Why Clothes Wear Warm or Cool: A Primer

[C.Lee]

It seems that not enough has been written about why clothes wear warm or cool, as many men continue to be hung up on cloth weight as a determinant of thermal comfort. In any case, I hope the information below helps us all dress appropriately and comfortably through the seasons. We'll keep it simple the first go around, and opportunity permitting, cover the topic in greater detail at a later time.

Insulation of Clothing

Clothing acts as a barrier to heat and moisture transfer between the skin and the environment. The insulation of clothing depends largely on the air trapped in and on it. A piece of fabric has 1) air enclosed within it, between the yarns, within the weave, and 2) still air that is bound to its outer surfaces. The thermal comfort of clothing is in a large part determined by the movement of heat and moisture through these air layers.

Dressing in Terms of Air Layers

Air, and still air in particular, acts as an insulator, and the thicker the still air layer is, the greater the resistance to heat transfer. A thicker air layer can be achieved with a thicker fabric (more air trapped within the fabric), by layering several layers over each other (air trapped between multiple fabric layers), or with a looser fit (air trapped between skin and fabric). Disturbance of air layers either from movement of the clothing by the wearer or by wind reduce the effective thickness of the air layer.





Thermal Comfort Is Dependent on a Fabric's Air Permeability

Air permeability is a measure of how easy it is to flow air through a fabric. You can gage it roughly by placing the fabric over your mouth and trying to blow through it. The higher the air permeability of a given fabric, the more easily its air layers are disturbed.

Air layers also act as a barrier to the transfer of evaporated sweat from humid skin to the environment. Water vapour transport through a fabric is mainly dependent on the fabric's air permeability. When the body is warm, it cools itself through sweating, and water vapour permeability is the number one factor in thermal comfort.

The concept of air layers and their resistance to disturbance is an important one. To keep cool, we want clothing with thin air layers, and which have high air permeability, such that the air layers are easily disturbed and water vapour transport from skin to environment is maximized. Alternatively, to keep warm, we want clothing with thick air layers which are not easily disturbed.

Examples
Let's put the above into practice and take a look at two examples picked from this forum:

viewtopic.php?f=4&t=11245
Lining a Coat for The Bitter Cold

Michael is lining a Loden coat with cashmere in the body and silk in the arms in an attempt to fend off bitter New York winters. The 375 gr cashmere is a step above a typical bemberg lining as it contains a thicker air layer. However, I am not sure what the cashmere's air permeability (and therefore resistance to air layer disturbance) is, and in any case it is likely higher than a tightly woven bemberg. I would suggest an additional silk lining layer between the Loden and cashmere lining to act as a barrier to any air flow, thereby maintaining the nice climate cultivated by the cashmere, and blocking out any wind or moisture which gets through the Loden.

To go even further, extend this additional silk lining through the sleeves such that the sleeves are lined in two layers of silk. The additional bulk is minimal, however the air layer gained is significant. Limbs play an important role in human heat regulation, and in focusing on insulating the trunk we have forgotten about our arms and legs.

Lining aside, fit plays a large part in creating air layers, and a loose fit where possible would increase air layer thickness and therefore thermal insulation. A lot of heat is lost through the shoulders and upper back as the cloth sits directly on the body (minimal air layer thickness) in those areas.

viewtopic.php?f=31&t=10830&hilit=Linen+ ... =30#p65857
Linen as a Basic Shirting, For Hot and Cold Weather

Well, the Lounge Linen Shirting at least. It is thick, airy, and has a surface finish which I can only describe as 'napped'. In the heat, the airiness lets the wind in and the water vapour through. The sheer heft of the material allows it to absorb significant amounts of sweat, thus keeping the wearer cool. As far as evaporative heat loss is concerned, whether the sweat is absorbed by the shirt or by the surrounding air is irrelevant to the wearer.

In the cold, the thickness, airiness of weave and surface nap of the linen form a thick air layer. So long as there is an outer layer limiting air permeability, heat is retained. A brushed cotton flannel could likely achieve the same effect, though the discussion in the thread referenced business cotton shirtings which are usually light, thin and tightly woven, i.e. having thin air layers. The surface nap of the linen also helps with creating a warm hand. A smooth-milled finish would have the characteristic cold hand of lesser linens.

