Wool is a single textile that offers multiple advantages, making it one of nature's most valuable fibres. It's naturally hypo-allergenic, breathable, insulating, moisture-wicking, flame resistant and soft---all qualities that make it ideal for use in bedding. Wool duvets are top blankets made of either cotton, cotton-bamboo blend or wool shell filled with natural wool fibres. They're manufactured in three weights---lightweight for hot months, mid-weight for most times of year, and heavyweight for cold months.
Unlike cotton and synthetic fibres, wool naturally resists mould, dust mites and mildew. Because it moves moisture through the core of the fibre, wool doesn't provide a hospitable environment for microorganisms, which require constant moisture for survival. Constructed of the fibrous protein called keratin, each individual strand also has scales along its casing that prevent dust or mould from lingering. Synthetic fibres and cotton tend to hold moisture, making them more accommodating to dust mites and other such microorganisms, and therefore not as clean or non-allergenic.
Wool has been called the most breathable textile available. "Breathability" is used to describe how a fibre or fabric responds to heat---body heat, specifically. Textiles that lack breathability, such as polyester, trap body heat next to the skin causing increasing warmth and sweating. Cotton is a more breathable fibre than polyester; however, when it's woven into a high-thread count fabric, it loses a certain amount of breathability. On the other hand, wool reacts to heat by pulling it into its fibres. The higher a body's temperature, the more heat the wool pulls into its fibres and releases out the other side. Because of this quality, a wool duvet can keep a sleeper warm on cold nights and cool on warm nights, as it does on a sheep's body.
Part of wool's breathability comes from its power to pull any moisture that's up against it into its fibres and release it through evaporation. Like sweating sheep, a sweating sleeper has an advantage with wool because it doesn't hold moisture against the skin, resulting in damp bedding. Wool is as much as 22 per cent more absorbent than cotton, and absorbs as much as 30 per cent of its weight in moisture before evaporation occurs. The fibres release heat as moisture evaporates; 27 heat calories are given off by a single drying wool fibre.
Wool is probably most well-known as an insulator. While this benefit may seem contradictory to its breathability and moisture wickability, all three of these properties are the result of fibre construction. In cold temperatures, wool still pulls moisture into the fibres' cores to keep dampness off the skin, which eliminate wetness while retaining warmth. A heavyweight wool duvet provides more insulation than a lightweight duvet, which is made specifically for use in warm weather.
Untreated or organic wool is naturally flame-resistant because the keratin it's made of, in combination with the moisture absorbed by the fibres at any given time, work hand-in-hand to resist catching fire. Wool has a much higher ignition temperature than other fibres, and burns slowly when it does catch fire. In certain cases, wool containing a high moisture content has even been known to self-extinguish. This doesn't hold true for chemically treated, or non-organic wool, since chemical treatments can easily catch fire and cause the wool to burn.