Baselayer Advice for Walking & Hiking
Fabric
Cotton can leave you cold and clammy and should generally be avoided. Polyester wicks moisture well but is prone to getting smelly unless it has an anti-microbial solution. Polypropylene is well known for its fast-wicking properties, but needs to be close-fitting to work well. Merino wool can be super-soft and naturally anti-microbial, but it's generally more expensive. Hybrid fabrics are a mix of wool and synthetic materials, which aim to give the best of both worlds: soft, odour-resistant, fast-wicking and quick-drying. Take your pick!
Style
Zip necks, crew necks, short or long sleeved - the best choice is down to individual choice. Most will be long-sleeved zip necks as they allow some flexibility for ventilation, warmth and sun protection. Dark colours will absorb heat from the sun, while white or light colours will help reflect heat but may be more transparent particularly when wet. Some brands do advanced technical fabrics called ColdBlackTM which whilst dark, are designed to remain cool in sunlight as they don't absorb the heat.
Fit
Again this is down to personal preference and there really is only one way to know for sure and that's to try it on. If it feel restrictive and impedes your freedom of movement, it's probably too small. Check that the collar, cuffs and wrist areas aren't too tight or too baggy either. Also, make sure the top is long enough and doesn't ride up when you bend forward or raise your arms, exposing your lower back and waist. Most base layers are cut with dynamic movement in mind to prevent this from happening and some also have silicone inner bands that help your base to stay in place.
Rubbing
Check that the garment won't irritate your skin this is critical when selecting your walking base. Some fabrics are softer than others, whilst some labels and seams can cause irritation. If you're buying a top with a zipped neck, check that it's comfortable - in cold weather a metal zip can feel icy cold against your skin.
Most zipped tops come with storm flaps or chin guard / zip housing, which will add protection against draughts and prevent zips being next to the skin when fully closed. Flat-locked seams are less bulky and less likely to rub, while off-set shoulder seams are designed to avoid sitting under rucksack straps.
Look out for scratchy labels, too; heat transfers that print information directly onto the garment side-step this problem. Most manufacturers use these in the back of the neck but then add an additional label in the side seam. Beware! Sadly it's a legal requirement now to show every minute detail of production etc in every language.....many will have a dotted line to cut along so just cut them out with care, it won't impact your consumer rights if the product fails because it is instructing you to cut them......just don't snip through the fabric or stitching, you are unlikely to get your money back if you do.