Don't Insulate Over a Heated Floor
In-floor heating systems are great for heating a basement, but installing thick, heavy carpet and heat-resistant padding over them is a sure way to reduce their efficiency. High R-values are a good thing for walls and attics, but when you're covering in-floor heating, lower is better.
Most carpeting is made of similar materials, so the type isn't as important as the thickness—thinner is better. Carpet padding, however, is made up of different materials with different insulating properties. Waffle rubber padding is a good choice, with an R-value of 1 for a 3/8-in. pad. Urethane is a poor choice, with an R-value of 1.62 for a 3/8-in.-thick pad.