Mattress construction explained
Foam vs. spring mattresses
Both materials can work in a conventional bed. In a thin replacement mattress, however, the limited depth changes how each construction performs.
Why depth matters
A sofa bed leaves less room for springs to work
A standard mattress is thick enough to accommodate taller springs with more working travel. A sofa bed mattress is less than half as thick, leaving far less room for coils to compress before reaching their solid height.
Side-by-side
How the materials respond
Continuous, adaptive support
Foam compresses where needed across the entire sleep surface. It conforms to the sleeper, distributes pressure, and returns to shape after the load is removed.
- Works efficiently in a shallow profile
- Conforms without individual metal coils
- High-quality foam resists bottoming out
Responsive, but depth-dependent
An innerspring mattress relies on many coils moving independently. More active coils and greater working length generally improve how well it adapts to the body.
- Needs room for coils to compress
- Thin profiles limit working length
- Quality construction requires more material and space
A closer look
What determines spring quality?
Spring performance depends on its usable travel, number of active coils, material, and manufacturing process.
Working length and active coils
- Free length
- The overall length of a spring when no load is applied.
- Solid height
- The compressed height when adjacent coils touch.
- Working length
- The usable travel between free length and solid height.
- Active coils
- The coils free to deflect under load. More active coils can improve contouring, but also require adequate working space.
Permanent set and material fatigue
Permanent set occurs when a spring is deflected beyond the elastic limit of its material and does not fully return to its original shape. Adequate working length helps reduce this risk.
Coating, corrosion, and stress relief
Steel springs must balance corrosion protection with material integrity. Manufacturing also creates internal stress, so higher-quality springs may be heat treated to relieve it. These processes add complexity and cost to a spring mattress.
The practical takeaway
Foam is well suited to thin replacement mattresses
Springs can make a satisfactory conventional bed, but their need for working depth creates real limitations in a thin sofa bed mattress. Quality foam provides support across the entire surface without requiring the same mechanical travel.