My daughter wears this under her jersey for outdoor skating and she's never once complained about being cold. The double layer really does seem to trap the warmth, and it's thin enough that it doesn't bunch up under her gear.
Ruby V.★★★★★
Got this for my son's early morning hockey practices and he actually keeps it on the whole time now. The Acclimate Dry fibers seem to pull sweat away instead of leaving him clammy, and the two layers feel thicker than his other crews without being bulky.
Isabella A.★★★★☆
Bought this for early morning soccer practices and the double layer really does seem to hold warmth without bulk. My son says the sleeves don't ride up when he's running, which was my main worry. Only gripe is it runs a bit snug, so size up.