Muslin cotton is woven in a loose, open weave, which is what makes it feel light and lets air move through easily. That's ideal for warm, humid climates, but an air-conditioned room works the opposite way: it pulls warmth out of the air, and a single-layer muslin outfit won't trap much heat against your baby's skin. Our clothes use a double layer of muslin, which adds a bit more warmth than a single layer, but it's still an open, breathable weave.
In a cold room, dress your baby in muslin as a base layer and add something over it: a swaddle, a light cardigan, or socks and a hat if the room runs cold overnight. If you're deciding between styles for extra coverage, it helps to compare a muslin cotton onesie against a romper, since a romper covers more of the legs and arms in one piece. Check that your baby is warm enough by feeling their neck or chest rather than their hands and feet, which run cooler naturally.
Muslin's breathability matters most for babies prone to heat rash, so it's still a sound choice for hot afternoons even if the nursery runs cool at night. Wash it cold on a gentle cycle and skip the fabric softener so the weave stays soft and keeps its breathability. Our sizes run from newborn through 4 years, so you can size up for extra room to layer underneath as your baby grows.