From the manufacturer

Boys' HeatGear Armour Long Sleeve

HeatGear Armour is our original performance baselayer—the one you put on first and take off last. So we made it extra comfortable by wicking sweat and stretching so you can move no matter what.

• Fitted: Streamlined fit that’s not too tight or too loose.
• HeatGear fabric is ultra-soft & smooth for extreme comfort with very little weight
• Strategic mesh panels for added ventilation where you need it
• 4-way stretch material moves better in every direction
• Material wicks sweat & dries really fast
• Ergonomic flatlock seams for added comfort & mobility