This paper informs on the flashover strength of three materials: Delrin (polyoxymethylene), HDPE (high-density polyethylene) and Ultem (polyetherimide) with smooth surface finishes, in zero-grade air at -0.5, 0 and 0.5 bar gauge, and at <; 10%, ~50% and >90% relative humidity (RH). Both negative and positive polarity impulse voltages were applied to investigate the potentially asymmetrical electrical performance of the geometrically-symmetrical electrode arrangement. In all tests, high voltage (HV) impulses with a nominal 100/700 ns wave-shape were applied. Each test conformed with the ASTM D3426-97 standard of `step up' testing, to find the average flashover voltage for each set of conditions. For negative polarity, each solid dielectric material demonstrated a decrease in flashover voltage as the RH was increased. For positive polarity, however, the flashover voltages were similar for all levels of RH, with the exemption of HDPE. A decrease in flashover voltage was found as the permittivity of the material increased for negative polarity, irrespective of humidity and pressure. Overall, the highest flashover voltage recorded for negative polarity was ~200 kV with an HDPE spacer, at 0.5 bar gauge and <; 10% RH. The poorest performance was for a Delrin spacer in negative polarity, at -0.5 bar gauge and >90% RH, at a value of ~53 kV. For positive polarity, the highest flashover voltage was for a Delrin spacer at ~50% RH and 0.5 bar gauge, with a voltage of ~180 kV; the lowest flashover voltage of ~60 kV was recorded with an HDPE spacer, at -0.5 bar gauge and >90% RH.