The commonly recommended range for a baby’s room is around 16–20°C, with 18°C often cited as a comfortable midpoint. Overheating is a recognised risk factor in unsafe sleep, so this is worth taking seriously rather than guessing by feel.
A simple room thermometer (no smart features needed) is one of the cheapest, highest-value purchases for a nursery, and pairs naturally with choosing the right tog-rated sleep bag for the season.
If the room runs warm in summer, a fan pointed away from the cot (never directly at baby) to circulate air is generally considered safer and more effective than closing the room up entirely.