Loose against close fitting in heat
Short answer: Loose clothing is cooler outdoors, because air can move between fabric and skin. Close fitting is better as a base layer, because the fabric can move moisture away from the skin. Choose by where in the layers the piece will sit, and you get the most comfortable result on warm days.
Why loose clothing cools
The air between fabric and skin is swapped out when you move. It works like slow ventilation, which is why a loose garment feels cooler in the sun. The fabric does not lie flat against the skin, so heat can escape, and a light breeze can slip in and carry the warmth away.
When the air does the work
The effect is biggest outdoors, where the air moves and the sun heats the outer surface. A loose garment acts like a small room where air can circulate. Indoors without a draught the difference is smaller. A loose cut is therefore a good choice for summer days spent walking or sitting on a terrace. See bamboo t-shirt dresses.
Why close fitting works as a base
The fabric has to touch the skin to move moisture away. A loose base layer touches the skin in only a few places, so sweat stays put instead of being led out to the outer surface. A close fitting base has contact almost everywhere and can pull moisture from the skin so it can evaporate.
Moisture and skin
Sweat is not the problem in itself, it feels clammy when it stays on the skin. A base layer in close contact takes up the moisture and passes it to the next layer. That is the job you ask for when you wear an undershirt under a shirt. See also bamboo clothing for women.
The combination
The best result often comes from combining a close base layer with a loose outer layer. The inner layer moves moisture away from the skin, and the outer one lets air in so the moisture can evaporate. It works best on days with both sun and movement, when you want to stay dry and cool down.
Loose in the shade
In the shade the difference between loose and close is smaller, because the sun does not heat the fabric directly. Loose is still slightly cooler, because the air moves, but the effect is not as clear as in the sun. If you are mostly indoors or in the shade, fabric weight and breathability matter more.
The base layer should not squeeze
A close base layer should touch the skin without squeezing. If it squeezes, it can feel warm and unpleasant, and you gain nothing. Choose a size where the fabric lies against the body, but you can still breathe freely and move without feeling the seams, especially on warm days.
Two loose layers
Two loose layers also work if the inner one still touches the skin. If both are too loose, the sweat has no way out, and you feel clammy. Choose a base layer that sits close enough to touch the skin in most places. If you want items with certification, see certified products.
How to choose
Think in layers and in jobs. The outer layer should give air, and the base layer should move moisture. If you wear only one layer, choose by whether you are in the sun and moving, where loose is coolest, or want skin and fabric close, where close stays dry more easily. Go with what feels good.
Frequently asked questions
What is coolest in the shade
Loose is still slightly cooler, because air can move between fabric and skin. The difference is smaller than in the sun, so in the shade fabric weight and breathability matter more.
Should the base layer fit tightly
It should touch the skin without squeezing. The fabric needs contact to move moisture away, but if it is too tight it can feel warm and unpleasant, so choose a size that lies close without pressing.
What about two loose layers
It works if the inner layer still touches the skin in most places. If both are too loose, the sweat stays put and is not led out, so choose a base layer with a slightly closer fit.
You can find the full range at Bamboo-Clothing.com.
