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Super Insulated Earth at Calliope Farm in Olympia, WA

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Super Insulated Earth at Calliope Farm in Olympia, WA
Calliope Farmhouse, photo by Joseph Becker
Building Envelope
Straw Clay Insulation with Exterior Cork Wrap
Thurston County, WA
Rowland Zoller, Building Official, Thurston County Planning
Aaron Weaver, Calliope Farm
Joseph Becker, ION EcoBuilding


Joseph Becker, Olympia-based natural builder, utilized light straw-clay insulation, perforated plywood shear wall, and expanded cork insulation in the earthen wall system of Calliope Farm’s new residential farmhouse. Using thermal resistance R-values from existing code supplements and references from the cork manufacturer, Becker proved his innovative wall system would exceed WA Energy Code. Sufficient evidence showed that a vapor-permeable wall system adequately managed moisture, thus annulling the required vapor barrier.

Permitting Process

This project was permitted through a component performance path to show the wall system met the Washington State Energy Code. Default insulative values for light straw/clay, set at R-1.6 per inch, were established by the Canadian Housing and Mortgage Corporation study and were explicitly recognized in New Mexico and Oregon code supplements. Insulative values for the expanded cork material was assumed to be R-4 per inch based on European standards and the manufacturer’s claims. Using the above values, Joseph Becker, with the help of the Washington State University (WSU) energy extension office, was able to compare the total area r-values (u-values) between a hypothetical home with straw-clay plus rigid cork insulation to a conventionally insulated home; the results of which proved that the rigid cork with straw-clay insulation system yielded thermal resistance values better than that required by the WA state energy code. With the performance requirement exceeded, the combination of materials intended for this project was approved.

While the residential code also requires a vapor retarder in this climate, sufficient evidence provided to building officials proved that the vapor permeable wall system, composed of straw-clay, cork, and a custom perforated or ‘holey’ plywood layer, managed moisture inside the home and throughout the wall assembly without the need for a vapor retarder.


Code RequirementCompliance Path

2015 Washington Residential Code, Section R702.7 Vapor Retarders

Reliance on Oregon and New Mexico code supplements for straw clay building guidelines.
2012 Washington State Energy Code, Residential Provisions, Section R405 ‘Simulated Performance Alternative’ Washington State University energy extension office assisted in technical aspects of completing a component performance pathway worksheet.
2015 IRC Appendix R: Light Straw Clay Construction (Newly added to 2015 edition) This section of IRC helped convince building department officials of viability of straw/clay as an insulative material and acted to guide specifications for this project’s approval.

Project Details:

Installation of holey plywood shear wall

The Calliope Farm owners/residents wanted an insulative wall system that reflected their values (promoting local resiliency, use of ‘least toxic materials,’ etc.) and therefore the decision to utilize locally sourced straw/clay and the sustainably manufactured cork was only natural. Becker’s familiarity with installing earthen wall systems and his experience working with city officials to get similar projects approved) only strengthened the decision to use straw/clay and cork as the primary insulative materials for this project (See related case study, Permeable Wall System at Port Townsend EcoVillage).

Becker drew upon a combination of modern and ancestral building technologies/techniques in order to construct an earthen wall system that both exceeds insulative standards and that manages moisture without the use of a vapor barrier. In conventional buildings, a vapor barrier is required to prevent the transfer of moisture through a wall, floor, or ceiling system with the intent of preventing water damage to structural and/or aesthetic aspects of the building. In ‘natural buildings,’ and in this project’s wall system, vapor permeability is used as a strategy to balance indoor humidity levels, outdoor moisture levels, and deal with the inevitability of moisture inside the wall assembly itself. The idea is that by using vapor permeable materials (natural plasters, straw/clay mix, a custom perforated plywood layer, and cork insulation) moisture will be able to move through a wall system and transpire into the outer atmosphere easily rather than hitting a vapor barrier, pooling, and causing water damage, mold, etc. within the wall system over time.

The anatomy of this particular wall system, moving from inside the home outward includes: an interior natural plaster ‘finish’, applied to a 6 inch thick section of light straw/clay (infilling a 2 x 6 structural wall), followed by a section of ‘holey’ or perforated plywood, a weather resistant barrier, the 3 inch exterior cork wrap, and finally a wooden rainscreen cladding material.

