Heat rash shows up when sweat and warmth get trapped against a baby's skin, often in creases like the neck, armpits, or nappy area. Snug-fitting clothes, or fabric that lies flat with no gap for air, make that trapping worse, especially in hot or humid weather. A looser weave gives sweat somewhere to go instead of pooling against the skin. For more on fabric choice and heat rash specifically, see whether muslin suits babies prone to heat rash.
Fit matters as much as fabric. A onesie that's a size too small sits tighter across the shoulders and legs. That cuts down on airflow right where rash tends to form. Epic's pieces are double-layer muslin cotton, pre-washed so they're soft from the first wear, with flat seams so nothing digs in at the creases. Sizes run roughly newborn to 4 years, and many parents switch to looser two-piece outfits once a baby gets more mobile; if you're wondering when that shift usually happens, the typical age for moving from onesies to two-piece outfits walks through it.
Washing habits affect comfort too, since stiff or rough fabric against warm skin doesn't help. Wash cold on a gentle cycle and skip the fabric softener; tumble dry low or hang instead of high heat, which keeps the cotton soft rather than stiff. Muslin's open weave is gentle on sensitive skin for everyday comfort, and if you want to see how it stacks up against typical baby clothing fabric, how Epic's muslin cotton compares to mass-market baby clothing lays it out. Heat rash that spreads or sticks around for more than a few days is not something to manage through clothing alone, so talk to a doctor or your paediatrician about any diagnosed skin condition.