Fleece is a synthetic (polyester) alternative to wool. Woven wool base layers are similar to polyester base layers (Polartec Power Dry and Polartec Power Stretch) and shearling wool is similar to fleece insulation (Polartec 100, 200, 300, Thermal Pro, and Thermal Pro High Loft). Wool has a 35% moisture regain and polyester 2% (moisture absorbed inside the fiber). Wool feels dry when up to 35% wet because the moisture is stored inside the fiber. If similar thickness fleece and wool garments are saturated, the wool garment will take up to 5x longer to dry. Primarily for this reason, Natick Labs only uses fleece for any military base layers or insulation layer. The inherent odor proof properties of wool are now similar to fleece that has had a silver salt bath during the manufacturing process (Polygiene).
Polartec manufacturers over 300 variations of fleece and so I will over simplify the different types. In the order of their warmth / weight ratio: base layers; hard faced fleece / membrane fleece; classic fleece (100, 200, 300); Thermal Pro; and Thermal Pro high loft. Again using Natick Labs recommendations, the 3 of 300 variants recommended are a woven base layer, a gridded bi-component base layer, and a Thermal Pro Hi Loft insulation layer. When very active and the temps are above 0F, Natick Labs recommends 1 or 2 base layers combined with a Level 4 windshirt or Level 5 softshell. Their Level 3 fleece is .5mm thick and 1.3 clo; it is recommended when highly active from 0 to -25F or higher temps for lower activity. Fleece is an ideal active insulation because the air permeability is very high (~240 CFM); as a result, it doesn't lower the CFM of the Level 4 windshirt (~25 CFM) or Level 5 softshell (~5 CFM). Static use is not ideal for fleece because the amount of insulation required to be thermo-neutral goes up nominally 4x when static and the warmth to weight ratio of conventional puffys is dramatically higher than fleece.
With the advent of Thermal Pro and Thermal Pro high loft, the old classification mechanism of (g/m2 areal density) is no longer viable to determine the relative warmth. For an insulation layer, it is the thickness that determines the warmth. Natick Labs determined that Thermal Pro Hi Loft with .5mm thickness and 1.3 clo was adequate for active use to -25F. Fleece available in the commercial market is typically; 100 weight = .3mm; Thermal Pro 200 = .4 mm; 200 weight = .4mm; 300 weight = .5mm; Thermal Pro High Loft = .5mm - .7mm and 1.8 clo max. Examples of .7mm Thermal Pro High Loft include the MH Monkey Man, TNF Radium, Patagonia R3, and Melanzana.
A fleece vest covers 75% of the body surface area as a fleece jacket; so, the MH Monkey Man vest = .75 *1.8 clo = 1.5 clo. This is equivalent in warmth and function to the Military level 3 at about 1/2 half the weight and volume.
Surprisingly, you can compare the relative thickness of fleece garments by placing the wrist opening of the fabric between your thumb and first finger. First feel the thickness of your reference fleece and then immediately feel the thickness of a new fleece. I was taught this trick by a US Special Forces operator after I told him I thought the thickness of the Special Forces Level III was the same as the ECWCS Level III. He told me he could feel a difference using this technique and I could also after I tried it. I then tested both garments with a lab based laser thickness measurement device. Although the difference was only .1mm, it was detectable using this finger-to-thumb subjective method.
