The Physics of Insulating – A Technical Tutorial
How your Body Uses and Loses Heat
Scientists have found that the human body sitting at rest consumes 4.7 kilocalories/hour/square foot of body surface area and transfers this energy into heat. When thinking about heat loss‚ it is important to remember that heat loss is affected by:
- body temperature
- outside air temperature and wind speed
- distances between the insulation fibers
- thickness of insulation and fiber size
- number of fabric layers
Mechanisms of Insulation
There are three kinds of heat transfer: conduction (exchange of heat through contact), convection (movement of air), and radiation. Air has a low thermal conductivity but is very mobile. There are thus two elements that are important in protecting from the cold:
- stopping the wind from penetrating and replacing the layer of warm air close to the body;
- setting up a layer of still air which serves as insulation, by the use of fibers (wool, etc.).
Another important factor is humidity. Water is a good conductor of heat, thus if clothes are damp — because of sweat, rain, or immersion — water replaces some or all of the air between the fibers of the clothing, causing heat loss through conduction and/or evaporation.
Thermal comfort is achieved by balancing three factors:
- the rate of heat production by your body
- the insulation value of your clothing (clo)
- environmental temperature
Of those three‚ it’s easiest to alter the insulation value of your clothing. Insulations work in two ways: the first‚ by trapping air. The more air trapped‚ the more efficient the insulation. Second‚ by reflecting back the body’s radiant heat.
Thermal insulation is thus optimal with three layers of clothing:
- a layer near the body, whose role is to get rid of sweat from the skin;
- an outer woven shell layer — preferably impermeable and breathable;
- and between the two, an insulating layer that separates and traps the air.
The three layers of air between the skin and the exterior layer also play an insulating role. Perhaps most importantly. up to 70% of heat loss can be through the head. It may go without saying, but wearing a hat is crucial to keeping the body warm.
Shell Technology and Staying Dry
Melting snowflakes, a light rain or generation of moisture from the inside are all bad news when trying to keep warm. When it comes to external factors, the best way to keep moisture out is to use a waterproof and breathable shell (this also implies that it is windproof). The days of one brand domination in this technology are long gone. There are two main technologies to make a woven fabric waterproof and breathable: polyurethane coatings and fabric laminations or membranes. The effectiveness of this function is measured by two units, the first for water resistance and the second for breathability, reported numerically such as 5,000mm/10,000gr/m2. This is where it gets pretty technical. Water resistance is determined by lab testing essentially measuring water leakage. which can vary with pressure (i.e. a driving wind). Breathability is also determined by laboratory testing. Breathability increases in colder ambient temperatures. Suffice it to say that the higher the numbers the better. For cold weather fishing and hunting outerwear, a minimum rating of 3,000mm/3,000gr/m2 is suggested.
10,000/10,000? 20,000/20,000? What do the rating numbers actually mean?
Manufacturers typically describe the waterproof breathability of fabrics using two numbers. The first is in millimeters (mm) and is a measure of how waterproof a fabric is. In the case of a 10k or 10,000 mm fabric, if you put a square tube with inner dimensions of 1” x 1” over a piece of said fabric, you could fill it with water to a height of 10,000 mm (32.8 feet) before water would begin to leak through. The higher the number, the more waterproof the fabric.
The second number is a measure of how breathable the fabric is, and is normally expressed in terms of how many grams (g) of water vapor can pass through a square meter (m2) of the fabric from the inside to the outside in a 24 hour period. In the case of a 20k (20,000 g) fabric, this would be 20,000 grams. The larger the number, the more breathable the fabric.
Why isn’t outerwear completely waterproof?
The truth is that all outerwear designed for active winter sports has various degrees of water resistance, but will eventually leak given enough water, time and pressure. Manufacturers define “waterproof” according to different standards, and testing is not standardized. A rubber raincoat is completely waterproof, and may be the ideal garment for standing in a downpour waiting for the bus, but if you tried to ski or snowboard in it, you’d be wet in no time from your own perspiration. The trick is to balance protection from rain and snow on the outside with the ability to let water vapor (warm perspiration) escape from the inside.
Also important to shell technology is seam leakage protection. You might have the best fabric in the world but if the seams leak it does not do much good. Pinholes in the fabric where thread passes through can create these leak points. This is addressed by applying polyurethane tape to the inside of the seam on the garment shell to create a flexible seal of these pinholes. Be aware that some manufacturers only seam tape in “critical areas” to save on production cost.
Effect of Activity Level and Ventilation
The insulation required by an individual to maintain comfort is markedly affected by activity level. This factor must be considered when choosing a garment for a particular activity. Ventilation can be as important as insulation as it relates to removing moisture from inside the garment shell. You can have massive insulation in a garment and still get cold due to moisture remaining inside the garment from normal or excessive perspiration. This is particularly true with flotation linings which do not breathe on their own.
Clothing insulation may be expressed in clo units. The clo has the same units as the R-value used to describe insulation used in residential construction. The clo value is also similar in magnitude to the R-value.
1 clo = 0.155 K·m²/W = 0.88 R
This is the amount of insulation that allows a person at rest to maintain thermal equilibrium in an environment at 21°C (70°F) in a normally ventilated room (0.1 m/s air movement). Above this temperature the person so dressed will sweat, whereas below this temperature, the person will feel cold.
Normal Clo Values
|Naked body||clo= 0.0|
|Summer clothing||clo= 0.6|
|Typical business suit||clo= 1.0|
|Downhill ski suit||clo= 2.0|
|Light polar eqpt||clo= 3.0|
|Heavy polar eqpt||clo= 4.0|
Determining Effective Clo
Total clo is estimated by adding the clo values of the components and correcting for surface area‚ fit‚ and air layer effects (Ref: American Society of Heating‚ Refrigeration and Air-conditioning Engineers).
Sureflote Flotation Lining and Clo
Sureflote flotation lining can add clo value of about 2.0 to the garment. The number of effective Sureflote layers in a garment may vary according to style and location. For example, additional Sureflote is typically used in the knees. The additive clo value is determined based on average flotation in the body of the jacket.
Striker Brands Technical Apparel - Why is it Superior?
One simple way to compare outerwear garments is to look at grams of thermal insulation in the garment. Of course, this comparison only tells a small part of the story and can be misleading or practically meaningless without considering clo value, moisture control, shell efficiency, venting, and layering. Simply stated, Striker Brands outerwear with Sureflote lining far surpasses effective insulation (clo value) of its competition. As impressive as this is, there is even more good news! On top of that, these additional important performance features are found in ALL Striker outerwear apparel and contribute to the premium technology level inherent in Striker products:
- Hydrapore™ Waterproof/Breathable shell systems (minimum 5,000mm/5,000gr/m2).
- Thermadex™ Insulation corresponding to improved warmth due to loft and fiber size.
- Sureflote™ flotation lining providing full windproof and additional insulation (+clo=2.0).
- All outerwear garment seams are specially taped for maximum water resistance.
- All outerwear garments have zippered vents for ventilation, even through the float lining.
- Mesh hems in cuffs and bottoms of garments release trapped moisture between layers.
It is these premium features that truly set apart Striker products from the competition. Striker Brands has patents pending on various technical features of its garments.