What is CMHR foam? The support foam in UK mattresses, explained
If you read a UK-made mattress spec sheet you will meet "CMHR foam", usually with a number after it. It is not marketing; it is the workhorse support foam of the British bed trade. Here is what it is and what the numbers mean.
What the letters mean
CMHR stands for combustion-modified high-resilience foam.
- High-resilience (HR) foam is a polyurethane foam made with a more open cell structure and a different chemistry from standard foam. It is springier, supports more evenly, and recovers its shape better and for longer. Sit on it and it pushes back; standard foam just squashes.
- Combustion-modified (CM) means fire-retardant additives are built into the foam so it meets UK fire regulations (the Furniture and Furnishings (Fire) (Safety) Regulations and, for mattresses, BS 7177) without needing a fibreglass sock or a heavy chemical treatment on the cover. This is the main reason it is a British thing: our fire rules are stricter than most of Europe's.
So CMHR is a springy support foam that passes UK fire tests on its own merits.
What it does in a mattress
CMHR sits between the comfort layer and the springs, or forms the core of a foam mattress. Its job is transition and support: it stops you feeling the springs through the memory foam, spreads your weight across the spring unit, and gives the mattress its "hold". In a good build it is the layer that keeps the whole thing feeling supportive in year eight.
It is not a comfort material. It is firmer than memory foam and does not mould. It is also not a base foam; that job goes to reflex foam.
The three foams on a spec sheet
| Foam | What it is | Where it goes | Typical density |
|---|---|---|---|
| Memory foam (viscoelastic) | Softens with heat, moulds, pressure relief | Top comfort layer | 40 to 85 kg/m³ |
| CMHR foam | Springy, supportive, fire-modified | Between comfort layer and springs; foam-mattress cores | 35 to 50 kg/m³ |
| Reflex foam | Firm, dense, resilient | Base under the springs; solid cores | 30 to 40 kg/m³ |
"Convoluted CMHR" is the same foam cut with an egg-box profile so air can move through it; it is used in cooling builds.
What the number means
The number is density in kilograms per cubic metre: how much material is in the foam. For CMHR:
- Under 30 kg/m³: not really CMHR; a standard foam with a fire additive. Softens quickly.
- 35 kg/m³: entry-level, fine under a thin comfort layer in a value mattress.
- 40 to 45 kg/m³: the good range for a mattress meant to last eight to ten years.
- 50 kg/m³ and up: heavy-duty; used in firm edges, rails, and builds for heavier sleepers.
Density decides how long the foam keeps its resilience. Hardness (how firm it feels) is a separate figure, in newtons, and a maker should give both if asked.
Why it matters when you are buying
Most mattresses do not name their support foam at all; "high-quality foam" or "comfort foam" is as far as it goes. When a spec says CMHR and gives a density, you know two things: the maker is building to UK fire rules without fibreglass, and they are willing to be measured. When a spec says nothing, assume a low-density foam.
How we specify it
Every Chapter build lists its CMHR layer with its density: 35 kg/m³ on the entry build, 40 to 45 on the everyday, support, cool and Hotel builds, and a 50 kg/m³ rail where an edge needs to be sat on. It is on every product page under "Every layer, by density". Compare the chapters.
The short version
- CMHR is a springy support foam with fire retardancy built in; a British standard material.
- It sits under the comfort layer and gives the mattress its hold.
- 40 to 45 kg/m³ is the range to look for; under 30 is not the real thing.
- If a spec sheet names it with a number, that is a good sign about the maker.
What BS 7177 on a mattress label means, and what "low hazard" is
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