In response to James Penmans great question, on our Ask The Expert section:
Like most things in outdoor these days, the insulation or fleece layer is split between organic and synthetic products.
By far the most popular is synthetic fleece, which makes up a good 80% of the product used on the hill in the UK and further afield. Whilst most of the major brands have their own in house fleece mini brand (TKA from The North Face, for example), the market leader is Polartec. The main Polartec offering ranges in weight from 100 to 300, with the larger number being a thicker, more insulative garment. A Polartec 300 weight fleece will keep you nice and snug in all but the coldest of condtions. A great example of this is the Berghaus PolarPlus fleece.
Other synthetic insulation layers can include heavier duty materials like Primaloft, which is also used in syntetic sleeping bags. Garments like The North Face Redpoint Jacket or the Mountain Equipment Fitzroy Jacket are great examples of this. They can be used over a base layer in mild to cold conditions or under a shell jacket when it gets really cold. One thing to look out for here, is the hood. Many of the better synthetic fill jackets have an insulated hood, but you need to ensure it will work with the hood on your shell jacket.
Finally, more Organic offerings using Merino wool are available. Icebreaker (a Webtogs favourite) make a 320 range of Merino Wool mid layers, the Rock Zip being a good example. Merino wool garments have a better warmth to weight ratio, so a 300 weight synthetic fleece will be thicker than an equivalent Merino wool garment offering the same insulative value.
On more of a technical side note, when it’s really cold, nothing beats down based products for cold (but dry) insulative value. Down’s primary method of keeping you warm is the loft effect, where the down seperates and traps air, which in turn provides excellent insulation. It’s for this reason that down doesn’t actually work that well as a mid layer, as any layer on top will prevent the loft and hence hamper the insulative effect.
So, short answer, the warmest mid layer out there will be one of the synthetic fill jackets, closely followed by a good 300 weight fleece. When buying a fleece it is worth paying the extra for top fabrics like Polartec, they will last longer and are much more resistant to pilling, which is where the fleece bubbles into small knots, harming it’s insulative effect.
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