The Brushed Cotton Effect
On my way in to work this morning, I was aware of a distinct chill in the air and was glad of my Kendal (80% cotton, 20% wool) shirt – it got me thinking…
In the Spring when the weather is generally warmer, we wear our lighter weight summer (eg.Zephyr) shirts without giving it much thought. Likewise, in the Autumn/Winter, we normally gravitate towards our heavier (eg.Kendal) shirts – this makes perfect sense as the “lighter” weight fabrics seem to keep us cooler and the “heavier” fabrics make us feel warmer – how do they do that? In order to answer this, we have to look at what we want the fabric or shirt to do, we need to look at textile performance.
How a textile performs is assessed by looking at several attributes: durability; comfort; aesthetic appeal; maintenance and protection. Obviously how crucial these attributes are depend on the intended end-use of the textile, so in terms of shirting design, we are more concerned with the first four and for the purpose of this article, we are particularly interested in comfort.
A shirt needs to look good and the fabric should have a nice handle, it needs to have a degree of pleasantness to the senses – it needs to have aesthetic appeal. A shirt needs the ability to retain its physical integrity under conditions of stress, it needs to remain looking like a shirt whilst wearing and after washing – it needs to be both durable and have a reasonable degree of maintenance. Most importantly, a shirt needs to have the ability to provide the wearer with freedom from pain or discomfort, the ability to maintain a neutral state – the shirt needs to be comfortable.
People are comfortable in their garments when they are unaware of them both psychologically and physiologically. Awareness of clothing usually leads to an expression of discomfort (too hot, too cold etc). It is unusual to hear expressions of positive comfort unless asked specifically how a fabric “feels”. In general, people consider themselves comfortable when they do not need to take off or put on additional clothing.
Heat is always being transferred depending on the difference in temperature of our bodies and the surrounding environment, always flowing to the lowest level. Our bodies need to maintain an internal temperature of 37 degrees (98.6F) so in a cold environment, heat will flow from our bodies, making us feel cold. The rate at which we loose this heat depends on how we can prevent some of the heat from escaping, or insulating ourselves by wearing clothes. Fabrics are perfect for this purpose as they are excellent insulators, compared to other types of material such as metal.
It is a misnomer that heavier fabrics are warmer than lighter fabrics – all textiles are very good insulators. This explains why we feel more comfortable wearing clothes in a cold environment rather than not wearing clothes – but why reach for the wool rather than a sheer voile shirt as the weight of the fabric doesn’t determine a fabric’s effectiveness against the cold ?
The crucial element is Air and the reason why some fabrics are “warmer” than others is related to the amount of air contained within the structure of the fibres, yarns and constructions (weave) of the different fabrics.
Air is even more resistant to the flow of body generated heat than fibres are, up to eight times more effective and is the best thermal insulator available. As fabrics are composed of both fibres and air, two materials that have low thermal conductivities, they are excellent thermal insulators.
Fabrics also slow the rate of convective heat loss from the body. The fibres and yarns divide the volume of air into small spaces or intersects. When these intersects are small, air movement is prevented and heat loss through convection is minimal. Fabric is an excellent insulator as it has a large surface area to which air can adhere – boundary layers of air form at the fibre and yarn surfaces as well as at the fabric surfaces. When a fabric is placed over the skin, a boundary layer of air is also created on the skin surface. Finally, fabrics are poor heat radiators and do not absorb or emit radiant heat.
In conclusion then, my Kendal shirt with its bulkier, hairier yarns, contains more air trapped within the fibres, the yarns and the intersects formed in the weave of the fabric than my Zephyr shirt made from finer, smoother yarns. This prevents heat loss through conduction, convection and radiation and helps us to maintain our body heat in a cold environment – we feel warmer – we feel more comfortable.