Enjoy autumn from the saddle.

With the changing leaves and air as crisp as a new-crop apple, fall is one of the best times to ride a bike. But it can also be chilly, especially if your schedule doesn't always allow you to wait for the warmest part of the day. To address this, we turn to Castelli's Flanders Warm Half-Zip Base Layer. While we're not sure if the name means that it's as warm as Flanders (which… what?) or if it means that it's warm enough to wear while cycling in Flanders (more likely), we do know that it fends off cold like Museeuw fended off late-stage attacks in the race of the same name.

The Flanders Half-Zip is comparable to its non-zip sibling in every respect except for the zipper. It's made with the same durable SottoZero Fleece that insulates and wicks, and includes the same mesh inserts in the underarms. The flatlock stitching and flat hems mean the Half-Zip will fit as comfortably as the non-zip. Castelli recommends it for use in 23 to 46 degrees Fahrenheit, and, while this range may seem excessive, we find that the Flanders gets the job done in all conditions when layered with the right outer layer.

The Castelli Flanders Warm Base Layer - Men's is available in six sizes ranging from X-Small to XX-Large in the color Grey.

  • Reviews
  • Q & A

The Half-Zipper Seals the Deal

  • Familiarity: I've put it through the wringer
  • Fit: True to size

No matter how cold it is when I start out, I always am too warm after about 10 minutes of riding. This long-sleeve base layer features a zipper that really helps solve this problem. With my other base layers, I unzip the outer layer yet still sweat due to the warmth. With this puppy, you just unzip the top like a jersey and you are cool as a cucumber once again. This is my favorite base layer out of the four that I own. If you need help cooling when it is cold out, pick this base-layer up.

Warmth Without Weight

  • Familiarity: I've put it through the wringer
  • Fit: True to size
  • Size Bought: Medium

Lightweight and thin, soft against the skin and warm. Works well under a jersey between 52 and 62 degrees along with arm warmers. Below 52°, add a base layer; above 62°, too warm. IMPORTANT: Do not tumble dry. The fleece is effectively collected by your dryer's lint filter and the fabric becomes too thin to retain heat.


    Very comfortable and cozy. It feels good on your skin. 4 stars only because I had to return it for a larger size. I'm usually an extra large and returned for an XXL.

    Definitely Upsize, Quite thin

    • Familiarity: I've used it once or twice and have initial impressions
    • Fit: Runs small
    • Size Bought: Large

    Listened to other reviews, and went up a size. I am 5’8” and a fit 175 lbs. Normally wear medium t-shirts, etc, but purchased the large. Still a tighter fit, might have even been able to do an XL.

    Material is surprisingly thin for the temperatures it claims to handle. I expected something more substantial. Honestly My initial impression is to be concerned about long term durability — it’s so thin I’m afraid I might tear it. Otherwise, it is soft and pleasing to wear. Stitching appears high quality. Underarm venting is a nice touch.

    I did an 8 mile ride in 35 degree weather with just this and a Gore Bike Wear Windstopper jacket and I wouldn’t ride any in any colder temps without more layers or a thicker jacket.

    Price to performance is lower than I’d like. I get similar warmth/moisture wicking performance from $20 wool blend long underwear ... though granted it isn’t near as nice to look at as this shirt is.

    Multipurpose Awesomeness!

    • Familiarity: I've used it once or twice and have initial impressions
    • Fit: Runs small
    • Size Bought: XL

    Form fitting, flexible and extremely comfortable. It is surprising how warm this material keeps the body given that it is so thin.

    I used it while hiking in the mountains of Colorado and felt very comfortable wearing it. I'm sure it will serve me well on the bike come winter.

    Had to upsize to XL and it fits my semi-athletic body quite well. It is very soft to the touch, reminiscent of fine Merino Wool.

    Quality Base Layer

    • Familiarity: I've used it several times
    • Fit: True to size

    This is by far the best base layer I have used. Its very warm and the vented armpits make it very breathable. A perfect combo.

    Great Base Layer/ Holds up great

    • Familiarity: I've put it through the wringer
    • Fit: Runs large
    • Size Bought: Lg

    This base layer def runs large which is rare for Castelli. I got this as a gift and requested an XL(my usual Castelli top size) and mistakenly was given the LG. It was a good mistake because it is perfect. It is strangely form fit but loose at the same time. What I mean is there is no compression at all but at the same time there is absolutely no extra fabric anywhere. Fits just like a base layer should so it can do its job. You wont notice it or think about it while riding. The fabric is very thin but warm with a fleecy material on the inside. If you are looking for a heavy feeling thick type base layer this is not the one you want, although it will probably do just as good a job if not better, with less weight. The half zip is a nice feature on this when putting on and taking off. The fabric under the arms are a thinner mesh material which is also a nice feature that I am sure helps this layer do its job. G

    Nice material. Don't like the zipper.

    • Familiarity: I returned this product before using it
    • Fit: Runs small
    • Size Bought: XXL

    I ordered this base layer because it sounds like what I need for deep winter riding under my Giordana FormaRed Carbon jacket (which I love). I thought about the same base layer without a zipper but CC was out of it at the time, so I ordered this half zipper. The material feels great and "extra fluffy" while being thin which is what I am looking for but the zipper pokes at my neck which is a admittedly a 17 inch neck. I tried to unzip it a bit to make that go away and could tell it was not going to make me happy. Others with skinnier necks may feel completely different about it. I am returning this one and getting the non-zip one now that it is in. That is the only reason for two stars.

    Keep the ride toasty

    • Familiarity: I've used it several times

    This has warmth without the bulk, so even if you need to add a 2nd base layer your good depending on the temps! The half zip keeps the neck warm too along with a Castelli winter jacket and your set! The XL size fits me fine.I am 6'.0" 165lbs

    soft and warm base layer

    • Familiarity: I've used it once or twice and have initial impressions

    This is a great base layer for colder weather. I've layered it with a poly long sleeve jersey for 50* and thought it was a bit too warm. Which is a good thing. Lets me wear it in lower temps and be ok or as a base layer with my winter jacket. It is also super soft against the skin! Sizing was great - XL for me, as I'm 6'1", 195lbs.

    Castelli's inconsistent sizing

    • Familiarity: I've used it several times
    • Fit: True to size

    Castelli makes some of the best cycling attire in the market. Almost all my cycling gears are Castelli. However, this Flanders Warm Half-Zip Base Layer (long sleeve) exposes yet again my frustration with Castelli's inconsistent sizing. Last year, i bought two XL Castelli Flanders Long Sleeve Base Layer (long sleeve). They were way too small so they had to be exchanged with the largest size available: XXL. I am 6'2" and weigh about 190 lbs. The XXL Flanders Long Sleeve Base Layer was a little too short on the sleeve and i could barely put my head through. It was with this understanding that i ordered an XXL Flanders Warm Half-Zip Base Layer (Long sleeve). Turned out, this is one size too big for me. An XL would've fitted me much better.

    What i like about this base layer is the ability to unzip the top half of it if you are feeling a little too warm. The zip also helps when you are putting it on or taking it off.