Innersprings 101: Open, Continuous, and Fabric-Encased Coils

Posted by Mark Kinsley on

Benefits of innerspring mattresses.

Innersprings are the primary component in the majority of mattresses sold in America. Whether open or pocketed, the engineering of coils offers exceptional support. With a breathable design, coils allow a lot of airflow, keeping the sleep surface up to 28% cooler than foam mattresses. Coils also offer the ability to customize a mattress to individual preferences by adjusting coil counts, wire gauges, and coil turns within the unit. And, because they’re made of steel, coils are extremely durable and resist sagging, settling, or taking a body impression.

Coil counts can factor into the comfort and support level of a mattress, and affect the price of a mattress, though what’s more important is wire gauge and coil system construction. Coils can typically be found in four coil system types. Three of these are open coils: Bonnell, offset, and continuous coils. The fourth system consists of pocketed coils, which are encased in lightweight, breathable fabric.

What are open coil units?

The majority of mattresses are open-coil units. Coils are helically laced using a thin-gauge spiral wire that holds the coils together in rows. The unit is finished with an outer rod that keeps the unit together and offers additional strength to the edge of the bed.

Bonnell coils have an hourglass shape with rounded tops and bottoms. This basic coil design means Bonnell coil units are typically affordable and are often found in 300- to 400-count mattresses. Bonnell coils are made with heavy gauges, using more steel. The knots at the tops and bottoms provide stability for a supportive surface.

Offset coils have a similar shape as Bonnell, but the tops and bottoms are squared off so the coils can fit together in rows. The squared ends of offset coils offer a more conforming feel than Bonnell coils and will deliver firm support and durability. Knotted offset coils will be more stable, while open-end offset coils offer more bounce.

Continuous wire coils are formed using only one piece of wire for an entire row of coils. This can amount to higher coil counts, thinner gauges, and less steel used in manufacturing the units. These mattresses are generally affordable, and usually quite comfortable, but they can allow for more motion transfer than other open coils.


What are pocketed coils?

Pocketed coils are made by enclosing individual coils in their own flexible, breathable fabric pocket, resulting in reductions in motion transfer and noise. The fabric pockets are ultrasonically welded or glued together so coils can move independently, cradling and conforming to the sleeper. Before they’re placed in the pocket, the coils can be pre-compressed, giving them more firmness.

Because pocketed coils can be customized to almost any specification and comfort-level, they can produce a variety of unique feels. When used as a comfort layer, like NanoCoil™, pocketed coils can offer higher coil counts and a resiliency that other comfort layers, like foam and latex, can’t provide. Quality coils are durable, meaning they won’t sag or settle, so they’re ideal for a long-lasting comfort layer.

Since each coil is individually wrapped, coil heights can vary within a row, as seen in Leggett & Platt’s Hi-Low® Varying-Height Coil System. This construction allows for zoning in a conventional pattern, a checkerboard, or a cavity set-up. The cavities can be filled with gel or other comfort layers to adjust the mattress’ firmness.

Pocketed coils can also replace foam encasements that line the edge of some beds. Using coils around the edge of the bed, like the Quantum® Edge Steel Perimeter, provides a higher level of edge support so sleepers don’t feel like they might roll off their bed at night. And again, the durability of steel plays an important role, as it ensures the edge of the bed won’t break down, causing sagging or rounded edges.

Now that you have a better understanding of open, continuous, and fabric-encased coils you can guide shoppers to the innerspring option that’s best for their sleep needs.



About Mark

Mark Kinsley is Leggett & Platt's Staff Vice President of Marketing for the Bedding Group. Outside the office he enjoys spending time with family, mountain biking, hiking, and snow skiing. Connect with Mark on Twitter via @markkinsley. View all articles by Mark

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