In a hot bedroom, the sack comes off. From 26°C the app suggests a single short-sleeve bodysuit and no sack at all, a step lighter than the makers' charts, because a baby is safer slightly cool than too warm.
What the app suggests for sleep in a warm room, for a baby up to a year:
26–30°C: short-sleeve bodysuit. Warm room — a single light layer is enough, no sack needed. 24–25°C: short-sleeve bodysuit (0.5 TOG). A light 0.5 TOG sack over a short-sleeve bodysuit. 22–23°C: short-sleeve bodysuit (1.0 TOG). A 1.0 TOG sack over a bodysuit is a good start.
And awake, in the same room:
24–30°C: short-sleeve bodysuit. Warm at home — one light layer is likely plenty. Bare legs are fine. 22–23°C: short-sleeve bodysuit and socks. Comfortable room — a light top and socks.
The Lullaby Trust recommends a room of 16–20°C for a baby's sleep. Above that, cooling the room a little, if you can, helps most. A fan can move the air, just not pointed at the baby.
In real heat, the Lullaby Trust says a nappy alone can be enough. Never a hat, and no covers on top, however warm or cool the room feels.
Awake in the same room, one light layer is plenty, as the second table shows.
0–3 months, 27°C: short-sleeve bodysuit. 3–6 months, 27°C: short-sleeve bodysuit. 6–12 months, 27°C: short-sleeve bodysuit. 1–3 years, 27°C: short-sleeve bodysuit.
Never a hat for sleep — a baby cools through the head.
A sleep sack, never a loose blanket.
Safer slightly cool than too warm: nothing makes the sack warmer than the chart.
A fan can move the air — just not pointed at the baby.
Check the neck — warm but not sweaty means just right.