Chamber A at Johnson Space Center is 16.8 metres wide and 27.4 metres high and is the only "room" big enough to hold the giant successor to Hubble in conditions like those in deep space.
Engineers have completed eight years of work to design and rebuild the facility at Houston, Texas, to make it suitable for cryogenic testing.
A three-week test that lowered the temperature inside to 11 Kelvin (-262 C) showed that the systems perform better than imagined.
The JWST, which is due to launch on an Ariane 5 rocket from Europe's spaceport at Kourou, French Guiana, in 2018, is the size of a tennis court, and with a cost of $8 billion it is important to ensure it will work in its harsh space environment.
Johnson Space Center's expertise and facilities supports a variety of other commercial applications, environmental testing, and pressure systems design.
Chamber A has been used to test all space vehicles and components for major space programmes since the Apollo Moon missions but to make it cold and clean enough for the JWST with its honeycomb of 18 perfect mirrors, several modifications had to be made.
Most significantly, the entire system for supplying liquid nitrogen to the chamber interior was redesigned and re-plumbed.
The number of valves was cut from more than a hundred valves, all presenting potential reliability risks, to fewer than two dozen of a new, more efficient design.
The result will not only keep working even during a power cut or a hurricane, but uses less than half the amount of liquid nitrogen as before.
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