Does a radiant barrier needed air on both sides or just one?

A double sided radiant barrier does not need an airspace on both sides of it, just one. You can have an airspace on both sides, but it doesn’t really change the effectiveness of the foil, just the options for airflow/ventilation.
 
For the single sided radiant barriers, the foil side must face an air gap. The other (non-foil) side may also face air (in the case of the white vinyl product) or it can go right up against another material like insulation or OSB.
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