Insulation and Thickness

It is intuitive to think that thickness equals insulation, but that isn't quite the whole story. Lets look at the four types of heat loss:

1. Different materials have different rates of conduction even if their thickness is the same. For example air may conduct 1/10th as well as nylon (materials' conductivities).
2. Convection currents effect insulation. Take for example double glazing. Once the air gap becomes too big convection currents inside make it less effective. You may also have experienced the lack of insulation that a thick conventional fleece offers in windy conditions.
3. Heat radiation is all about the surface emissivity of a material and isn't related to thickness but instead is based on its chemistry.
4. Limiting evaporation (with low breathability) slows the loss of latent heat.

In conventional garments the materials are the same or very similar, so conduction and radiation provide little differentiation between types (unless we are talking about Blizzard Survival Equipment, foam or phase change fabrics).

However, convection and evaporation are greatly effected by a fabrics design and garment features. For example:

• Thick fabrics with little wind resistance can be much cooler than thinner windproof fabrics.
• Thin fabrics with poor breathability may cause more overheating than thicker fabrics with better breathability.
• Vents can increase breathability and convection inside a garment, reducing overheating and the effects of thick, windproof insulation.
• Loose fitting garments (or sleeping bags) may allow more convection inside and feel cooler.

Some of these factors may be why I often feel more comfortable in FurTech garments than conventional waterproofs, even in the summer months, when intuition would claim the opposite. It would be interesting to get some experimental data on this phenomenon to quantify my experience.

Comments

[Andy]

Resistance to radiant heat loss is increased by more of a material. This may mean that a thick material is better and retains some performance even when compressed.