Layering Systems & Underwear
Stay warm and dry with thermal underwear and base layers built on the military's layering approach, from thin moisture-wicking tops to heavyweight thermal knits. This section covers the Rothco pieces that sit closest to your skin, the foundation of any cold-weather system.
The ECWCS (Extended Cold Weather Clothing System) group includes the Gen III Silk Weight Underwear Top and Bottoms, the ECWCS Gen III Mid-Weight Underwear Top and Bottoms (Level II), a Level II crew top, Level II breakaway bottoms, ECWCS poly crew neck tops, zip collar shirts and poly bottoms. These synthetic layers pull sweat off your skin so you don't chill when you stop moving.
For classic warmth around camp, the house or a job site, choose from:
- Thermal knit tops and bottoms in midweight, heavyweight and extra heavyweight
- The Rothco Union Suit, a one-piece full-body layer
- Single layer poly underwear tops
- Moisture wicking performance boxer shorts in regular and long length
The section also carries Rothco's "Booty Camp" booty shorts and tank top in olive drab, woodland, ACU digital, black camo and pink camo, a fun option for lounging and gym days.
A note for service members: DA PAM 670-1, paragraph 4-7, lets Soldiers wear silk weight (cold weather) moisture-wicking undershirts under the combat uniform coat in place of the tan 499 undershirt. ECWCS outer garments are a separate matter and need commander approval. Check color and unit policy before you buy; details are below.
How to Choose Thermal Underwear and Base Layers
The right base layer depends on how hard you'll work and how long you'll sit still. Sweat is the enemy in the cold: a damp layer against your skin pulls heat away the moment you stop.
Silk Weight vs. Mid-Weight ECWCS Layers
Silk weight is the thinnest option. Wear it on cool days, under a uniform or work shirt, or as the first layer on a hard hike or ruck where you'll generate heat. The Level II mid-weight top and bottoms add warmth for colder, slower days such as hunting stands, range details and guard shifts. Many people own both and swap based on the forecast. The breakaway bottoms are worth a look if you want to take a layer off without pulling off boots and pants.
Thermal Knit Underwear Weights
Waffle-style thermal knits trap warm air in their texture. Midweight works for everyday winter wear, heavyweight for long outdoor days, and the extra heavyweight bottoms for real cold where you'll be fairly still. The Union Suit puts top and bottom in one piece, so nothing rides up or leaves a cold gap at the waist.
Everyday Underwear
The moisture wicking performance boxer shorts suit long days in boots and uniforms. The long length version gives more leg coverage to cut chafing on rucks and runs.
Thermal Underwear Sizing and Layering Tips
Base layers should fit close without squeezing. Too loose, and they can't move sweat; too tight, and they restrict blood flow and movement. Size your outer layers to fit over your thermal underwear, not the other way around. Wear the layer that wicks closest to your skin, add insulation over it, and finish with a shell such as a jacket from our military jackets, which includes the Gen III Level 3 ECWCS fleece jacket.
Thermal Underwear and Military Uniform Regulations
AR 670-1 lists the undershirt and undergarments as parts of the combat uniform, and DA PAM 670-1 has specific rules for each.
Undershirts Under the Combat Uniform
DA PAM 670-1, paragraph 20-32, requires the tan 499 undershirt with utility uniforms. Paragraph 4-7 then authorizes silk weight (cold weather) moisture-wicking undershirts under the combat uniform coat in place of the tan 499 or foliage green undershirt. That paragraph doesn't name a color, so check what your unit expects before you buy.
ECWCS as Outer Layers
Paragraph 4-7 also says ECWCS jackets and trousers, including the layer III fleece jacket, may be worn as outer garments only when the commander authorizes it. Under the uniform is one thing; on top of it is the commander's call.
Drawers, Bras and Other Undergarments
Under paragraph 20-32, male Soldiers may wear commercial brief or boxer drawers in white, brown, tan, sand or other neutral colors, and female Soldiers wear commercial brassieres (sports bras count) and underpants in white, black or neutral colors. Neutral-colored performance boxers fit that rule; the camo booty shorts are for off-duty wear. Commercial items must still meet the standard, and unit policies can be stricter than the regulation.