What Is Memory Foam? Density, Heat and the Honest Drawbacks
Memory foam is polyurethane foam with the chemistry adjusted so it softens with heat and recovers slowly. Press it and the dent stays for a second or two. That slow recovery is the whole product.
What it does well
Because it softens where your body is warmest and heaviest, it spreads load rather than concentrating it. At the shoulder and hip — the two narrow points that carry everything when you lie on your side — that is the best pressure relief available in a mattress.
It also absorbs movement almost completely, which is why all-foam beds feel so still.
Density: what the number means
Density is measured in kilograms per cubic metre and it describes how much material is in the foam, not how firm it is. Those are different properties and the trade confuses them constantly.
| Density | What it means in practice |
|---|---|
| Under 40 kg/m³ | Budget. Contours at first, softens permanently within a few years. |
| 45–55 kg/m³ | The useful range for a comfort layer in a domestic mattress. |
| Over 60 kg/m³ | Very durable, very slow to recover, sleeps warmest. |
Firmness is measured separately, usually as ILD. A high-density foam can be soft and a low-density foam can be firm.
The two real drawbacks
It sleeps warm
The property that makes it contour is the same one that makes it insulate: dense, closed cells, and a surface that wraps around you reducing airflow over the skin. Gel infusions and open-cell variants delay it; they do not remove it. Putting the memory layer over a spring unit rather than a foam block makes far more difference than any additive — see sleeping hot.
It is slow to respond
Turning over means climbing out of the shape you were in. Some people find this comforting and some find it exhausting. If you move a lot at night, latex does the same job of relieving pressure while springing straight back.
The smell
New foam off-gasses for 24 to 72 hours. It is harmless and it goes away with a window open. A rolled mattress smells strongest in the first hour after unwrapping.
Who should avoid it
- Anyone who overheats at night.
- Front sleepers — the hips sink and the lower back arches.
- People with reduced mobility, who can find getting out of a deep memory surface genuinely difficult.
- Very light sleepers under about 55 kg, who often never compress it enough to feel the benefit.
Common questions
Is memory foam the same as viscoelastic foam?
Yes. "Viscoelastic" is the technical name.
Does it need turning?
No — the layers are built in one order. Rotate it head to foot instead, twice a year.
How long does it last?
Eight to ten years over a spring unit. The comfort layer is doing most of the work, so it is the part that degrades.