In Closing

To close, let's look at what dressing for warm weather, or staying cool in general, looks like. The goal here for the dresser is to maximize the air permeability of the clothing ensemble they have on so that heat and water vapour can move from the skin to the environment. It all begins at the shirt, and if air can not easily pass through here, then we are off to a bad start and it only gets warmer as we put on the coat. Pick a shirt with an open weave which let's air through. Linen, as covered above is an excellent choice. Cotton voile is another. You can always do the rough air permeability test with your mouth to find the relative difference between fabrics. You may be surprised by the results.

Now onto the coat. Its fit, fabric selection, and construction will be to maximize air flow or air permeability. A tighter fit gives a thinner air layer between body and coat, minimizing insulation. The coat's fabric, like the shirt, will also have a weave suitable for air flow. Construction is oft overlooked, to the detriment of a warm wearer. Each layer and piece of material used in the construction of a coat - the lining, padding, hair cloth, flannel pieces, pocketing - all go towards adding layers and increasing the thickness of air layers within the coat. This is not a good thing if the goal is to minimize insulation to keep cool. Consider a light construction with minimal chest canvass construction, shoulder padding, pocketing, and lining.

I hope the above has added to your view of dressing to stay warm or cold. Air layers and air permeability are parameters we can use to maximum effect, so long as we keep them in mind.

Regards.

[Gido]

Thank you. That was an good read, and I feel a deeper digging follow-up piece would be very interesting; I hope I can encourage you to write it.

[ajbjasus]

Surely the cooling effect is greater if the sweat evaporates rather being absorbed by the shirt, as its takes its latent heat of vapourisation from the body in the former case ?

[Gido]

I'd think that that's depending on the air circulation. Water cools quickly and effectively once it has cooled itself. A little bit of wind or even movement makes all the difference. Right?

[ajbjasus]

Gido - if water evaporates it draws heat from its surroundings in order for the molecules to gain the kinetic energy to enter the gaseous phase, this is the basis of the cooling effect on the body. If liquid in the form of sweat is simply absorbed by let's say a shirt, it is merely removed from the skin. Its subsequent evaporation may have some cooling effect, but it is probably, less because it can also draw its latent heat from the surroundings - eg warmth of the sunshine.

[Luca]

On the shirt weaves, I think it's worth mentioning the Panama weave which is often, for a given weight, quite breathable and, because of its 'springiness' shows wrinkles less easily than poplin.

[couch]

Yes, this is nice as far as it goes, but one might be cautious about treating all fibers as if they have similar thermal properties. Fine wool fibers, and to an even greater extent cashmere fibers, for instance, have crimps that cause the spun yarns to trap a great deal more air at a micro level than fibers such as silk or cotton. This is why cashmere wears warm for its weight, and displays loft. So yes, with a strong enough wind, this trapped air can be disturbed and lose heat; however, the tightly shrunk weave and napped finish of good loden were developed precisely to be wind and water resistant and I think Michael will be just fine without an additional layer.

Also, for reasons you indicate, weave or knit structure can play a role in thermal performance that can be counter to other of your generalizations. Piqué knit in cotton polo shirts wears cooler than an equivalent weight of jersey knit or plain weave cloth, even when the piqué is thicker and/or heavier, not mainly because it can absorb more moisture or is more "open" (which is debatable depending on how you measure openness) but also because its physical structure allows it to maintain more open air channels when moist rather than clinging to the skin more evenly. A larger area of skin remains open to the air when a smaller area of cloth is in contact with it.

And, Gido, and ajbjasus, the point not yet made about evaporation is that it is a phenomenon affected by equilibria. If the air near the wet skin becomes saturated with water vapor, and is not displaced, very little new evaporation will occur, and thus little more heat will be removed. If however, the porous cloth, the breeze, and the body's motion allow the saturated air to be displaced by less saturated air, evaporation will be accelerated and more heat will be removed.

There are members here much more knowledgeable than I about fiber properties and physics, and I encourage them to correct and amplify the above, but it's important not to oversimplify the model too much if the phenomena are to be understood well enough to guide our judgments.

[alden]

C. Lee

Great post and entertaining read on an important subject.

Cheers

[alden]

the tightly shrunk weave and napped finish of good loden were developed precisely to be wind and water resistant and I think Michael will be just fine without an additional layer.

Hi Couch

Yes that was my idea in using the loden and cashmere combination. Loden's wind and water resistance is tremendous, especially in the 700 gms plus weight range. The addition of 375 gms of cashmere as lining should make the coat a stove. I hope so.

Cheers