The invasive fungal pathogen that causes white-nose syndrome (WNS), Pseudogymnoascus destructans, has decimated bat populations in the United States, causing significant mortality since the winter of 2006–2007. Temperatures inside many bat hibernacula are ideal for fungal growth, yet data are limited on the effects of humidity on the development of the fungus. The aim of our study was to determine optimum relative humidity (RH) levels for vegetative growth and sporulation of P. destructans growing at optimal temperatures. We cultivated P. destructans isolate MYA-4855 at 13 °C in individual humidity chambers where RH was maintained between 70.5 and 96.5% using supersaturated salt solutions. We quantified vegetative growth and conidia formation for 3 weeks and implemented single linear regression and ANCOVA analyses to ascertain the effects of RH. Mycelial growth increased significantly with increasing RH by 2 and 3 weeks post inoculation (P < 0.001, r = 0.49, 0.61). This effect was most pronounced up to 81.5% RH, after which no significant increases in growth were detected. Conidiation increased linearly with increasing RH by 3 weeks post inoculation (P < 0.05, r = 0.33). Similar to mycelial growth, there was no difference in conidia production once RH exceeded 81.5%. The RH range permissive for significant mycelial development is fairly wide, and RH levels at 81.5% and above at 13 °C support similar levels of vegetative growth. However, our results indicate that at 13 °C, RH of 70% or lower impedes mycelial growth, which could restrict infection severity and/or colonization of organic matter. Finally, lower RH does not restrict production of conidia, which serve as important transmission propagules. Our study suggests that lowering RH could stem infection serverity but may be of limited value as a means of mitigating pathogen dispersal from infected to healthy bats.