How to Wash Thermal Clothing: The Method to Keep the Heat

Here in El Bierzo, when the cold bites, thermal clothing is our second skin. Whether for going to work in winter or for a route through the Ancares, we all have that t-shirt or tights that save our lives. But, as a washing machine technician, I see a constant mistake: people wash these technical garments as if they were cheap cotton t-shirts.

Result: in three washes they don't heat the same, smell weird, or are full of pilling balls. If you've spent good money on technical clothing, you need to know how to wash thermal clothing correctly so it lasts for years with its properties intact. Let's see how it's done, differentiating between the two main types: synthetic and merino wool.

Golden Rule: Zero Fabric Softener

First of all, burn this into your mind: fabric softener is the number one enemy of thermal clothing. It doesn't matter if it's synthetic or wool.

Thermal clothing works because its fibers are hollow or have a special structure that traps warm air and expels sweat. Softener fills those gaps with an invisible wax, plugging the fiber. The effect? The garment stops breathing (you get soaked in sweat) and stops heating. Also, that layer traps bacteria, generating bad odors.

How to Wash Synthetic Thermal Clothing (Polyester)

It is the most common (like Uniqlo Heattech or sportswear). It is resistant, but has its tricks.

Step by Step in Washing Machine:

1. Turn them inside out: Always. Friction with other garments creates pilling on the outer surface. Washing inside out protects the visible face.
2. Cold or Warm Water: Maximum 30ºC or 40ºC. Excessive heat "relaxes" the elastic fibers (elastane) and the garment loses its fit, ceasing to be tight against the skin (which is what keeps you warm).
3. Synthetics or Sports Program: Medium spin (800 rpm). If you spin at 1,200 rpm, you will stretch the fibers unnecessarily.
4. Liquid Detergent: Better than powder, as it dissolves better at low temperatures and leaves no residue between the fibers.

How to Wash Merino Wool Thermal Clothing

Merino wool is the crown jewel: it warms even when wet and doesn't pick up odors. But it is delicate.

• Frequency: Do not wash it after every use. Wool is antibacterial and self-cleaning. Air out the garment and wash it only when it is really dirty.
• Wool Program: Always use the wool or hand wash cycle of your washing machine, totally cold.
• Specific Soap: Use a detergent for wool or delicate garments with neutral pH. Normal detergents have enzymes that "eat" wool proteins and end up making holes.
• Drying: NEVER hang it wet, it will deform due to the weight of the water. Dry it flat on a towel.

The Problem of Bad Odor in Thermals

Synthetic thermals are famous for picking up a sweat smell that doesn't go away when washed. If this happens to you, do not use more soap or temperature. Do a pre-wash or soak with cold water and a cup of white vinegar. Vinegar kills the odor-causing bacteria without damaging the technical fiber.

Dryer? Better Not

As a technician, I advise you to avoid the dryer for these garments. Direct heat is aggressive for technical fabrics and elastane. Thermals dry very quickly in the air (indoors or outdoors in the shade). If you are in a big hurry and use a dryer, set the cold air or minimum temperature program.