How Effective Is Sheep’s Wool Insulation Compared to Fiberglass?

Recent Trends in Natural Insulation

Over the past few years, demand for natural insulation materials has risen steadily, driven by tighter building energy codes and growing consumer interest in low-carbon home upgrades. Sheep’s wool—long used in clothing and bedding—has gained attention as a high-performance alternative to conventional fiberglass batts. Installers and homeowners are now weighing wool’s thermal, moisture, and lifecycle qualities against well-established fiberglass products.

Background: Two Very Different Materials

Fiberglass insulation, made from spun glass fibers, has dominated residential and commercial construction for decades. It is lightweight, inexpensive, and widely available. Sheep’s wool insulation is produced from raw wool fibers that are cleaned, treated with a natural fire retardant (often borate salts), and formed into batts or rolls. Each material takes a fundamentally different approach to managing heat and moisture.

Key User Concerns: Performance, Cost & Safety

When comparing the two, homeowners and builders typically consider four areas:

• Thermal efficiency: Both materials offer similar R-values per inch (roughly R-3.5 to R-4.0 for wool, versus R-3.0 to R-3.7 for fiberglass). Wool’s natural crimp traps air effectively, but fiberglass batts can match or slightly exceed wool in standard lab tests when perfectly installed.
• Moisture & air handling: Wool can absorb up to 35% of its weight in moisture without feeling damp or losing insulating ability. Fiberglass, by contrast, wicks moisture through fibre channels and loses R-value when wet. Wool also buffers indoor humidity, while fiberglass does little to regulate airborne moisture.
• Fire & safety: Fiberglass is inherently non-combustible, but its fine airborne fibres can irritate skin, eyes, and lungs during installation. Wool naturally smolders rather than sustains open flame, and the borate treatment further improves fire resistance without creating toxic smoke. However, wool is not classified as non-combustible in all building codes.
• Cost & installation: Sheep’s wool insulation typically costs two to three times more per square foot than fiberglass. It is also easier to handle without protective gear, compresses slightly less over time, and can be cut with scissors. Fiberglass remains cheaper and faster to source, but requires careful fitment to avoid gaps and reduced thermal performance.

Likely Impact on Building Practice

Industry observers expect wool insulation to capture a growing niche—particularly in retrofit projects where moisture management is critical, and in high-end, eco-conscious new builds. Fiberglass will likely keep its dominant share in standard new construction and large commercial projects due to price and familiarity. Wool’s superior moisture buffering may also reduce overall ventilation loads in tight, well-sealed homes, a factor that could offset its higher upfront cost over the building life cycle.

What to Watch Next

Several developments could shift the balance in the coming years:

• Changes in fire codes: If natural materials gain acceptance under mass timber or passive-house fire-safety frameworks, wool’s market may broaden.
• Supply chain scaling: More wool-processing facilities could reduce cost premiums, especially in wool-producing regions.
• Long-term performance data: Independent studies on real-world R-value retention, settling, and microbial resistance in wool over 20–30 years are still limited. As more buildings age, clearer comparisons will emerge.
• Embodied carbon policies: Carbon-conscious building standards may favor wool’s lower manufacturing emissions over fiberglass, influencing specification choices in public and commercial projects.

For now, the choice between sheep’s wool and fiberglass hinges on project priorities: budget, moisture risk, installation ease, and environmental values.