The best base layers are designed for the solemn business of road cycling, but don’t let that put you off because the roadies know their stuff when it comes to layering. Base layers are much more of a staple for the road than MTB and something most serious riders don every time they go out. Road apparel brands also have bigger ranges, do more R&D on them and ultimately pack more technology into their line ups.

The Castelli mesh base layer might look a bit Rab C in the car park, but under your riding jersey nothing works better

Design and specifications

This Castelli mesh Tee might have the most ridiculous look of any base layer I’ve tested, but don’t be put off by the fruity styling. This top is Castelli’s lightest and airiest base layer, designed to wick the maximum moisture away from your skin on hotter days.

The open mesh design uses a relatively thick fabric and this helps hold the jersey up and away from the skin and separate the layers. This in turn allows for better airflow for cooling and drying on hot days, or you can use it with a tighter jersey on cold rides to trap air and provide a layer of insulation.

The mesh lofts outer layers of clothing away from your body, helping you stay cool

The big openings in the Core Mesh 3 weave allow air to move freely over the skin so this top is especially good for warm conditions, and with a more permeable outer riding jersey on there’s an excellent cooling effect on the skin.

This version 3 model is upgraded to a more open neck for better cooling and uses a whole new stretchy ‘Magic Mesh’ that hugs the body snugly and uses elastane for good freedom of movement and flexibility. The overall construction is minimal and lightweight and uses reduced low-profile seaming for maximum comfort.

The original Brynje base layer is still one of the best bits of kit around

Performance

The first time I was exposed to the ‘string vest’ base layer was when bike test editor Alan Muldoon rocked up to ride in one about 20-years-ago, at Afan trail centre. I’m not sure where he’d got the idea, and it definitely looked as bad as when I wear one, but he was evangelical about a Brynje of Norway string vest worn by the Norwegian army to keep soldiers warm and dry on operations. The string vest concept was presumably later adopted by road cyclists who wanted to stay dry and cool doing long, high intensity efforts.

Flat seams and facings to the sleeves, neckline and bottom mean there is no discomfort from chaffing

Like with (too) many things, Muldoon was dead right, and these bizarre aerated garments do seem to make the perfect first layer for biking in. With steep climbs and descents, mountain biking takes you through multiple temperature ranges in every ride and, depending on the terrain and your physical effort, it can be a really sweaty business.

The very open weave here pumps air onto the skin and keeps your jersey from clinging and sticking once you’re hot and sweaty, while the polyester mesh material dries exceptionally quickly to keep it working effectively. The key to the mesh base layer system seems to be the size of the gaps in the mesh and also the use of fabric thick enough to loft clothing off skin. Castelli has nailed both aspects here and also uses a fabric that doesn’t absorb moisture.

Castelli has opened out the neck for this V3 base layer

Because the Core Mesh 3 top doesn’t absorb your sweat in a way that makes it sodden, there’s always an insulating layer of dry air on the skin. This means it’s kind of magically self-regulating for your temperature and keeps you cosy if it’s under a tighter layer like a long sleeve thermal or fleece, or keeps you really cool if it’s under a more perforated or airy riding jersey.

Castelli’s top works so well, I can manage with a thinner outer layer in colder weather

Castelli’s top works so well, I can manage with a thinner outer layer in colder weather and leave off more restrictive, thicker jackets or waterproofs in winter. You can also cover it with something like a thin merino jersey or tea bag style top on the hottest days and let the air flow onto your skin for excellent cooling. This dual-purpose effect also means I don’t need to constantly add or remove layers to manage temperatures while riding.

The 10% elastane in the fabric makes it really flexible and easy to move about in with no pinch points under arms or across the back, although the coarser material does make it a tiny bit scratchy against skin compared to something like a cushy merino base layer. The top is long enough to tuck into shorts too so you don’t ever end up flashing too much mesh if your jersey rides up.

Road riders are slim beasts, so you might want to upsize if you want to actually fit in this base layer

Castelli only rates this Tee (it also offers a vest version) for 20-30° on its website, but I find it much more versatile than that and basically use it all the time. The only possible drawback here is that even though the fabric isn’t very permeable, it does start to smell pretty fast if you use it on multiple hot days in a row, on a trip for example. In my experience, it’s not possible to achieve this insulating/cooling effect as effectively with a flatter fabric that might use an anti-bacterial treatment or be made out of a more absorbent natural material like wool.