Waste cellulose fiber from agriculture or forestry is coated in a light slurry of a clay- or lime-based binder and tamped in 4-6 inch deep lifts within temporary formwork on a wood-framed wall, roof, or floor. The forms are removed and the material is allowed to dry (3-6 weeks, typically), creating a continuous, rigid insulation. Bonded cellulose systems are typically finished with plaster applied directly to the insulation, but they can also be clad with board or sheet materials. There are two variations of bonded cellulose insulation: If Hempcrete or hemp-lime — The lightweight core of the hemp plant, called the hurd or shive, is lightly coated with a hydraulic lime binder. The lime binder protects the insulation from fire, insects, and mold. Light straw/clay (LSC) — Hollow grain straw (from wheat, oats, barley, rice, or other cereal crop) is lightly coated with a runny clay slip. The clay protects the straw from fire and insects. CRiTERiA CONSiDERATiONS Ecosystem impacts: Farming practices for ^ hemp and straw will affect impacts. Clay binder will have lower impacts than lime. Embodied carbon: Natural fibers sequester more carbon than is produced. Hemp sequestration will outweigh carbon footprint of lime binder. Energy efficiency: Moderate to good thermal properties. The ratio of binder to insulation, the density of the mixture, and the degree of compaction in the cavity will result in a wide potential range of R-values. Indoor environment quality: Inert. Anti-microbial qualities of lime binder give hempcrete excellent properties. Long drying period can result in elevated humidity during construction, and drying must be complete before enclosing insulation to avoid potential mold problems. Building code compliance: LSC is recognized in an appendix of the International Residential Code 2015. LSC and hemp-crete require alternative compliance. Material costs: Vary widely. Research material sources for specific costs. Labor: Very few contractors. Owner-builder can install.
There are many types of rigid foam board and spray foam insulation. Though often sold as a “green” material because of their reasonable degree of energy efficiency, they are incompatible with many sustainable building criteria:
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Ecosystem impacts: The full “chain of custody” for foam products needs to consider the wide range of ecosystem impacts of oil exploration, extraction, shipping and pipelining, refining and processing. Foam building products typically contain flame retardants that are very dangerous to the human nervous system, soil, and water. Embodied carbon: Foam insulation will contribute significantly to the carbon footprint of your building.
Indoor environment quality: Off-gassing of foam products is a contentious topic. Many independent researchers have noted issues of strong concern; the foam industry says there is nothing to worry about. The precautionary principle reminds us that we have often underestimated the dangerousness of chemicals we create and use that industries and regulators have deemed “safe.”
Waste: During the construction phase, the cutting and shaving of foam insulation can generate a high volume of foam particulate that is difficult to contain and typically ends up polluting the ground (see concerns about flame retardants). During renovations and at the end of life, spray foam is difficult to separate from other materials, and all foam products will add substantial volume to landfills.