Castelli Diagonal Women’s Jersey FZ

Castelli's Diagonal Women's Jersey FZ is brilliant for dry, cool-to-cold rides. It does a great job of keeping you warm without excess bulk, and works well with layers to increase its weather-window of performance. Riders committed to performance-focused training will appreciate the levels of breathability this thing offers.

With some practical layering, a thermal jersey like this can be used for virtually every single training ride from autumn through to spring. The FZ is a snug fit, if not quite the race-fit of something like Sportful's Thermal Jersey, and the fabric is really not bulky either.

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Consequently, while it performs perfectly well on its own, it can easily take a base layer (or two) beneath or a jacket, gilet or a waterproof on top.

I followed Castelli's sizing chart to test a medium (that's not me modelling it). It's a good fit all over, with a decent sleeve and body length and no tightness around the chest or shoulders. If you're between sizes, I'd say sizing up or down is probably pure preference - whether your want a bit of breathing room, or a very snug fit.

The collar is a good height and sits close to the neck; I can't get a neck warmer on and zip it up without feeling choked. It's not fleece lined here, but the fabric is soft enough (it's the same as the rest of the jersey), if a little less cosy. There's no zip guard either, though the doubled-over material running the length of the zip ensures your neck is not irritated.

The cut at the rear is generous, with an added strip of fabric to protect the lower back.

In a riding position, this jersey feels really comfy – there's no pulling or tightness anywhere. It moves well with the body, in and out of the saddle. The silicone strip at the rear hem helps prevent the jersey shifting under the weight of loaded pockets.

Performance

On its own, the FZ performs well in temperatures between 12 and 15 degrees. The polyester has a brushed, fleecy inner layer, so it's soft enough against bare skin too. Castelli suggests it can be used up to 18 degrees; obviously, I haven't managed to test this in the last four weeks, though I really wouldn't want to be exerting myself in it above 15 degrees. I started adding layers below 12 degrees.

With two decent layers, this is actually a decent bit of kit in temperatures as low as three degrees, as it offers exceptional breathability. Head out for some intense riding in colder temperatures and it really does shine. I found absolutely no moisture build up inside the jersey in such scenarios. Naturally, this is also a little dependent on the choice of base layer.

The FZ doesn't handle rain, but stick on a good waterproof and it continues to perform in terms of breathability. On the occasions I did get caught out (without a waterproof), there wasn't an immediate chill. You certainly wouldn't want to be caught in a prolonged cold shower though; there's very little in the way of windproofing.

Practicalities

I'd say Castelli has got the storage volume and positioning right on this jersey; the three rear pockets are generous and easily accessible. It's worth noting there's no zipped pocket, though.

There's only a small amount of reflective piping at the rear, too - not really enough to have an impact. The colour I tested scores highly on the visibility stakes however, although in foul weather I found myself having to use stain remover to get it sparkling again.

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Still, I'd rather this than ride in dark kit that can render you invisible on a grim day. If you do want to be a little more subtle than this (or the light blue Celeste), Military Green or Light Black may appeal.

Value

Of the three thermal jerseys I've tested so far this autumn/winter, Castelli's offers – by a good margin – the best fit and performance. No surprises it's the most expensive, though. It's a tenner more than Sportful's Bodyfit Pro Women's Thermal Jersey and a whopping £50 more than Altura's Women's Nightvision. I keep harping back to Liv's Race Day Thermal Jersey which impressed me with its performance, even if the cut was a little niche; the latest version is now £74.99.

The Castelli may seem rather a hefty investment, then, but you can still splash out more on the likes of Rapha's Women's Classic or Assos' UMA GT at £125 each.

Overall

I've been impressed with the breathability and versatility of the Diagonal Thermal Jersey FZ, and its great fit is the icing on the cake. It's a significant investment but, with supporting kit, it will serve you extremely well for at least three of our four seasons.

Verdict

Close fitting fleecy jersey with outstanding breathability – it's expensive but worth it