Tank top against t-shirt as an inner layer
Short answer: A tank top covers only the upper body and does not show at the neck. A t-shirt also covers the armpits and absorbs sweat that would otherwise hit the outer layer directly. The choice depends on what you wear over it and how long you want the outer layer to last between washes.
The tank top
The tank top does not show under an open shirt, and it adds nothing under an outer layer. It is light and thin and gives a smooth surface under the fabric. On the other hand it does not protect the armpits, and that is exactly where the outer layer gets dirtiest. See bamboo tank top for men.
The t-shirt
The t-shirt covers the armpits and absorbs sweat, so the shirt or sweatshirt on top can be worn for several days. Its sleeves sit right under the sleeves of the outer layer, and it becomes the layer that takes the moisture. It is a little warmer and can show at the neck if the neckline is not deep enough.
Armpits and sweat
The armpits are where sweat most often meets clothing. With no layer between, it goes straight into the shirt or jumper, and then the outer layer must be washed more often. A base layer with sleeves takes the sweat first and is easier to wash. That makes it the practical choice if you want to wear the outer layer for several days.
When the neck must stay clean
Under an open shirt the neckline can give the base layer away. A tank top has a deep neckline that does not show, so it looks neat. A crew neck t-shirt can be seen if the shirt is open, and then a deeper V-neck can be a good solution. See bamboo undershirt for men.
Warmth and air
A tank top covers less and is coolest, because more skin gets air. A t-shirt covers more and is a little warmer, but it also takes up more moisture. In summer the tank top can be pleasant under a light outer layer. In winter the t-shirt gives a little extra warmth without adding much bulk.
What you wear on top
If you wear a thick sweatshirt on top, the t-shirt is often a good choice, because it protects the sweatshirt from sweat. If you wear a thin, open shirt, the tank top can look neater, because it does not show. So the choice of outer layer decides what is best, not the fashion of the base layer.
For women
For women there are both tops with straps and tops with short sleeves that can be worn under blouses and jumpers. Choose a top whose neckline suits what you wear over it. See bamboo tops for women to see how a simple top can work as a base layer.
Washing and lifespan
A base layer is washed more often than the outer layer, and that is the point. It takes the sweat so the outer layer can hang to air. Wash the base layer by the care label and let the outer layer rest between wears. That way the outer layer wears more slowly, and you get more from the pieces on top.
How to choose
Tank top under an open shirt, t-shirt under everything else. If the outer layer should last several days, the t-shirt is the right choice, because it takes the sweat before it hits the shirt. Choose a size that lies close without squeezing, so the base layer does not ride up or bunch under the outer layer.
Frequently asked questions
What extends the life of the outer layer most
The t-shirt, because it absorbs the sweat in the armpits before it reaches the outer layer. Then the shirt or sweatshirt can be worn several days between washes, and you wash the base layer more often instead.
Does a tank top show at the neck
No, if the neckline is deep enough. A tank top usually has a wide, deep neckline, so it does not show under an open shirt, which is one of the reasons many choose it.
What is coolest
The tank top, because it covers less and lets more air reach the skin. The t-shirt is a little warmer, but it takes more of the sweat and protects the outer layer better.
