Mid Layers
The mid layer is anything between the base layer and the outer layer and is the most complex layer, with different fabrics and materials to consider. You should be thinking about volume, weight, warmth and breathability. Let’s look at how each of these attributes work in the different materials.
Fleece, the miracle fabric of the 80s, was the new kid on the block in the decade of disposable extravagance and clothing manufacturers went nuts for it. But fleece is made of polyethylene terephthalate or PET, so buying one is the equivalent of buying 90 plastic bottles. Not the most environmentally friendly choice, (unless you’re going for recycled PET) but it is light, warm, soft, hydrophobic (repels water) and machine washable. The down sides are that it has a tendency over time to pile, so the fibres bunch, which will diminish its insulating capability and because it’s hydrophobic it’s difficult to wash, so, like the polyester baselayer or an old bath, it will build up a unique ‘scent’ over time.
Insulation puffy jackets are the padded down or synthetic neon jackets that are usually sported by freshly retired couples who want a sporty active look as they do the shopping. That is to malign their effectiveness though. They are incredibly light for the amount of warmth they provide, perfect for colder days, and they are low volume so pack down well. They are a great choice if you are expecting big temperature fluctuations or planning a high energy output day. Their kryptonite, however, is moisture. If it rains or snows on them or you sweat heavily in them, they will just absorb and hold the moisture and then not only is their warmth reduced, but they also become heavy. So, the key to using insulators is having a good shell and pre-empting when you are going to sweat so you can take them off and them and stash them away in your pack.
Merino is just as good as a mid layer as it is for base-layers. All of the same attributes apply; it is soft, warm, light, and very breathable. Like fleece and down, it comes in different weights so you can choose a thickness that will work for your thermostat, but unlike the other two the natural temperature regulation means you won’t have add and remove layers as often. One new innovation is the use of merino as a down-style fill, since it is light and warm and offers a more ethical alternative to duck or goose down.