Men's Thermal Underwear Lock in warmth before venturing outdoors with men's base layers from Rainier Sportsman. Shop lightweight base layer tops and bottoms, midweight base layer tops and bottoms, heavyweight base layer tops and bottoms, scent-control base layers, silk-weight base layers and performance underwear.Rainier Sportsman's supplies men's layer brands such as Rainier Sportsman, Sitka, True Timber, Under Armour and many others to trap vital warmth for all-day wear.