In the Insider, iconic Italian brand Castelli has an indoor-specific top that’s the ideal turbo training companion, says cycling enthusiast Charlie Allenby…

Castelli Insider Jersey

$124.99 / £90,

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  • Superior sweat-wicking
  • Great odour management
  • Extremely breathable


  • Mesh design is quite revealing
  • Two rear pockets limit outside potential

Fit: 4.5/5
Comfort: 4.5/5
Breathability: 5/5
Overall: 4.5/5

Over the past 10 years, indoor training has gone through something of an evolution. The days of sweaty static bike sessions in the spare bedroom have been replaced with an accessible, enjoyable and interactive form of exercise thanks to the rise of the smart turbo trainer and apps such as Zwift. You might still wear the same cycling shoes but jerseys can benefit from being a bit more indoor-specific.

This growth of the indoor training market hasn’t been restricted to training tech. Clothing manufacturers have been keen to cash in on the trend with indoor-specific kit too. Castelli’s Insider jersey is a prime example.

Designed to be lightweight and breathable above all else, Castelli claims that it should be able to survive ‘the rigours of indoor training’. They can often be seriously hot affairs – particularly when undertaking an interval session and your fan isn’t very powerful. But how did it fair in the heat of the moment? And is there any noticeable difference compared to any other lightweight, non-indoor-specific jersey? I donned an Insider and got to work in my pain cave to find out.

Castelli Insider performance

The hot and humid environment of a high-intensity turbo trainer interval workout is the ideal testing ground for an indoor cycling jersey. It doesn’t usually take me long before I’m unzipping a top completely to get maximum airflow and dabbing sweat from my brow with a towel.

While I was still left reaching for a towel, I was blown away by the performance of the Castelli Insider jersey. At no point did I feel the need to unzip the jersey, and other than slight damp patches at the armpits, it wicked sweat from other hotspots and stayed remarkably dry after an hour of indoor riding. In fact, it was so breathable, at the start of each session I had to wear an extra layer over the top to keep me warm before I’d come up to temperature.

Product shot showing rear of a Castelli cycling jersey
Two pockets on the Insider might not be quite enough for outdoor riding

I was testing the jersey during the autumn and winter in the UK so I didn’t have a chance to put its warm weather credentials to the test. But I don’t doubt that it would be a great companion on the hottest summer rides.

The only negative was the two pockets. When training indoors, I tend to have everything I need in front of me on a desk, so the inclusion of storage in an indoor-focused jersey seemed superfluous. They’d be useful on those hot summer rides, but in those cases I’d probably want three pockets.

Castelli Insider fit

To successfully wick sweat away at source, the jersey has a skin-tight fit, but at no point did I find it restrictive – there was enough stretch in the material that it fit to me rather than the other way around.

This close fit does have a side effect though when combined with the jersey’s lightweight mesh material. While Castelli claims that it shouldn’t become too transparent, I found that it didn’t leave much to the imagination in certain settings and might be too revealing for some – particularly if used outside. Also, the material doesn’t offer any UPF protection – fine for indoor cycling but something to be aware of if wearing outdoors.

Castelli Insider value

Cycling isn’t a cheap hobby and the idea that you need to invest in even more kit when you’ve already got perfectly good short-sleeve summer jerseys might be a step too far. But at $124.99 / £90, the Insider is a relatively affordable Castelli jersey and a similar price to the likes of Rapha’s Core and Maap’s Training jerseys – both entry-level models. Plus when you factor in its performance, if you train indoors a lot, then £90 starts to sound quite reasonable.

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