Radon is a natural radioactive gas accounting for approximately one in ten lung cancer deaths, with substantially higher death rates in sub-Arctic communities where smoking is more prevalent. Radon transport is significantly reduced in permafrost, but permafrost is now thawing due to climate change. The effect of permafrost thawing on domestic radon exposure is unknown. Here we present results from radon transport modelling through soil, permafrost and model buildings either with basements or built on piles. We find that permafrost acts as an effective radon barrier, reducing radiation exposure to a tenth of the background level, while producing a ten-fold increase in the radon activity behind the barrier. When we model thawing of the permafrost barrier, we find no increase in radon to the background level for buildings on piles. However, for buildings with basements the radon increases to over one hundred times its initial value and can remain above the 200 Bq/m³ threshold for up to seven years depending on the depth of the permafrost and the speed of thawing. When thawing speed is taken into account, radiations remains higher than the threshold for all scenarios where 40% melting occurs within 15 years. This new information suggests that a significant sub-Arctic population could be exposed to radon levels dangerous to health as a result of climate change thawing of permafrost, with implications for health provision, building codes and ventilation advice.