Dairy cattle are sensitive to heat stress, which occurs when they cannot dissipate heat to maintain thermal balance. This stress can negatively affect milk production, reproduction, and health, resulting in lost revenue for dairy farmers. To study heat stress in dairy cows, the temperature-humidity index (THI) is commonly used which is expressed as a single value representing the combined effect of air temperature and humidity. Above certain THI thresholds, dairy cows are at greater risk of heat stress. This study explores changes in temperature, humidity, and THI in the past and future projections, both outside and inside dairy barns in major dairy producing regions across Canada. In-barn THI was measured in nine dairy barns in a range of locations and with contrasting barn designs. These data enabled relationships between outdoor and indoor conditions to be derived. Historical hourly T and RH data were used to compare the frequency of THI above critical levels from the 1960s to 2019, and climate change models were used to understand projected changes for the future under a variety of emission pathways.