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I want to know a bit more about this material, it keeps coming up, is it as durable as sandwiched pertex insulation or are there sacrifices on durability for weight saving?
It's a lose polyester knit, brushed & cropped for as much loft-to-weight possible, coupled with a degree of stretch - either two or four-way, depending on application. Some are designed to be layered with either one, or two fabrics (usually woven), others can be used on their own, 'direct'.
Popularised by Polartec as Alpha, most mills have versions out there now. Primaloft call theirs Active, confusingly the same name as one of their non-woven synthetic insulation categories. Weight varies from ultralight ~75gsm, to extremely warm, chunky 200gsm - for comparison Thermoroubaix (typical stretch fleece used on winter tights and winter jerseys) is 250gsm.
The open, loose construction means it has exceptionally high air-permeability (CFM = cubic feet per minute) over a regular brush-backed fleece. It excels where environment and activity varies; if you are generating loads of heat/high-exertion, the fabric can dump heat/sweat through air-movement, and you can keep wearing the garment. When the temperature/activity drops, adding a windproof layer - or zipping up if garment has a windproof front - means all that previously venting space in the knit traps warmth and insulates. The lightweight, stretch, and broad range of temperature use has elevated it over a traditional mid-layer fleece for performance use.
Drawbacks are durability (elbows, cuffs), fibre shedding (I will be installing a washing machine filter), and fabric integrity for features (but then again, what 100gsm fabric would you put hand pockets on?).
I want to know a bit more about this material, it keeps coming up, is it as durable as sandwiched pertex insulation or are there sacrifices on durability for weight saving?