Double-Knit vs Single-Jersey for Sports Bra Shells
The choice between single jersey and double knit quietly decides how a cut-out back holds its shape, how much the garment grows in wear, and how many returns come back for a neckline that has lost its line. It is also one of the few construction decisions that shows up in unit cost immediately, so it should be made deliberately rather than inherited from whichever swatch the mill had on the shelf.
How the two behave
Single jersey is one set of needles and one face: lighter, softer, more extensible and cheaper per meter. Its weaknesses are structural. It curls at raw edges, it has poorer dimensional stability, it can spiral after laundering, and it grows more under load. Double knit, which in this category means interlock, ponte or a double-faced construction, runs two sets of needles: it is stable, does not curl, resists spirality, gives better opacity at the same weight and recovers more consistently. It costs more per kilo and usually dries more slowly, because there is more fiber holding water.
What it means for a cut-out back
Edge behaviour is usually the deciding factor. A large hollow back panel with open lock-hole vents at the side has a lot of cut edge. In single jersey, every one of those edges needs binding, tape or adhesive film to stop it curling and waving after washing, which adds trims, operations and bulk in exactly the area the wearer feels against the shoulder blade. In double knit the edge is stable enough to finish cleanly with less reinforcement, and the opening keeps its shape through repeated laundering. If the design carries any open or lightly finished edge, double knit is usually the right call.
Support and recovery
Medium impact support depends on resistance to movement, and double knit delivers more of it at the same gsm and elastane content, because the structure itself contributes stability. That matters most at the upper end of B/C grading, where containment demand rises with cup volume. Single jersey can be used there, but it generally needs a containment lining or power mesh under the cup, which moves the cost back up and adds a layer to a garment sold on being light.
Stability testing
Spirality and skew are the failure modes buyers notice late. Test skewness change to AATCC 179 and spirality after laundering to ISO 16322-1, and set a maximum, typically around 3 percent for a bra shell, because a twisted body is unwearable regardless of its other properties. Pair that with AATCC 135 dimensional change within plus or minus 3 percent and recovery to ASTM D4964, and the two structures separate clearly on paper before you cut a single sample.
Cost, weight and where single jersey still fits
Double knit in the 220 to 280 gsm range is the standard medium impact shell. Single jersey still has a place in linings, pad pockets, light fashion layers and cost-led programmes where the support work is done by structure and by a high-power lining rather than by the shell. If you take that route, budget for the extra stabiliser and for the edge operations, and check pilling to ASTM D3512 and abrasion to ASTM D4966, because lighter single jersey constructions wear faster at the underband and underarm.
Pick the structure for the edge behaviour you need, then buy the gsm and elastane content that delivers the power. Doing it in that order saves the most development time.
