Thermal Insulation Explained

In brief: Thermal insulation is the property that lets clothing slow the loss of body heat, mainly by trapping a layer of still air. The more still air a layer holds, and the better it stops that air moving, the warmer it is. Insulation can be natural, such as down and wool, or synthetic, such as polyester fills. Warmth is commonly measured in two units: clo, used more in America, and tog, used more in Britain and Europe.

What is thermal insulation?

Thermal insulation in clothing is the ability of a fabric or fill to reduce the rate at which heat escapes from your body. Your body is constantly producing heat, and in cold conditions that heat is lost to the surroundings. Insulation slows this loss, keeping you warmer for longer. The key to almost all clothing insulation is still air. Air is a poor conductor of heat, so a layer that traps a thick blanket of motionless air around the body dramatically slows heat loss. This is why lofty, fluffy materials insulate so well: they are mostly air, held in place by a fine network of fibres.

How insulation traps heat

Insulation works by holding air still. Heat moves away from the body in several ways, but in everyday clothing the biggest factor is convection, warm air being carried away and replaced by cold. By trapping air in countless tiny pockets, insulation stops that air circulating, so your body heat warms the trapped layer and stays there. The effectiveness depends on three things: loft, or how thick the still-air layer is; the fineness and number of fibres holding the air in place; and the construction, which keeps the insulation evenly distributed and stops cold spots. A thicker, more evenly lofted layer of fine fibres traps more still air and insulates better.

Natural vs synthetic insulation

Insulating materials fall into two broad camps. Natural insulations include down, the soft clusters from ducks and geese, and wool, both of which trap air extremely well. Down offers the best warmth-to-weight of any common insulation but loses loft when wet, while wool insulates even when damp and manages moisture well. Synthetic insulations are made from fine polyester fibres engineered to mimic down. They are typically cheaper, hypoallergenic, keep insulating when wet and dry quickly, but tend to be heavier and bulkier for the same warmth, and can lose loft over years of compression. Many modern garments combine materials or treat down to get the best of both.

Clo and tog: measuring warmth

Two units are commonly used to put a number on insulation. The clo is a unit defined so that one clo is roughly the insulation a resting person needs to stay comfortable in a normal indoor room, and it is used more widely in America. The tog is used more in Britain and Europe and is familiar from duvets and bedding. The two relate to each other directly: one tog is a smaller amount of insulation than one clo, with roughly one and a half tog being about equal to one clo. Both ultimately measure the same thing, how well a layer resists heat flow, just on different scales, which is why you will see tog ratings on duvets and clo values in technical outdoor and scientific contexts.

As a rough guide, one clo is about one and a half tog. Both measure resistance to heat flow.

Choosing the right insulation

The best insulation depends on how and where you will use it. For cold, dry conditions and maximum warmth with minimum weight, high-quality down is hard to beat. For damp climates, active use, or anyone who wants reliable warmth in the wet, synthetic insulation or treated hydrophobic down is the more dependable choice. Wool suits those who want natural fibre and steady warmth that copes with moisture. Beyond the material, fit matters: insulation only works if it can loft, so a layer that is crushed flat or worn too tight will not trap the air it needs. Layering several thinner insulating pieces is often warmer and more adaptable than one thick layer.

Thermal insulation at OD's Designer Clothing

At OD's Designer Clothing we stock insulated outerwear across the full range of technologies, from natural down to advanced synthetic fills, from premium brands chosen for genuine warmth and quality. Whatever the conditions you are dressing for, we can help you weigh up warmth, weight and wet-weather performance to find the right layer. We offer next-day delivery and free click and collect, and customers in the North West are welcome to visit our St Helens store to compare insulation types in person.