, heat stays inside in

winter and outside in summer. Therefore, strawbale homes require less

energy to stay warm (or cool).

strawbale guest house with lime plaster

thick clay plaster covers the inside of insulating strawbale walls

Cheat Sheet

STRAWBALE WALLS
Strawbale walls use large bricks, made from straw fibers, stacked to form a super-insulating wall. Both sides are typically finished with breathable plasters, made from clay or lime. Walls are thick (averaging 16" to 26" depending on bale size & orientation).

Techniques are quick to learn and use simple tools. However, attention to details must be followed to ensure walls stay dry (and thus are built durably)

AT A GLANCE
R-2.7 per inch of thickness
meets all fire safety codes
meets hurricane wind-load codes
can be used in wet/humid climates
inexpensive material
modest labor to install

BEST USE
use as insulation to reduce heating or cooling energy
(wherever you plan to use heating or air conditioning systems)

LINK TO GLOSSARY
Read the glossary description here.

strawbale walls can be curvy...especially if they are independent of the structure

strawbale walls create deep window sills

Infill Strawbale Walls

Strawbale infill construction does not rely on bales to carry any of the building loads (other than the weight of the bales themselves). The bales infill around any framing, meaning you can use any type of structural system that holds up the floors and roof. Infill construction simplifies the building permit process and can be less intimidating to builders and contractors, because structural elements are conventional.

The basics of strawbale infill construction are simple to learn and require no expensive tools. The skillset required to successfully install strawbales includes: retying bales to a specified length, cutting notches to fit around framing elements, and plastering.

The Process

1. Construct all structural elements (including foundation, walls, floors, roof)

2. Construct window & door framing plus interior walls (non-structural).

3. Dry-in roof and install finish roofing material.

4. Install strawbale walls.

5. Install doors & windows.

6. Rough-in and inspect electrical & plumbing.

7. Apply first coat plaster on interior & exterior.

8. Install all trim.

9. Apply second coat plaster on interior & exterior; shape walls tight to trim

10. Install any drywall; tape & float. Tape is also applied at connections

between plastered walls & drywall, including joint to ceiling.

11. Install any cabinets, countertops.

12. Apply finish plaster to interior & exterior, pigment as desired.

13. Install electrical & plumbing fixtures.

Schedule project-specific elements & inspections as appropriate.

Benefits

• High insulating qualities
(R-2 to 2.7 per inch, minimum)

• High sound absorption coefficient

• Simple, easy-to-learn construction techniques

• Structure can be completely standard

• Inexpensive material that is often available locally

• Natural and completely biodegradable materials, that create a healthy indoor space

• Renewable agricultural waste material, needing only a single growth season

• Requires very little energy to produce

• Diverts farming waste material

• Aesthetics of a thick-walled building with large widow sills

Challenges

• Requires careful detailing to prevent liquid water infiltration

• Requires breathable finishes, usually natural plasters, which may necessitate research or hiring a consultant

• Necessitates educating yourself, the builder, and (often) building permit officials

• Requires more interaction with permit officials in regions that do not currently have building codes for strawbale construction in place

• Hired labor can be expensive, especially for plaster finishes

Performance

SIZE

The size of strawbales depends on the

specific dimensions of the baling machine

that made them. There are 2 common

bale sizes in the USA:

18" wide X 14" tall

24" wide X 18" tall

INSULATION

Insulation is measured in R-value, which

quantifies how well a material resists heat

flow. The higher the R-value, the greater

the insulation.

Third-party testing of strawbales puts the

R-value between R-2 and R-2.7 per inch of

wall thickness. That means strawbales have

the following R-values:

18" bales = R-36 to R-48

24" bales = R-48 to R-64

For reference, model energy building code

requires that walls have R-19 insulation,

minimum.

Higher insulation means less heat flows

through the walls. Strawbale structures,

therefore, use substantially less

energy to heat and cool than

conventionally framed structures.

RELATIVE COSTS

Materials for strawbale wall systems

cost less than materials used in

conventional construction. (Material

costs make up about 1/2 of conventional

construction costs, compared to only 1/4 of a

natural home.) However, labor costs are

substantially higher for a natural building.

This accounts for many owner-built

strawbale structures costing a fraction of

what the same house would cost using

conventional materials.

As a general rule, however, the exact

same structure will cost the same

whether built conventionally or

naturally. (Assuming you pay for all labor

costs.) The difference is that a natural

building pays more for labor and

conventional construction pays more for

materials.