Cork’s thermal resistance value is considered about R-4 per inch. A three inch exterior layer was used on this project making for an R-12 value in cork alone.  Light straw-clay’s thermal resistance is roughly R-1.6 per inch (at medium density), and at 6 inches (thickness used in this project) this equates to about R-9.6. The combined R-value for both cork and the light straw-clay comes to slightly over R-21.6 for the total wall, 0.6 units over that required by the Washington State Energy Code (see Table R402.1.1 for details).

Cork’s permeability (measured in ‘US perms,’ the US standard unit of permeability or rate of transfer of water vapor through a material) is rated 2.15 US perms at 1.5 inches thick, 2.04 US perms at 2 inches, and 1.26 US perms at 4 inches (decreasing in permeability with thickness). For comparison, American building codes generally classify ‘vapor barrier’ at or below 1 US perms. Cork falls slightly above the ‘vapor barrier/retarder’ designation and lies in the ‘semi-permeable’ category (1-10 perms). When comparing permeability of cork to other exterior or interior ‘natural’ finishes, lime-sand plaster is roughly 9-12 perms, earthen plaster is 16-20 perms, and a 1:1 cement-lime plaster mix clocks in at around 7.12 perms (a seven-fold increase from cement stucco, 1.16 perms) (See Natural Building Companion, page 101). The semi-permeable character of the cork mixed with its ability to both insulate and not degrade when exposed to exterior conditions (rain, wind, etc.) made cork an attractive alternative to both conventional insulation materials and natural-plaster finishes alike.

Additional Resources

(2011) Oregon Reach Code: Code supplement for straw/clay standards, requirements, and inspection.  See Relevant Chapter 13- Residential Provisions; see section 1307 for ‘Alternative Methods and Materials’ (i.e. straw/clay standards, requirements, etc.) State of New Mexico Construction Division Clay Straw Guidelines
Sierra-Perez et al.  "Environmental Implications of the use of agglomerated cork for use as thermal insulation in buildings."  From Journal of Cleaner Production 126 (2016) 97e107 Light Straw-Clay Construction Code supporting documentation website, by Paula and Richard LaPorte


The farm owners collectively affirm in their mission statement that, “[practicing] conservation and [providing] clean food for [their] community and family is what motivates [them].” It becomes clear that this sentiment extends beyond their agricultural pursuits and into the ‘alternative’ approach to meeting the insulative needs of their project. Becker’s knowledge of light straw clay and expanded cork provided the idea and facilitated the installation of this ‘alternative’ approach.

The materials for the straw/clay mix were gathered and harvested locally (onsite and within Olympia area) and therefore were an obvious choice from an environmental vantage point. The fact that light straw/clay is also a healthy and safe alternative to contemporary insulation material added to its social value.

Exterior cork insulation wrap
Expanded insulative cork is produced in such a way that virtually no pollution and/or ‘waste’ occurs during the manufacturing process. This along with the carbon sequestration potential of the cork oak tree has caused some to claim that the process of producing/manufacturing cork is a carbon ‘net negative’ and can act as a climate-change mitigation strategy. While these attributes/claims should be tempered with realistic energy/carbon calculations of post-production practices (i.e. shipping), overt dismissal of cork in green-building circles because of its need to be shipped by boat from Portugal requires comment. Most building materials, even the ‘greenest’ insulations and siding, still require highly energy intensive manufacturing and shipping processes (often more energy/carbon intensive than cork). It seems irrational, therefore, to make the case that cork should be abandoned from use for this reason alone. Those interested in cork insulation should take into consideration the environmental and economic ‘shipping costs’ but also recognize that it is not alone in the current necessity of globalized shipping.

Additionally, cork is hypoallergenic (doesn’t absorb dust), is a fire retardant (prevents the spread of fires, insulates surrounding area from heat, doesn’t off-gas VOCs during either combustion or normal use), and allows for vapor and moisture permeability (see more complete list of characteristics, here). The combination of environmental and social benefits weighed strongly in favor of this material being chosen for this project.

Design / Build Process:

The permit was obtained in October of 2015 and by November, the project had broken ground. Joseph, along with owner/contractor Aaron Weaver and a small crew of skilled builders, got the structure dried-in with the completion of a roof just in time for Spring. With Olympia’s wet seasonal weather patterns timing was a crucial consideration for this project. Joseph’s strategy was to get the structure essentially dried-in, or protected from excessive rain/moisture, by the spring so that the straw-clay wall system could adequately dry out over the course of the summer months. He left little to chance either, immediately putting fans on the straw/clay walls after their completion in order to speed up the drying process.

