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 conditions. Click here to see a list of all our fleece, from lightweight summer fleece top to heavyweight winter hooded fleece jackets.
Other synthetic insulation layers can include heavier duty materials like Primaloft, which is also used in synthetic sleeping bags. Garments like The North Face Redpoint Jacket or the Mountain Equipment Fitzroy Jackets 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. SmartWool are also in the mix for merino base layer and they make some really cleaver, warm and comfortable merino products, including socks and underwear.
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 separates 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 insulating 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 integrity.
In our opinion if you were to start from the beginning then ideally you’d have a close fitting, long sleeve merino wool base layer then on top you’d put a mid to heavy weight Polartec fleece – you can choose between a 1/4 and full zip, the 1/4 will be warmer but the full zip is easier to take off or put on. The above the fleece a lightweight synthetic jacket like the Montane Fireball or a forth coming Polartec Alpha product. If it’s really cold (and dry) then add a down jacket to seal in that warmth and block out the wind.