Insulation: MATERIALS ENCYcLOPEDIA
Applications for this system
Infill insulation in frame structures
Borax (as an anti-fungal, if required)
Temporary wooden formwork
Ratings Chart for straw/clay insulation
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- INDOOR AIR QUALITY
- FUTURE DEVELOPMENT
Loose straw is tossed with a runny clay slip until lightly coated. The straw/clay mixture is then placed in the wall cavity between temporary form boards. The insulation is lightly tamped in lifts as it is built up in the cavity. Slip forms can be moved up the wall as straw/clay is added. The material is stable in the wall immediately after tamping.
The finished insulation is left exposed on both sides of the wall in order to dry. The straw/clay provides a dense, flat surface and makes an ideal substrate for plasters. Sheet barriers and more conventional siding and wall covering can also be used.
Environmental Impact Rating
Harvesting — Negligible to Moderate
Clay harvested from the site will have negligible impacts. Clay and straw harvested locally will have low impacts. Bagged clay is mechanically extracted from pits and dried using fossil fuel heat, and will have moderate impacts, including air and water pollution.
Manufacturing — Negligible
Transportation — Negligible
Sample building uses 7,920 kg of straw:
11.9 MJ per km by 15 ton truck
Sample building uses 581 kg of clay soil:
0.9 MJ per km by 15 ton truck
Installation — Low
Formwork may be reused elsewhere in the building.
Biodegradable/Compostable — All soil and straw
Chart of Embodied energy & carbon
R-value: Depends on density of material. Tested per-inch values range from:
– R 0.9 at 705 kg/m3 to R 1.9 at 164 kg/m3 (Forest Products Lab results)
– R 0.69 at 700 kg/m3 to R 1.44 at 300 kg/m3 (Gernot Minke)
Where insulation value is a critical design factor, sample mixtures can be created, dried and weighed to estimate the R-value.
Straw/clay is not an air or vapor control layer, and will require a sheathing material to achieve insulation values.
Material costs: Low
Both straw and clay soil are low-cost components, and though volumes required are high, costs are relatively low.
Labour Input: High
There are three components to the labour input for slipstraw insulation:
- Mixing the material involves making slip from water and clay soil, and then tossing the straw with the slip.
- Creating and moving formwork. Type of formwork will depend on framing style of the walls.
- Tamping slipstraw into wall cavities.
Each of these phases is fairly labour intensive, resulting in a system with significant embedded labour time.
Skill level required for homeowners: moderate
Slipstraw mixing and placing is well within the capabilities of novice builders, as long as they take care to create consistent mixes and tamp evenly. Requiring only basic carpentry skills, formwork can be built by beginners.
Sourcing & availability: Easy to moderate
The ingredients for straw/clay insulation are widely available in most regions. Straw must typically be sourced directly from farms or straw brokers. Clay for slip making can be found in some site soils or obtained in bagged form from pottery supply outlets. Materials for formwork (typically plywood or planks) can be sourced via building supply stores. A simple straw/clay tumbler can be built or adapted from recycled or found materials.
Durability: moderate to High
Straw/clay insulation has lasted for hundreds of years in a number of different climates. Clay coatings have a beneficial effect on straw, wood and other natural fibers. The hydrophilic clay can help moderate the moisture content of the straw and restrict the likelihood of mold growth. Fire resistance of straw is dramatically increased when lightly coated in clay.
Straw/clay is not prescribed in any North American building codes. Some lab testing of the material has been carried out, but results vary widely and have not originated in certified laboratories. The variability of the mixtures means that it is difficult to accurately quantify insulation value, fire resistance and other elements of code requirements. The time-proven history of straw/clay, however, speaks to its viability, and many code-approved buildings have been able to use straw/clay insulation based on existing test figures and engineer or architect’s approval.
Indoor air quality: moderate to high
Both straw and clay have excellent moisture handling and storage capabilities, and the straw/clay mix has not shown itself to be prone to mold growth (as long as adequate drying times have been allotted). In conjunction with a natural plaster finish, the material can contribute to a building with very good IAQ.
Resources for further research
Baker-Laporte, Paula, and Robert Laporte. Econest: Creating Sustainable Sanctuaries of Clay, Straw, and Timber. Salt Lake City, UT: Gibbs Smith, 2005. Print.
Thornton, J. Initial Material Characterization of Straw Light Clay. Ottawa, ON: Canada Mortgage and Housing Corporation, 2004. Print.
Minke, Gernot. Building with Earth: Design and Technology of a Sustainable Architecture. Basel, Switzerland: Birkhäuser, 2012. Print.
Straw/clay does not readily lend itself to large-scale production, and is likely to remain a choice of dedicated owner-builders and professionals committed to its strengths as a natural insulation. While bulk mixing of the material would be easy to achieve, the long drying times required do not mesh well with typical construction schedules. It is likely that further testing of the material will make it easier to justify its use, but the choice to do so will remain that of the enthusiast.
Tips for successful straw/clay insulation
1. There are plans for simple, homemade straw/clay mixing devices that are low-cost and increase productivity dramatically. Even for a single building, it is well worth the time to construct a mixer.
2. Work out accurate systems for measuring quantities of straw and clay slip for mixing. An overly wet mixture compacts a lot when tamped, lowering insulation values and increasing drying times.
3. Allow sufficient time in the building schedule for drying. This process can take up to one week per inch (2.5 cm) of wall thickness, depending on weather conditions. This can require scheduling adjustments for builders unused to such long drying times.
4. As the insulation value of the material depends on the degree of compaction that occurs when the straw/clay is tamped into the forms, ensure consistent installation procedures throughout the process, especially when larger crews are involved.
5. Use hex-head screws to secure formwork. Forms are moved frequently and typical screw heads get covered in clay and can be hard to locate and use.