In order to capitalize on community interest in natural building while simultaneously providing hands-on education and a community ‘building’ experience, Joseph recruited 25 volunteers to help mix, fill, and form the straw-clay walls. The sheer number of workers made this process reach completion much faster than if Joseph and his three-person crew had been acting alone. Joseph and his crew at Ion Ecobuilding, along with Aaron and some friends worked through the summer and fall to complete the more detailed straw/clay installation, cork wrapping, and all other remaining components of the wall system.

One interesting design element unique to this project was the use of a customized ‘holey’ or perforated layer of plywood set in-between the straw/clay and cork sections of the wall system. This layer acts as a backing for the exterior cork wrap, but is perforated (strategically fabricated with holes) so as to maintain vapor permeability throughout the wall assembly.

Cost / Benefit

When assessing the economic factors behind any construction project, whether conventional or naturally built, it becomes apparent that the owner’s priorities (see triple constraints triangle), resources, and values ultimately weigh substantially into the overall equation, along with financial costs and benefits. As discussed above, the owner’s value-set: promoting earth conscious, eco-social health through embodied/applied ethics, was a significantly motivating force for this project and likely weighed in favor of certain materials and methods being used over other cheaper (and more ecologically/anthropologically damaging) materials/methods.

Making an apples-to-apples economic comparison between conventionally built and naturally built structures can be very difficult, primarily due to the highly predictive material/labor costs of conventional construction and highly variable or sometime unpredictable material/labor costs associated with many naturally built homes. The current trend seems to indicate an inverse correlation between material and labor costs where conventional construction has high material costs (and therefore lower labor costs) and ‘natural building’ has high labor costs (with generally lower material costs). In other words, conventional construction often tends to be more economically ‘efficient’ on the labor side but requires a higher investment in highly industrialized materials whereas naturally built structures tend to be much more labor intensive but often acquire their materials (straw, clay, plasters, timber, etc.) cheaply, sometimes even for ‘free’ (NBC, 122).

Becker’s skill organizing educational ‘work-parties’ provided cheap/volunteer-based labor for the initial LSC mixing and installation. With Weaver acting as the contractor, this project also utilized help from friends, family, and a few hired (skilled) laborers to assist Joseph for the more detailed aspects of the wall system. These techniques kept labor costs low while also providing an opportunity for community building, education of natural building, and exposure to the owner’s for-profit farm business. From a ‘triple-bottom-line’ perspective, this project seems to have achieved social, ecological, and economic viability.

The expanded cork insulation was the largest material expense, costing roughly $3 per board foot. One online resource included a figure of roughly $5 per square foot for cork insulation. That’s roughly 2-4 times that of conventional polyisocyanurate insulation ($1.10-$1.60 per square foot) or extruded polystyrene ($2-$2.25 per square foot) (figures found here under ‘Cost and Availability’). While this figure will ultimately fluctuate over time depending on one’s location and the factors of supply, demand, etc., the important piece to keep in mind is that the owner’s values of ecological and social sustainability made the higher cost bearable, even preferable to the cheaper alternatives.

Lessons Learned

Becker learned from a similar project that climatic considerations (humidity, temperature, season) in one’s locality largely affect the rate at which these wall systems dry-out. In order to catalyze and accelerate this process, Joseph used heating fans, which reduced the moisture content within the wall system to roughly 20% or below (when measured to four inches in from either side of the wall), which allowed for the application of a finish plaster to the surface of the interior walls (see OR Reach Code 13073.8.2 for more on moisture content specifics).


Currently no domestic (US) cork ratings exist; however, building science company RDH is conducting moisture/thermal testing on cork supplier and enthusiast Albert Rooks’ ‘exposed’ cork exterior building, Corkhaus. This is a three year study, set for completion by 2018, the results of which might bring to light more relevant and worthwhile information into the insulative and vapor permeable properties of this material.

Project Contacts
Builder: Joseph Becker ION Ecobuilding 360-402-2249 Supplier: Albert Rooks Small Planet Supply (360) 866-8779 : Wolfgang Kahler
Owner: Aaron Weaver, Owner Calliope Farm Approving Official: Rowland Zoeller, Building Official Thurston County Planning (360) 786-5490
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