The biggest difference, of course, is that

costs for energy use over time will be lower

(and sometimes zero) for a natural home.

PESTS

Pest infestations are absolutely not an issue

with walls that are monolithic, have no

nesting cavities inside, and are finished with

thick plasters on both sides. The same is

true for bugs...as long as walls remain dry,

the inside of a strawbale wall does not have

enough food source or moisture to create a

suitable habitat for bugs.

strawbale infill wraps around a structural skeleton

truth window let's you show that yes, really, your walls are made from straw

strawbales in the process of wall building

electrical boxes attach to a wooden wedge

wiring mounts on the surface, since plaster is over 1.5" thick

FIRE RESISTANCE

Loose straw is highly flammable. Which

leads people to believe that strawbale walls

must also be flammable. However, quite the

opposite is true.

Fire needs 3 things: fuel, oxygen, and a

spark. The difference between loose straw

and a tight strawbale, is that there is not

enough oxygen inside a strawbale to

support a flame. In addition, strawbale

walls are finished with thick plasters...the

same materials used for fire-proofing.

Testing supports the fire-resistance.

Strawbale walls pass a two-hour

rating in a small-scale fire test (ASTM-

119), outperforming most standard types of

construction. Flame spread characteristics

and smoke development (ASTM E-84) also

exceed code requirements, with a flame

spread index of 10 and a smoke

development of 350.

MOISTURE

By far, the biggest concern with strawbale

walls...as with any construction materials...is

moisture. Any material that stays

persistently damp can support mold growth.

When biodegradable materials remain damp

they begin to decompose. This is true for

wood as well as straw.

Moisture infiltrates a wall in one of

two ways: as liquid water (such as from

a leaky pipe or roof) or as vapor/humidity.

Preventing liquid water migration requires

careful detailing (at wall penetrations and

horizontal surfaces). Humidity only becomes

a concern if it condenses inside the wall.

Which means eliminating cold spots inside

the wall (where humidity condenses into a

liquid) and using only vapor-permeable

finishes that allow humidity to migrate freely

through a wall.

Bales should be dry before

installation (15% or less moisture) and the

tops of all walls must be protected from rain

during construction.

INSTALLING ELECTRICAL

Installing wiring is actually more

simple with strawbale walls than with

conventional studs. This is because there is

no need to drill studs and snake the wires

through. Wires are simply surface

mounted to the bales using 4" landscape

staples or tucked between the bales. Plaster

is a minimum of 1-1/4" thick, which meets

the code requirement for wire location

behind the wall surface.

Junction boxes are attached to structural

elements or are screwed to wooden wedges

driven tightly into the monolithic bales.

light bounces off clay plaster

small passive solar strawbale cottage with lime plaster

Building Permits

Most habitable buildings in the USA require a building permit. And that's not a bad thing. Building codes ensure that all structures perform at a baseline for safety,

durability, and energy efficiency.

Several states and counties throughout the USA have adopted building code amendments for strawbale construction. However, you absolutely can get a permit for strawbale buildings anywhere in the USA. Even if you do not live in a jurisdiction that specifically spells out strawbale methods in their code. This is because strawbale wall systems meet all of the performance requirements for wall systems everywhere in the U.S.

Every building code begins with a paragraph that basically states something to this effect: You may use alternative materials and methods that are not specifically laid out in this code...IF...you demonstrate compliance with the INTENT of the applicable codes. Demonstrating compliance means supplying testing data that explains safety & energy performance. Luckily, testing data is available for free online, thanks to the

generosity of many strawbale pioneers in the 1990's.

How to communicate with permit officials

The most important thing to keep in mind when getting a building permit is that the peopleworking in the permit office are there to collaborate with you.

1. Educate yourself well on what and how you want to build. The permit officials will need to feel comfortable entrusting you with this non-standard construction method. If you do not feel confident with your knowledge, hire a professional that is acquainted with strawbale construction to assist you.

2. Start an early dialog with your building officials. You can start with a phone call, but often face-to-face meeting is most effective. Be friendly and have a collaborative (not combative) attitude. If they are not already familiar with strawbale construction, provide them with printed information and additional resources. Do not overload them! If they ask you something you don't know, tell them so, and offer to provide additional information.

3. Give them at least a week to review the information you provided. Then follow up and ask if they have any additional questions or concerns. Ideally ask them to itemize any concerns in writing.

4. Address each concern in a clear, concise, and informed manner, and submit your response in writing. Again, get professional assistance if you are not confident with your knowledge.

strawbales wrapped around timber framing