The rule that decides it
Natural fibres breathe; most synthetics do not. Polyester satin and polyester chiffon look like their silk counterparts and cost a fraction as much, but they trap heat against the skin. In a hot climate that is the difference between comfortable and miserable, regardless of how the dress looks.
The good options
- Linen — the coolest fabric there is. It wrinkles, and no amount of pressing changes that. Accept the wrinkles as the character of the cloth or choose something else.
- Cotton lawn and voile — light, crisp, breathable, cheap, easy to sew. Excellent for day dresses.
- Rayon challis — drapes far better than cotton and stays cool. It shrinks, so it must be prewashed, and it shifts while being cut.
- Silk crepe de chine or habotai — expensive, but genuinely cool and beautiful. Worth it for an occasion dress.
- Cotton–linen blends — most of the coolness, noticeably fewer wrinkles. A sensible compromise.
For a summer wedding guest dress: rayon challis or silk crepe if the budget allows, linen if you do not mind wrinkles, cotton voile over a lining if you want floaty without the heat of polyester chiffon.
What to avoid
Polyester satin, polyester chiffon and anything described only as "silky" without a fibre content. Also avoid a full lining in a non-breathable fabric — if the dress needs lining, ask for cotton batiste or bemberg rayon rather than the standard polyester.
Shapes that help
Anything that does not sit flat against the body. An A-line or fit-and-flare with a defined waist allows air to move; a fitted sheath in the same fabric will feel hotter. Sleeveless and cap sleeves are obvious, but a loose elbow sleeve in linen is often cooler than bare skin in direct sun.