Stay cool in summer? Wear wool!
Yes, I know this seems counter intuitive. You don’t put a sweater on to stay cool in the summer, after all. But, a wool diaper cover breathes and lets air pass through the material of your baby’s diaper cover.
Breathable natural fibers will keep your baby cooler in summer. Avoid diapers or diaper covers that are made with non-breathable plastics that keep the heat and the sweat locked in. Avoid disposable diapers altogether. Paper and plastic are not as breathable and comfortable as cotton. A simple cotton diaper will do the simple diaper job.
If you are just hanging around the house or yard, forget the cover completely, and let your baby run around in just a cute diaper. From experience, though, I suggest that you watch closely to avoid a baby with a wet diaper sitting on your nice, light-color carpet.