These tees from #Gru. are specially made to generate more heat than a regular cotton tee shirt.

It’s moisture wicking properties keeps sweat off your tee and prevents moist patches on your tee even during your most intense workouts.

Some other properties are :
Quick drying
More flexible 
Wrinkle and Shrink resistant.

And ... it’s made from 100% recycled polyester ( also known as rPET) which helps reduce plastic landfill and is made from recycling existing plastics.

So you prevent the landfill of about 5 soft drink PET bottles when you buy 1 Gru. XL tee shirt.

It also consumes about 69% LESS energy to produce and eventually resulting in a reduced 32% CO2 emissions than a regular virgin fabric made tee.