In a featured embodiment, a gas turbine engine has a first compressor rotor driven by a first turbine rotor, and a second compressor rotor driven by a second turbine rotor. The second compressor rotor is upstream of the first compressor rotor and the first turbine rotor is upstream of the second turbine rotor. An air mixing system taps air from a location upstream of the first compressor rotor for delivery to an environmental control system. The air mixing system receives air from a first air source and a second air source. The first air source includes air at a first pressure upstream of the first compressor rotor. The second air source includes air at a lower second pressure. At least one valve controls a mixture of air from the first and second sources to achieve a predetermined pressure for the environmental control system.