Type of underpad on concrete

I am finishing my basement. I have a concrete floor. I will be laying under pad on the concrete and carpet over the under pad. I was told at home depot to buy only a red coloured under pad that has white dots on it, because it acts as a vapour barrier as well as an under pad. Does anyone know what this under pad is called ?? As well, when laying the under pad, do I have to staple it to the concrete floor before laying the carpet or can I just lay it down and lay the carpet right over it ? Thanks. From Ottawa, Ontario.
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Duct tape,glue or spray glue pad seams and around edges.
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